A loft conversion is one of the most cost-effective ways to add a bedroom or bathroom to a UK home — you are using space that already exists rather than digging foundations or pushing into the garden. In 2026, a straightforward Velux conversion starts at around £20,000–£35,000 nationally and rises to £70,000–£110,000 or more for a full mansard in London. Where you live, what your roof structure allows, and how ambitious your fit-out is will all shift that number significantly.
This guide breaks down costs by conversion type and region, explains what drives the price, covers the planning and building regulations you need to understand, and walks through a realistic worked example so you can sense-check any quotes you receive.
Loft Conversion Costs by Type (2026)
The conversion type is the most important variable. Your existing roof shape — particularly whether it is a cut roof (traditional timber rafters) or a modern trussed roof — will determine which options are structurally feasible.
| Conversion Type | National Range | London Range | Best For |
|---|---|---|---|
| Velux / roof light | £20,000–£35,000 | £30,000–£50,000 | Adequate existing headroom (2.2 m+), minimal structural change |
| Dormer | £35,000–£60,000 | £55,000–£85,000 | Most common; adds usable floorspace and standing height |
| Hip-to-gable | £40,000–£65,000 | £60,000–£90,000 | End-of-terrace or semi-detached with hipped roof |
| Mansard | £50,000–£80,000 | £70,000–£110,000 | Maximum space; near-vertical rear wall; common in London Victorian stock |
All ranges assume a standard two-storey house, include structural works, insulation, stairs, and basic first-fix electrics and plumbing. Fit-out (flooring, bathroom suite, joinery) is additional unless specified by your contractor.
Costs by Region
London labour and material haulage add a consistent premium of 30–50% over the national average. The South East (particularly Surrey, Kent, and the Home Counties bordering London) typically sits 15–25% above the national mean.
| Region | Approximate Premium vs National Average |
|---|---|
| London (inner) | +40–50% |
| London (outer) | +30–40% |
| South East | +15–25% |
| South West | +5–10% |
| East of England | +10–15% |
| Midlands | ±0–5% |
| Yorkshire & Humber | −5–10% |
| North West | −5–10% |
| North East | −10–15% |
| Scotland | −5–15% (varies widely by city vs rural) |
| Wales | −10–20% |
These are indicative multipliers — not guarantees. A sole trader in rural Yorkshire and a design-and-build firm in Leeds can quote very differently for the same job.
What Drives the Price
Conversion type and structural complexity
A Velux conversion avoids altering the roof structure entirely; the roofer cuts openings and installs window frames. A dormer requires a flat-roofed box to be built out from the slope, involving carpentry, roofing membrane, lead or EPDM waterproofing, and (usually) a parapet or cheek wall. A mansard is essentially a full roof replacement with near-vertical walls, closer to a small new-build in complexity.
Trussed roofs (common in houses built after the 1960s) must be re-engineered before any conversion can proceed. Structural timber must be rearranged or replaced, which typically adds £3,000–£8,000 to the base cost before fit-out begins.
Headroom and floor-to-ridge height
Building Regulations require a minimum of 2.2 m standing height over at least a portion of the converted space to classify as habitable. If your ridge sits below 2.4 m from the floor joists, a Velux is borderline and a dormer may be the only sensible option. Get headroom measured by a surveyor before commissioning any drawings.
Stairs
A compliant loft staircase — minimum 800 mm width, maximum 42° pitch under Building Regs — costs £2,500–£7,000 for a bespoke timber flight, or £1,500–£4,000 for a standard straight flight. Space planning for the staircase is often the trickiest part of a loft conversion layout and can significantly affect the design of the floor below.
Bathroom
Adding an en-suite or shower room in the loft requires plumbing runs back to the existing stack, extraction, and often a macerator pump if gravity drainage is not possible. Budget £4,500–£10,000 for a compact loft en-suite including sanitaryware and tiling, on top of the conversion cost.
