⚡ Key Questions Answered Upfront
📄 Contents
- Permitted Development Rights
- Prior Approval (Larger Home Extension)
- When You Need Planning Permission
- Types of Extension Explained
- Extension Cost Guide 2025
- Interactive Cost Calculator
- Additional Costs and Fees
- The Planning Application Process
- Building Regulations
- Party Wall Act
- Impact on Property Value
- Frequently Asked Questions
🏠 Permitted Development Rights: When You Don't Need Planning Permission
Permitted Development (PD) rights allow you to make certain improvements to your home without needing to apply for planning permission. These rights are granted by Parliament and apply to most houses in England. However, PD rights do not apply to flats, maisonettes, or listed buildings, and may be restricted in conservation areas or Article 4 Direction areas.
Single-Storey Rear Extensions
This is the most common type of extension. Under PD rights, a single-storey rear extension is permitted if:
- The extension does not exceed 4 metres in depth for a detached house, or 3 metres in depth for any other house (semi-detached or terraced)
- The maximum height does not exceed 4 metres
- The extension does not exceed the height of the existing eaves
- The extension does not extend beyond the side walls of the original house
- The materials used are similar in appearance to the existing house
- The extension does not include a veranda, balcony or raised platform
You own a detached house in Leicester. You want to build a single-storey rear extension 4 metres deep and 5 metres wide, with a flat or low-pitch roof no higher than 3.5m. This falls within Permitted Development — no planning application required. You will still need Building Regulations approval before starting work.
Double-Storey Rear Extensions
A double-storey rear extension can also be built under PD rights, but with stricter limits:
- Must not exceed 3 metres in depth beyond the rear wall of the original house
- Must be more than 7 metres from the rear boundary of the property
- The roof pitch must match the existing house as far as possible
- Upper-floor windows on the side elevation must be obscure-glazed
- Any upper-floor side windows must be non-opening, or openable only above 1.7m from floor level
- Must not exceed the height of the existing house
Side Extensions
Single-storey side extensions are permitted under PD rights provided:
- The width of the extension is no more than half the width of the original house
- The maximum height does not exceed 4 metres
- It is a single storey only (maximum 4m height)
Note: In designated areas (conservation areas, National Scenic Areas), side extensions generally require planning permission even if they meet these limits, as PD rights are reduced.
Loft Conversions Under PD
A loft conversion can be carried out under Permitted Development if:
- The additional roof space created does not exceed 40 cubic metres for terraced houses
- The additional roof space does not exceed 50 cubic metres for detached and semi-detached houses
- No extension is higher than the highest part of the existing roof
- No front roof extension is built (dormers to the front require planning permission)
- Side-facing windows are obscure-glazed and non-opening below 1.7m from floor level
- The materials match the existing house
Garage Conversions Under PD
Converting an integral garage to living space typically does not require planning permission under PD rights, as it does not involve an external addition. However, building regulations approval is required. If you are converting a detached garage, this may require planning permission depending on the works involved.
General PD Rules That Always Apply
- No extension can be built beyond the principal elevation (front wall) of the house
- Total extensions and outbuildings must not cover more than 50% of the total area of land surrounding the original house
- Extensions must not be used as a separate dwelling
- In designated areas (conservation areas, AONB, National Parks), cladding the outside of the house with stone, artificial stone, pebbledash, render, timber, plastic or tiles requires planning permission
✓ PD Typically Allowed
- Single-storey rear extension up to 4m (detached) / 3m (other)
- Loft dormer to the rear
- Side extension up to half house width
- Integral garage conversion
- Outbuilding in rear garden
✗ Usually Requires Planning Permission
- Any extension beyond front wall
- Extension exceeding PD size limits
- Work on flats or maisonettes
- Balconies or raised platforms
- Work on listed buildings
- Extensions in Article 4 Direction areas
⚠ Restricted PD in Designated Areas
- Conservation areas: side extensions require permission
- AONB / National Parks: stricter limits
- Article 4 areas: PD rights removed by council
- Some new-build estates have PD rights removed by planning condition
📄 Prior Approval: The Larger Home Extension Scheme
The Larger Home Extension scheme (also called the Neighbour Consultation Scheme) is a form of Prior Approval that allows you to build bigger single-storey rear extensions than standard PD rights permit. It was made permanent in 2019 and applies throughout England.
