House Extension Costs UK 2025: Complete Guide
Building an extension is one of the most effective ways to add space and value to your home. Costs vary enormously depending on the type of extension, its size, location, specification level, and the complexity of the build. This guide provides up-to-date cost data for 2025 and the key considerations for every extension project.
Cost Per Square Metre by Location (2025)
Build costs exclude design fees, planning fees, and VAT. These are contractor costs for the structural shell and internal fit-out to a habitable standard.
London
£2,000 – £4,000/m²
Higher labour costs, restricted access, premium materials
South East England
£1,800 – £3,000/m²
Includes Surrey, Kent, Essex, Hertfordshire
Rest of England
£1,400 – £2,200/m²
Midlands, North, East Anglia, South West
Scotland & Wales
£1,200 – £1,800/m²
Generally lower labour and material costs
Typical Extension Project Costs (2025)
- Single-storey rear extension (30m²): £45,000 – £120,000 total including VAT and fees
- Double-storey extension: Add 30-50% for the second floor structure
- Side return extension: £30,000 – £80,000 (narrow passageway infill, typical for Victorian terraces)
- Wrap-around (side and rear combined): £60,000 – £150,000+
- Over-garage extension: £25,000 – £60,000 depending on structural requirements
Additional Costs to Budget For
Beyond the build cost itself, most extension projects incur additional professional and regulatory fees:
- Architect / designer: £2,000 – £6,000 for design, planning drawings, and building regs drawings. Some charge 8-15% of build cost.
- Planning application fee: £206 for a householder application in England (from April 2024)
- Building regulations: £900 – £1,500 (local authority building control or approved inspector)
- Party wall surveyor: £700 – £2,000 per surveyor if neighbour dissents
- Structural engineer: £500 – £1,500 for calculations and drawings
- VAT: 20% on all contractor costs (standard rate for extensions to existing homes)
- Contingency: Always budget 10-15% above the quote for unforeseen issues
Underpinning and Foundations
If your soil is clay-heavy or the existing foundations are shallow, you may need additional foundation work. Underpinning can add £5,000 – £15,000+ to the project cost. A structural engineer can advise after a site investigation.
Planning Permission vs Permitted Development
Understanding when you need planning permission saves time and money. Under Permitted Development (PD) rights for houses in England:
- Single-storey rear extensions can extend up to 3m from the rear wall for semi-detached and terraced houses, or 4m for detached houses — without planning permission
- Under the Neighbour Consultation Scheme (Prior Approval), you can extend up to 6m (semi/terraced) or 8m (detached)
- Side extensions require planning permission if they would create a wing wider than half the original house width
- PD rights do not apply in Conservation Areas, Areas of Outstanding Natural Beauty, or National Parks
- Flats and maisonettes have no PD rights — all extensions require planning permission
Always check: Even if your extension appears to fall within PD rights, confirm with your local planning authority before starting work. PD rights can be removed by Article 4 Directions. A Certificate of Lawful Development costs £103 and gives legal certainty.
How to Find a Reliable Builder
- Get at least 3 written quotes from FMB (Federation of Master Builders) registered contractors
- Ask to see examples of similar completed extensions locally
- Check Companies House for the contractor's trading history
- Verify public liability insurance (minimum £2m)
- Use a written JCT (Joint Contracts Tribunal) contract — never pay more than 25% upfront
- Agree a clear payment schedule tied to project milestones
- Check the builder is registered with a professional body (FMB, NHBC, LABC Warranty)
Does an Extension Add Value to Your Home?
A well-designed and executed extension typically adds 10-20% to a property's value. A double-storey extension adding a bedroom and bathroom can add more — particularly in markets where bedroom count is a key driver of value.
However, you can over-extend. If your extended property becomes the most expensive on the street by a significant margin, you may not recoup the full cost. Always research local sold prices for similar extended and non-extended properties before committing to a major project.
Extensions that add the most value: kitchen-diner extensions opening to garden, master bedroom with ensuite, home office (post-pandemic demand), additional reception rooms in family homes.
Frequently Asked Questions
How much does a house extension cost in the UK in 2026?
A typical single-storey rear extension of 30m² costs between £45,000 and £120,000 depending on location and specification. Cost per square metre ranges from £1,200 in Scotland/Wales to £4,000 in London for high-end finishes. Always add VAT (20%) to contractor quotes and budget a 10-15% contingency.
Do I need planning permission for a house extension?
Many single-storey rear extensions up to 3m (semi/terraced) or 4m (detached) depth fall under Permitted Development rights and do not need full planning permission. Extensions over these limits, side extensions, or properties in conservation areas require a planning application (fee £206). Under the Neighbour Consultation Scheme, extensions up to 6m or 8m may be approved without full planning.
How much does an architect cost for a house extension?
Architect fees for a house extension typically range from £2,000 to £6,000 depending on the project complexity. This usually includes initial design, planning drawings, and building regulations drawings. Some architects charge a percentage of build cost (typically 8-15%). A design-and-build contractor may include design costs in their quote but will have less design flexibility.
How much value does an extension add to a house?
A well-designed extension typically adds 10-20% to a property's value. A double-storey extension adding a bedroom and bathroom can add more in competitive markets. The value uplift depends on the local property market, the quality of work, and whether the extension improves the property's functionality and kerb appeal. Over-extending relative to local comparables can reduce ROI.
What is a party wall agreement and how much does it cost?
If your extension work affects a shared wall with a neighbour (or excavation within 3-6m of their foundations), you need to serve a Party Wall Notice at least two months before work starts. If the neighbour agrees in writing, it's free. If they dissent, a Party Wall Surveyor must be appointed — costing £700-£2,000 per surveyor. You typically pay your neighbour's surveyor fees too.
How long does it take to build a house extension?
A typical single-storey extension takes 8-16 weeks to build once work starts on site. Add 4-12 weeks for design and planning, plus 2-8 weeks for building control approval. A double-storey extension may take 16-24 weeks on site. Material procurement lead times and builder availability can extend overall timescales — many reputable builders are booked 3-6 months ahead.
Should I get a fixed price or day rate quote for an extension?
Always request a fixed price quote for major extension work. This gives cost certainty and protects you from budget overruns. Ensure the quote includes all contingency, VAT is clarified (20% applies to most extension work on existing homes), and that payment milestones are clearly defined in a written JCT contract. Avoid contractors who insist on day rates for the entire project.