Loft Conversion Cost Guide 2025: UK Prices, Types & Planning

Everything you need to know about loft conversion costs in the UK — from Velux to mansard, planning permission to ROI.

By Mustafa Bilgic • Updated 20 February 2026 • 12 min read

Loft Conversion Costs at a Glance (2025)

1. Velux / Rooflight Loft Conversion: £20,000–£30,000

A Velux or rooflight conversion is the simplest and cheapest type. The roof structure remains unchanged — contractors simply strengthen the floor, add insulation, install roof windows and fit a new staircase. Because there is no external alteration to the roofline, these almost always fall within Permitted Development rights.

The main limitation is headroom. Your existing ridge height must be at least 2.2 m from the floor joists to the underside of the ridge beam to create usable space. This makes Velux conversions most suitable for detached and semi-detached properties with a traditional pitched roof.

• Floor strengthening: £2,500–£5,000
• Velux windows (×2–3): £3,000–£6,000 installed
• Insulation (warm roof or cold roof): £2,000–£4,000
• Staircase: £3,000–£6,000
• Plastering, electrics, decoration: £4,000–£8,000

2. Dormer Loft Conversion: £35,000–£60,000

The dormer is the UK's most popular loft conversion type. A box-shaped structure extends vertically from the existing roof slope, dramatically increasing headroom and usable floor area. Dormers can be added to the rear (most common, usually Permitted Development) or the front (usually requires planning permission).

A full-width rear dormer on a Victorian terrace can add a bedroom and en-suite or a large open-plan studio. The cost includes structural steelwork, roofing felt and tiles, new dormer walls, windows and full internal fit-out.

3. Hip-to-Gable Conversion: £40,000–£65,000

On a hipped roof (where the end slope is pitched rather than vertical), a hip-to-gable conversion builds out the sloping end to create a vertical gable wall. This significantly increases the internal volume. It is common on detached and semi-detached properties and is often combined with a rear dormer for maximum space.

The additional cost over a standard dormer reflects the more complex structural work required to rebuild the roof end. In London and the South East, the combined hip-to-gable and dormer conversion can approach £70,000 for a premium specification.

4. Mansard Loft Conversion: £45,000–£70,000

A mansard conversion alters almost the entire roof structure, creating near-vertical walls (at least 72°) and a near-flat top. This delivers the maximum possible living space and is particularly popular in London. Because it substantially alters the roofline, mansard conversions almost always require full planning permission — expect 8–13 weeks for the application process.

Despite the higher upfront cost, mansard conversions add the most value in urban areas. A well-specified mansard in Inner London can add £100,000–£150,000 to a property's value.

5. L-Shaped Dormer: £45,000–£75,000

An L-shaped dormer combines a main rear dormer with a smaller side or return dormer, creating an L-shaped footprint on the roof. This is especially suited to Victorian and Edwardian terraced houses that have a rear outrigger (back addition). The result is two separate dormer spaces connected by an internal corridor, often creating two bedrooms and a bathroom.

Key Factors That Affect Loft Conversion Costs

Location

Labour costs vary considerably across the UK. London and the South East command the highest rates. A dormer conversion costing £45,000 in Yorkshire may cost £58,000 in Surrey and £62,000 in central London.

Property Type and Existing Roof Structure

Modern houses built after the 1960s often have trussed rafters rather than traditional cut-timber roofs. Converting a trussed-rafter roof requires replacing the entire roof structure, adding £8,000–£15,000 to costs. Always have a structural engineer assess your roof before budgeting.

Head Height

As noted, a minimum of 2.2 m of available height is required for a comfortable habitable space. Properties falling below this threshold need a dormer or mansard conversion rather than a Velux option, significantly increasing costs.

Number of Bedrooms and Bathrooms Added

Adding an en-suite bathroom adds £5,000–£10,000. A separate utility or storage room adds £2,000–£4,000. Each additional partition wall and associated plumbing/electrical work increases the final cost.

Staircase Design

A standard straight staircase costs £3,000–£5,000 installed. A bespoke glass-and-oak feature staircase can cost £8,000–£15,000. Space constraints in smaller properties may require a space-saving alternating tread staircase (£2,500–£4,500).

Planning Permission for Loft Conversions

Most loft conversions in England are covered by Permitted Development (PD) rights, meaning you do not need to apply for full planning permission, provided your conversion meets certain conditions:

• The additional roof volume does not exceed 40 m³ for terraced houses or 50 m³ for detached and semi-detached properties
• No extension beyond the existing roof plane on the principal elevation (front)
• No verandas, balconies or raised platforms
• Materials used are similar in appearance to the existing house
• The property is not in a designated area (Conservation Area, AONB, National Park, etc.)

Even if full planning permission is not needed, it is strongly advisable to obtain a Lawful Development Certificate (LDC) from your local planning authority (LPA) for approximately £100–£200. This provides legal confirmation that the work is permitted and will be required by solicitors when you come to sell.

If your property is in a Conservation Area or has had PD rights removed by an Article 4 Direction, you will need full planning permission. Front dormers and mansard conversions almost always require planning permission regardless of area.

Building Regulations

All loft conversions require Building Regulations approval, regardless of whether planning permission is needed. Building regs ensure the work is structurally sound and safe. Key areas covered include:

• Structure: Floor and roof structural integrity
• Fire safety: Protected stairway, smoke alarms, fire doors, escape windows
• Insulation: Thermal performance to meet current Part L standards
• Drainage: If a bathroom is included
• Electrics: All new electrical work signed off under Part P

Building regulations applications cost approximately £700–£1,000 and should be managed by your architect or structural engineer. A Building Control Officer will make site visits during construction and issue a completion certificate when satisfied.

Architects vs. Design-and-Build Contractors

You have two main routes to getting your loft conversion designed and built:

Hire an Architect Separately

An architect typically charges 8–15% of the build cost for full service (drawings, planning application, building regulations, contract administration). On a £45,000 conversion this equates to £3,600–£6,750. The benefit is an independent professional overseeing the contractor on your behalf.

Design-and-Build Package

Many specialist loft conversion companies offer a fixed-price design-and-build service where they handle everything from initial survey to completion certificate. This can be convenient but removes the independent oversight an architect provides. Always get at least three quotes and check reviews on Checkatrade, Which? Trusted Traders or TrustATrader.

How Much Value Does a Loft Conversion Add?

Nationwide research consistently shows that a loft conversion adds 15–20% to a property's value. On a £300,000 home this equates to £45,000–£60,000 in added value, against a typical conversion cost of £40,000–£55,000 — making it one of the best home improvement investments available.

The ROI is highest in areas where an extra bedroom meaningfully shifts the property into a higher price bracket (e.g. from a 2-bed to a 3-bed, or 3-bed to 4-bed in a competitive school catchment area).

Insurance Implications

A loft conversion increases your home's rebuild cost and floor area. You must notify your buildings insurer before work commences. During construction you should ensure your contractor carries public liability insurance (minimum £2 million, preferably £5 million) and that your own policy covers the works in progress.

After completion, update your buildings insurance with the increased rebuild value. The Association of British Insurers recommends using the BCIS House Rebuilding Cost Calculator to estimate the new rebuild figure. Failure to update your policy could result in being underinsured, meaning your insurer pays only a proportion of any claim.

Timeline: What to Expect

Frequently Asked Questions