What Are the Size Limits?
- Detached houses: Up to 8 metres deep (instead of 4m under standard PD)
- Semi-detached and terraced houses: Up to 6 metres deep (instead of 3m under standard PD)
- Maximum height remains 4 metres for single-storey extensions
- Extension must not extend beyond the side wall of the original house
How the Prior Approval Process Works
-
Submit a Prior Approval application to your local council
Provide a written description of the proposed extension, a plan showing the site and the proposed extension, and the addresses of any adjoining houses. There is no fee for this Prior Approval application.
-
Council notifies your neighbours (42-day consultation)
The council will notify the owners of any adjoining properties. Neighbours have 42 days from the date of the council's notification letter to raise objections about the impact on their amenity.
-
No objections — automatic approval
If no objections are received within the 42 days, you automatically have Prior Approval and can proceed with the build. The council must confirm this in writing.
-
Objections received — council assesses impact on amenity
If neighbours object, the council will assess the impact of the proposed extension on the amenity of adjoining properties. It can only consider the impact on light, outlook and privacy — not general planning matters. The council must make its decision within 42 days of the application date.
A homeowner wants to build a 7-metre-deep single-storey rear extension on a detached house. This exceeds the 4m standard PD limit but is within the 8m Prior Approval limit. They submit a Prior Approval application. A neighbour objects about loss of light. The council assesses the impact and decides the effect on the neighbour's amenity is acceptable. Prior Approval is granted within 42 days.
📌 When You Need Full Planning Permission
You will need to submit a full householder planning application (or sometimes a prior approval application) in the following circumstances:
Your Extension Exceeds Permitted Development Limits
If your proposed extension is larger than the PD limits allow — for example, a 5-metre-deep single-storey rear extension on a semi-detached house, or any two-storey side extension — you will need planning permission. There is no way around this: building without permission when required is a breach of planning control and can result in enforcement action.
Conservation Areas
Properties in conservation areas face additional restrictions. The following works require planning permission even if they would otherwise be PD:
- Any extension that is visible from a public highway (not just from the front)
- Side extensions of any size
- Cladding the exterior in any material other than the existing material
- Demolition of a chimney
To find out if your property is in a conservation area, use your council's planning portal or the Planning Portal postcode checker at planningportal.co.uk.
Listed Buildings
If your home is a listed building, you need both planning permission and Listed Building Consent for any extension — regardless of size or location. This includes internal alterations. The consent process is more rigorous and you will need to demonstrate that the extension preserves the character and special interest of the listed building. Unauthorised works to a listed building are a criminal offence with an unlimited fine.
Areas of Outstanding Natural Beauty (AONB) and National Parks
In these designated areas, the Permitted Development rules are the same as for conservation areas. Side extensions and certain roof extensions require planning permission. The Prior Approval Larger Home Extension scheme still applies, but the council may be less likely to grant it if the extension is prominently visible.
Article 4 Direction Areas
A local planning authority can remove PD rights in a specific area by issuing an Article 4 Direction. This is common in parts of cities where the council wishes to control the character of an area — particularly in HMO (Houses in Multiple Occupation) areas or in historic suburban streets. If an Article 4 Direction applies to your property, you must apply for planning permission for works that would otherwise be PD. Check with your council.
Flats and Maisonettes
Permitted Development rights do not apply to flats or maisonettes. If you live in a flat, any extension, additional room or structural change will require planning permission. In practice, it is very rare for the owner of a flat to have the right to extend, as the structure of the building (including external walls, roof and foundations) is typically owned by the freeholder or managed by a management company.
What Happens If You Build Without Permission?
Building without planning permission when it is required is called a breach of planning control. The council can serve an Enforcement Notice requiring you to undo the unauthorised work. If the breach occurred within the last 4 years (for operational development), the council can take action. This can be very costly — not only do you face demolition costs, but an unauthorised extension can make the property difficult or impossible to sell, as buyers' solicitors will discover the issue.
🏠 Types of House Extension Explained
Choosing the right type of extension depends on your home's layout, your budget, your planning situation, and what you want to achieve. Here is a detailed guide to each type.
Single-Storey Rear Extension
The most popular type of extension. Extends the back of the house to create a larger kitchen, dining area, or living space. Often opened up with bi-fold or sliding doors onto the garden. Works well on all house types.
£30,000 – £80,000 ✓ Often within Permitted Development (up to 4m detached / 3m other, or up to 8m/6m via Prior Approval)Double-Storey Rear Extension
Adds two floors to the rear of the house, typically providing a larger kitchen-diner on the ground floor and one or two additional bedrooms on the first floor. Excellent value for money as cost per square metre decreases. Transforms the living space significantly.
£60,000 – £100,000+ ⚠ Limited PD: max 3m deep, 7m+ from rear boundary. Most require planning permission.Side Return Extension
Common in Victorian terraced and semi-detached houses, which often have a narrow alleyway running alongside the kitchen. Filling in this side return creates a much wider, open-plan kitchen-diner. Often combined with a rear extension for a wrap-around effect.
£25,000 – £50,000 ✓ Often within PD (max 50% of house width). May need planning in conservation areas.Wrap-Around Extension
Combines a rear extension with a side return extension to create an L-shaped addition. Maximises ground-floor space and is particularly effective for creating a large open-plan kitchen-diner-living room with garden access. Popular in Victorian terraces.
£50,000 – £120,000 ⚠ Usually requires planning permission due to combined size exceeding PD limits.Loft Conversion (Dormer)
Converts the roof space into a habitable room by adding a box-shaped dormer window to the rear roof slope. Creates a bedroom with ensuite in most cases. One of the most cost-effective ways to add a bedroom and bathroom without reducing garden space.
£30,000 – £55,000 ✓ Usually within PD (up to 40m³ terraced / 50m³ detached & semi). Rear dormers only.Hip-to-Gable Loft Conversion
Converts a hipped roof (sloping on three or four sides) to a gable end (vertical wall on the side), creating much more usable loft space. Usually combined with a rear dormer. Common on 1930s semi-detached houses.
£40,000 – £65,000 ⚠ Changes the roofline — may require planning permission if it extends beyond PD limits.Mansard Loft Conversion
Creates a near-flat roof with steeply sloped sides, maximising the internal volume. Common in central London terraces. Often extends across the full width of the house. Provides the most space of any loft conversion type.
£50,000 – £90,000 ⚠ Changes the roofline significantly — almost always requires planning permission.Garage Conversion
Converting an integral or attached garage into living space is one of the cheapest ways to add a room. Popular uses include a home office, playroom, guest bedroom or utility room. The existing structure is retained; the main costs are insulation, flooring, windows and a new door.
£10,000 – £25,000 ✓ Usually within PD for integral garages. No external addition so often no planning required.Orangery or Garden Room
A premium garden room or orangery creates a bright, airy living space at the rear of the house. Uses more glazing than a traditional extension. An orangery has a partially solid roof with a central lantern, distinguishing it from a conservatory (which has a fully glazed roof).
£30,000 – £70,000 ✓ Often within PD if within size limits. Full conservatories with glass roofs may be exempt.Basement Extension
Creating a new basement or extending an existing one is the most expensive and complex type of extension, but adds significant space in properties where extending outwards or upwards is not possible. Popular in London where garden space is limited. Involves underpinning and significant structural engineering.
£100,000 – £200,000+ ⚠ Usually requires planning permission if it changes the external appearance. Always needs Building Regs.💰 Extension Cost Guide 2025
Extension costs in the UK have risen significantly since 2020 due to increases in labour and materials costs. The figures below are realistic 2025 estimates based on current market rates. They include labour and materials but exclude VAT (20%), architect fees, planning fees, and other professional costs which are covered separately below.
Cost by Extension Type and Size
| Extension Type | Size | Basic Spec | Standard Spec | Premium Spec |
|---|---|---|---|---|
| Single-storey rear | 20 sqm | £30,000 | £40,000 | £50,000 |
| Single-storey rear | 30 sqm | £42,000 | £57,000 | £75,000 |
| Single-storey rear | 40 sqm | £55,000 | £72,000 | £95,000 |
| Double-storey rear | 40 sqm (20 per floor) | £60,000 | £75,000 | £95,000 |
| Double-storey rear | 60 sqm (30 per floor) | £80,000 | £105,000 | £135,000 |
| Side return extension | 15–25 sqm | £25,000 | £35,000 | £50,000 |
| Wrap-around extension | 30–50 sqm | £55,000 | £80,000 | £120,000 |
| Loft conversion (dormer) | 25–40 sqm | £35,000 | £45,000 | £65,000 |
| Hip-to-gable loft | 30–50 sqm | £45,000 | £60,000 | £80,000 |
| Mansard loft | 40–60 sqm | £55,000 | £75,000 | £100,000 |
| Garage conversion | 15–25 sqm | £10,000 | £18,000 | £28,000 |
| Basement extension (new) | 30–60 sqm | £100,000 | £150,000 | £220,000+ |
| Orangery / garden room | 20–35 sqm | £35,000 | £50,000 | £75,000 |
Cost Per Square Metre Benchmarks
| Specification Level | Cost per sqm (excl. VAT) | What It Includes |
|---|---|---|
| Basic | £1,200 – £1,600 | Standard block/brick construction, basic kitchen or utility-level finish, UPVC windows and doors, plasterboard walls |
| Standard | £1,800 – £2,400 | Better specification brick or render finish, aluminium or timber windows, bi-fold or sliding doors, solid or engineered wood flooring, good quality kitchen units |
| Premium | £2,800 – £4,000+ | High-spec materials throughout, structural glass elements, bespoke joinery, underfloor heating, high-end kitchen, architect-designed features |
Regional Cost Variations
Extension costs vary considerably by region. The table below shows approximate multipliers relative to the national average (Midlands / North England baseline):
| Region | Cost Multiplier | Note |
|---|---|---|
| Inner London | +30% to +50% | Highest labour costs, access challenges, party wall complexity |
| Outer London / South East | +15% to +25% | High demand for builders; supply chain costs higher |
| South West / East of England | +5% to +15% | Above national average in many areas |
| East Midlands / West Midlands | Baseline (0%) | National average reference point |
| North West / Yorkshire | -5% to +5% | Near national average; some variation city vs rural |
| North East / Scotland | -5% to -10% | Lower labour costs but materials often similar |
| Wales | -5% to 0% | Varies by proximity to major cities |
🔐 Extension Cost Calculator 2025
Estimate your extension cost based on type, size, quality and location. Figures are guideline estimates including labour and materials but excluding VAT and professional fees.
📋 Additional Costs and Fees You Must Budget For
The build cost is only part of the total spend on a house extension. There are numerous additional costs that homeowners frequently underestimate. Here is a complete breakdown.
| Cost Item | Typical Cost | Notes |
|---|---|---|
| VAT (20%) | 20% of build cost | Charged on all labour and most materials by VAT-registered builders. New residential builds are zero-rated, but extensions are standard-rated at 20%. |
| Architect fees | 8%–15% of build cost | On a £60,000 extension, expect £5,000–£9,000 for a full service including planning drawings, building regs drawings and contract administration. |
| Householder planning application fee | £258 | Fee from April 2025. Previously £206. Applies to householder planning applications in England. Wales and Scotland have different fees. |
| Lawful Development Certificate | £103 | Half the planning fee. Recommended to confirm PD works are lawful. Gives you a legal document for when you sell the property. |
| Structural engineer | £500–£2,000 | Required for all extensions. Calculates beam sizes, foundation requirements and structural specifications. Fee depends on complexity. |
| Building regulations application | £500–£1,200 | Full Plans application fee (submitted before work starts) covers plan check and inspection visits. Fees vary by local authority and project value. |
| Party wall surveyor | £800–£3,000 | Required if your extension involves work on or near a shared wall with a neighbour. Each neighbour will need their own surveyor (whose cost you may be liable for). |
| Topographic / measured survey | £500–£1,500 | Your architect may require an accurate measured survey of your property before drawing plans, particularly for complex sites. |
| Drainage survey / CCTV drain survey | £200–£500 | May be required if the extension is near existing drains. Building over a sewer requires a build-over agreement with the water company. |
| Arborist report (trees) | £300–£800 | Required by planning if there are protected trees (TPO) near the proposed extension, or in a conservation area. |
| New kitchen or bathroom | £5,000–£25,000+ | If the extension includes a new kitchen or bathrooms, these are separate to the build cost. A good quality kitchen can add significantly to total project cost. |
| Contingency fund | 10%–15% of build cost | Essential. Unexpected costs are common: poor ground conditions, hidden structural problems, services that need rerouting. Always hold back 10–15%. |
Build cost ex. VAT: £63,000 | VAT (20%): £12,600 | Architect fees (10%): £7,560 | Planning fee: £258 | Structural engineer: £900 | Building regulations: £800 | Party wall surveyor: £1,500 | New bi-fold doors and kitchen: £15,000 | Contingency (10%): £7,500 | Total: approximately £109,000
Can I Reclaim VAT on an Extension?
In most cases, no. VAT at 20% is charged on extensions to existing homes and is not reclaimable by private homeowners. However, there are two important exceptions:
- Converting a non-residential building to a dwelling is zero-rated (0% VAT)
- Converting a property that has been empty for 2+ years is reduced-rated (5% VAT)
If you are a VAT-registered business and the extension is for business use, you may be able to reclaim VAT. Consult a VAT specialist if this applies to your situation.
📌 The Planning Application Process: Step by Step
If your extension requires planning permission (either because it exceeds PD limits or because your property is in a restricted area), here is what to expect from the full householder planning application process.
Step 1: Pre-Application Enquiry (Optional but Recommended)
Before submitting a formal planning application, most councils offer a pre-application advice service. For a fee (typically £50–£200 for householder enquiries), a planning officer will review your proposal and advise on its likelihood of success. This can save significant time and money by identifying issues early.
Pre-application advice is particularly valuable if:
- Your property is in a conservation area or has a listed building designation
- Your proposal is significantly larger than typical extensions in the area
- The design is unconventional (flat roof, modern materials in a traditional area)
- You have had a previous application refused
Step 2: Appoint an Architect or Architectural Technician
Unless your proposal is very simple, you will need professional drawings for your planning application. An architect will:
- Measure your existing house and produce scale drawings
- Produce site plan, block plan, existing and proposed floor plans and elevations
- Advise on design to maximise the chance of approval
- Submit the planning application on your behalf
- Liaise with the planning officer during the assessment period
Check that your architect is registered with the Architects Registration Board (ARB). You can also use an architectural technician (MCIAT) for less complex projects, often at a lower fee.
Step 3: Submit via the Planning Portal
Householder planning applications in England are submitted via the Planning Portal at planningportal.co.uk. The application requires:
- Completed application form (1APP)
- Site location plan (1:1250 scale OS map showing property boundary)
- Block plan (1:500 scale showing property and immediate surroundings)
- Existing and proposed floor plans
- Existing and proposed elevations
- Existing and proposed sections (if relevant)
- Design and Access Statement (required in some cases)
- Application fee: £258 (from April 2025)
Step 4: Validation and Registration (1–2 Weeks)
The council validates your application and checks all documents are correct. Once valid, it is registered and assigned a reference number. You can track its progress online. The 8-week statutory determination period starts from the date of registration, not the date of submission.
Step 5: Neighbour and Statutory Consultation (21 Days)
The council notifies your immediate neighbours in writing and, where relevant, statutory consultees (such as the Highways Agency, Environment Agency, or Historic England). Neighbours and the public have 21 days to submit representations. These are considered by the planning officer but are not a veto — even strong opposition does not automatically lead to refusal.
Material planning considerations that can influence a decision include:
- Loss of light to neighbouring properties
- Overlooking and loss of privacy
- Overbearing impact
- Design and appearance
- Impact on conservation area character
Non-material matters that cannot influence a decision include: the value of properties, personal circumstances of the applicant, private disputes between neighbours, loss of a view (unless protected), and competition between businesses.
Step 6: Planning Officer Assessment
The planning officer visits the site and assesses the proposal against local and national planning policies. The planning officer writes a report recommending approval or refusal, with reasons. For householder applications, the decision is made by the planning officer under delegated authority in the vast majority of cases (typically over 90%). Only complex or controversial applications go to a planning committee.
Step 7: Decision (8 Weeks)
The council must make a decision within 8 weeks for a householder planning application (or 13 weeks for more major applications). In practice, many councils take longer, and you can consent to extensions of time if the officer requests it. Once a decision is made, you receive a decision notice. If approved, you have 3 years to start the development (unless a shorter period is specified as a condition).
If Your Application Is Refused
If your application is refused, the decision notice will set out the reasons. You have two options:
- Revise and resubmit: If the issues can be resolved by modifying the design, you can submit a revised application. One resubmission within 12 months of the original decision is free.
- Appeal: You can appeal to the Planning Inspectorate within 12 weeks of the refusal decision. Around 30% of householder appeals succeed. Appeals take 6–12 months.
🔧 Building Regulations: Separate from Planning Permission
Building regulations are completely separate from planning permission and are almost always required for any extension, even those that are permitted development. Planning permission controls whether you can build; building regulations control how it must be built.
What Do Building Regulations Cover?
Building regulations cover the following technical requirements for your extension:
🏛 Structural Integrity (Part A)
- Foundation depth and type for ground conditions
- Wall construction and load-bearing capacity
- Beam and lintel sizing over openings
- Roof structure and loading
- Lateral restraint and stability
🔥 Fire Safety (Part B)
- Means of escape in case of fire
- Fire resistance of walls and floors
- Smoke alarms and heat detectors
- Separating walls between dwellings
- Distance from property boundaries (fire spread)
🏙 Insulation and Energy (Part L)
- U-values for walls, roof and floor
- Window and door thermal performance
- Thermal bridging at junctions
- Air tightness testing
- Low carbon heating options may be required
💧 Drainage (Part H)
- Foul water drainage connection
- Surface water drainage (SuDS in new extensions)
- Build-over agreements for work near sewers
- Rainwater disposal from roof
🌞 Ventilation (Part F)
- Adequate ventilation to habitable rooms
- Mechanical extract ventilation to kitchens and bathrooms
- Trickle ventilators in windows
- Whole-house ventilation in very airtight builds
⚕ Electrics and Gas (Parts G, J, P)
- Electrical installation testing and certification
- Safe positioning of boiler and gas appliances
- Sanitary ware and hot water safety
- FENSA certification for replacement windows
How to Apply for Building Regulations Approval
There are two routes for building regulations approval:
- Full Plans application: You submit detailed drawings before work starts. A building control officer checks the plans and approves them. This gives you certainty before you start building and is recommended for all but the simplest works.
- Building Notice: You notify the council that you are starting work, without submitting detailed plans. The building control officer inspects at key stages. This is faster but carries more risk if the build does not meet regulations.
You can use your local council's building control team or an approved inspector (private building control body). Fees are set by each council and are broadly similar in the private sector. For a typical 30 sqm extension, expect to pay £600–£1,200 for a Full Plans application.
Inspection Stages
A building control inspector will visit your site at key stages to check compliance:
- Commencement (you must notify within 2 days of starting)
- Excavation for foundations (before concrete is poured)
- Foundation concrete poured
- DPC (damp proof course) level
- Drainage installation (before backfilling)
- Structural frame (if steel frame used)
- Roof structure (before tile or felt is laid)
- Insulation installation
- Completion inspection
Completion Certificate
On satisfactory completion, the building control body issues a Completion Certificate. This is an important document: it confirms the works were built to building regulations standards. Keep it safe — solicitors will ask for it when you sell your property, and mortgage lenders may require it. An extension without a Completion Certificate can cause significant problems on sale.
🏠 The Party Wall Act: Your Rights and Obligations
The Party Wall etc. Act 1996 governs works that affect shared walls, boundaries and structures between neighbouring properties. If your extension involves any of the following, you must comply with the Act:
- Work on or to a party wall (a wall shared with a neighbour, including a wall that sits on the boundary)
- Excavation within 3 metres of a neighbouring building or structure, where the excavation goes deeper than the neighbour's foundations
- Excavation within 6 metres of a neighbouring building where the excavation is deeper than a line drawn at 45 degrees from the base of the neighbour's foundations
- Building a new wall on the boundary line
What You Must Do
If the Party Wall Act applies to your works, you must serve a Party Wall Notice on all affected neighbours at least 2 months before starting work (or 1 month for excavation works). The notice must describe the works and state the proposed start date.
Your neighbour can:
- Consent in writing — works can proceed under the Act without a surveyor being involved
- Dissent — you must both appoint surveyors (or agree on a single "agreed surveyor") who will produce a Party Wall Award setting out how the works are to be carried out
- Not respond within 14 days — this counts as dissent and triggers the surveyor process
Costs
If your neighbour dissents, you (as the building owner) are responsible for paying for both your own surveyor and your neighbour's surveyor. Typical costs:
- Agreed single surveyor: £800–£1,500
- Two surveyors (one each): £1,500–£3,500 total
- Complex cases (basement work, London terraces): up to £5,000+
The Party Wall Award also includes a schedule of condition of your neighbour's property before works start. This protects both parties: if the neighbour later claims your works caused damage, the schedule of condition is the baseline.
📈 Impact on Property Value and Tax
How Much Value Does an Extension Add?
A well-designed extension can add significant value to your home, but the return on investment varies enormously by location, type of extension and local market conditions. Here are typical value uplifts:
| Extension Type | Typical Value Added | Best For |
|---|---|---|
| Single-storey kitchen-diner | 5%–8% | Family homes; open-plan demand very strong |
| Double-storey (extra bedroom + bathroom) | 10%–20% | Properties where adding a bedroom moves to next price tier |
| Loft conversion (ensuite master bedroom) | 10%–15% | 3-bed to 4-bed conversions; very strong return |
| Garage conversion (home office or bedroom) | 3%–8% | Lower cost; good ROI if not losing off-street parking |
| Basement extension (London) | 5%–15% | Prime London properties with limited garden; high cost but high values |
| Side return (larger kitchen) | 4%–7% | Victorian terraces; transforms ground floor living |
A double-storey extension adding a fourth bedroom and a second bathroom costs £80,000 all-in (inc. VAT and fees). The property's value increases by an estimated 15%, adding £42,000 in value. Net position: -£38,000 short-term, but the homeowners gain a significantly improved home and may recoup more on sale over time as property values rise.
Council Tax Band Implications
Adding an extension does not immediately change your council tax band. The Valuation Office Agency (VOA) only reassesses a property's band when the property is sold after the extension is built. At that point, if the extension significantly increases the value, the band may be increased. If you are in the highest band (Band H in England), the band will remain unchanged regardless of the extension size.
Capital Gains Tax
If your extended home is your main residence, Private Residence Relief means you will not pay Capital Gains Tax (CGT) on any gain when you sell. However, if you rent out any part of the property or use it partly for business, the CGT position becomes more complex and you should seek professional advice.
Stamp Duty on Subsequent Purchase
An extension to your current home does not trigger any stamp duty. However, by increasing the value of your property, a future purchase of a more expensive property will result in a higher stamp duty bill. Use our Stamp Duty Calculator to understand the cost of your next move.
Insurance During and After the Build
Notify your home insurer before starting any building works. Most standard home insurance policies have exclusions for building works, and an extension being built increases your rebuilding liability. You or your builder should have Contract Works insurance during the build. Once completed, notify your insurer again to update the reinstatement value of your property.
❓ Frequently Asked Questions
-
Do I need planning permission for a rear extension?Not always. Under Permitted Development rights, a single-storey rear extension up to 4 metres deep is allowed on a detached house, and up to 3 metres deep on a semi-detached or terraced house, without planning permission. Under the Prior Approval Larger Home Extension scheme, you can extend up to 8 metres (detached) or 6 metres (semi/terraced), subject to a 42-day neighbour consultation with your local council. Planning permission is required if you exceed these limits, or if your property is a flat, listed building, or in a conservation area with additional restrictions.
-
How big can a rear extension be without planning permission?Under standard Permitted Development rights: up to 4 metres deep for a detached house, or 3 metres deep for any other house (semi-detached or terraced). The extension must also not exceed 4 metres in height. Under the Prior Approval Larger Home Extension scheme, these limits increase to 8 metres for detached houses and 6 metres for other houses, but you must notify the council and allow 42 days for neighbour consultation. In all cases, the extension must not extend beyond the side walls of the original house, and materials must visually match the existing house.
-
How long does planning permission take?A householder planning application for a home extension has a statutory 8-week determination period, starting from the date the application is validated (not the date of submission). In practice, some councils take longer, particularly if they are under-resourced or the application is complex. The council and applicant can agree to extend this period. Prior Approval under the Larger Home Extension scheme takes 42 days. If planning permission is refused and you appeal to the Planning Inspectorate, the appeal process typically takes a further 6 to 12 months.
-
How much does a single storey extension cost in 2025?In 2025, a single-storey rear extension costs approximately £1,400 to £3,000 per square metre for the build, excluding VAT. A typical 20 sqm extension costs £30,000 to £50,000, and a 40 sqm extension costs £55,000 to £95,000. Adding VAT (20%), architect fees (8–15%), planning fees (£258 if required), structural engineer fees (£500–£2,000), building regulations costs (£600–£1,200), and a kitchen or fitting-out costs, the all-in total for a 30 sqm extension in the South East is typically £80,000 to £130,000. London costs run 30–50% higher.
-
Does an extension add value to a house?Yes, in most cases. A well-designed single-storey kitchen-diner extension typically adds 5–8% to a property's value. A double-storey extension adding a bedroom and bathroom can add 10–20%, particularly if it converts the property from three bedrooms to four, which is a significant price threshold in most markets. A loft conversion adding an ensuite master bedroom is often one of the best-value extensions, adding 10–15%. However, the return on investment is not always immediate, and over-extending for the area (creating the largest house in the street) can mean you do not fully recoup the cost on sale.
-
Can neighbours stop my extension?If your extension complies with Permitted Development rights, neighbours cannot stop it. If you need planning permission, neighbours are consulted and their objections are considered by the planning officer, but objections alone are not enough to refuse permission — the planning officer must have planning grounds to refuse. Under Prior Approval, if a neighbour objects within the 42-day period, the council assesses the impact on amenity (light, outlook, privacy) only — and can refuse if the impact is unacceptable. Under the Party Wall Act, neighbours cannot stop works but may require a formal Party Wall Agreement to be in place first.
-
Do I need an architect for a house extension?You are not legally required to use an architect. However, an architect or architectural technician is strongly recommended for most extensions, because they will: produce the drawings required for planning and building regulations; advise on design to maximise space and light; ensure the structural and thermal performance requirements are met; and manage the project. Architect fees are typically 8–15% of the build cost. For a simple single-storey extension, an architectural technician (MCIAT) may offer a more cost-effective service. Avoid relying solely on the builder for design — builders are not qualified to advise on planning or building regulations compliance.
-
What is the difference between planning permission and building regulations?Planning permission controls whether you are allowed to build an extension at all — it considers the impact on the surrounding area, neighbours, and the character of the neighbourhood. Building regulations are separate and control how the extension must be built, covering structural safety, fire safety, insulation, drainage, ventilation, and energy efficiency. You can have planning permission but still need separate building regulations approval. Many PD extensions do not require planning permission but do require building regulations approval. Both are enforced by your local council (planning by the planning department, building regulations by the building control department or an approved private inspector).
-
How long is planning permission valid for?Householder planning permission is valid for 3 years from the date of the decision notice. You must begin construction within this 3-year period. "Beginning" means starting a material operation — typically digging the foundations. You do not have to complete the build within 3 years; once commenced, the permission does not expire. If you do not start within 3 years, the permission lapses and you must reapply. When selling a property, unexpired planning permissions are a valuable selling point, as they demonstrate permitted development potential to buyers.
-
Can I build an extension in a conservation area?Yes, but Permitted Development rights are reduced in conservation areas. Side extensions, extensions visible from a highway, and cladding of exterior walls all require planning permission in a conservation area, even if they would be PD elsewhere. Any planning application in a conservation area will be assessed against the duty to preserve or enhance the character and appearance of the area. The council will normally require materials and design to match the existing character of surrounding buildings. In practice, modest rear extensions that are not visible from the street are often approved, while large or visually prominent extensions may face resistance.
-
Do I need a party wall agreement for an extension?You need to follow the Party Wall etc. Act 1996 if your extension involves work on a shared wall with a neighbour, excavation within 3 metres of a neighbouring property (where your dig goes deeper than their foundations), or building a new wall on the boundary. You must serve a formal Party Wall Notice on affected neighbours at least 2 months before starting work. If neighbours consent in writing, you can proceed without a formal award. If they dissent (or do not respond within 14 days), both parties appoint surveyors to produce a Party Wall Award. You pay both surveyors' fees. Typical costs: £800 for an agreed surveyor, up to £3,500 for two surveyors.
-
What is a Lawful Development Certificate and do I need one?A Lawful Development Certificate (LDC) is a formal document from the local planning authority confirming that your proposed works (or existing works) are lawful under Permitted Development rights or do not require planning permission. It costs £103 in England (half the householder planning fee). While not legally required to carry out PD works, it is highly recommended because: it gives you certainty before spending money; it provides documentary evidence for when you sell the property (solicitors and mortgage lenders will ask for it); and it records the position at the time the works were carried out, which can be important if planning rules change in the future.