Specification and glazing
A large Velux or dormer window with integrated blinds costs considerably more than a standard fixed unit. Triple-glazed roof windows start at around £800 per unit fitted; juliet balconies and full-width glazed doors can add £3,000–£8,000.
Professional Fees and Consents
These are costs that often get omitted from headline quotes — confirm upfront whether they are included.
| Item | Typical Cost 2026 |
|---|---|
| Structural engineer | £500–£1,500 |
| Architect or designer (drawings and specification) | £1,500–£4,000 |
| Building Regulations application | £200–£500 |
| Party Wall surveyor (per neighbour) | £700–£1,500 |
On a mid-range dormer in the South East, professional fees commonly add £4,000–£8,000 to the build cost — around 10–15% of the construction budget.
Planning and Building Regulations
Permitted Development
Most loft conversions in England do not need planning permission under Permitted Development (PD) rights, provided they meet the following conditions (as set out in Schedule 2, Part 1, Class B of the General Permitted Development Order):
- The additional roof space does not exceed 40 m³ for a terraced house or 50 m³ for a detached or semi-detached house.
- No extension beyond the plane of the existing roof slope on the principal elevation (the front of the house).
- No raised platform, balcony, or veranda.
- No extension higher than the existing ridge line.
- Materials are similar in appearance to the existing house.
PD rights do not apply in Conservation Areas, Areas of Outstanding Natural Beauty, National Parks, or for Listed Buildings — you will need full planning permission in those cases. Always check with your local planning authority or obtain a Lawful Development Certificate before starting work, even if you are confident PD applies. This certificate provides legal protection if you sell or remortgage.
Scotland, Wales, and Northern Ireland have their own PD regimes which differ from England in scope and thresholds.
Building Regulations
All loft conversions — whether or not planning permission is needed — require Building Regulations approval. The key parts that apply are:
- Part A (Structure): The floor joists will almost certainly need to be strengthened with new timber or steel to carry the new loading. Your structural engineer will size these.
- Part B (Fire safety): A loft conversion creates a new storey, so fire escape routes must be upgraded. This typically means FD30S fire doors on rooms opening onto the escape route and a mains-wired smoke alarm system throughout.
- Part L (Conservation of fuel and power): The conversion must be insulated to current standards. For a warm roof construction, this typically means 150–200 mm of rigid insulation between and below the rafters.
You can submit either a Full Plans application (drawings reviewed before work starts) or a Building Notice (inspection during construction). Full Plans is strongly recommended for loft conversions given the structural and fire elements involved.
Party Wall Act 1996
If you share a wall with a neighbour — likely for a terraced or semi-detached property — work on or near that wall may trigger the Party Wall Act. A rear dormer on a flat that shares a separating wall, or any structural beam that bears on a shared wall, will usually require you to serve a Party Wall Notice at least two months before work begins. If your neighbour dissents or does not respond, a Party Wall Agreement must be drawn up by a surveyor — expect to pay £700–£1,500 per neighbour for this.
Worked Example: Rear Dormer in Outer London (2026)
A 1930s semi-detached house in Ealing, with a hipped roof and a ridge height of 2.7 m above the first-floor ceiling. The owner wants one bedroom and a shower room.
| Item | Cost |
|---|---|
| Hip-to-gable structural works + rear dormer | £68,000 |
| Architect (planning drawings + Building Regs pack) | £2,800 |
| Structural engineer | £900 |
| Building Regulations application (Full Plans) | £350 |
| Party Wall surveyor (one neighbour) | £1,100 |
| Bespoke staircase | £5,200 |
| En-suite (including plumbing, tiling, sanitaryware) | £6,500 |
| Electrics (first fix to finish, including new consumer unit circuits) | £2,200 |
| Flooring (engineered oak, 22 m²) | £1,800 |
| Decoration | £1,200 |
| Total | £90,050 |
This is a realistic mid-range figure for outer London in 2026 — not a budget project, but not premium either. The same job in Leeds or Bristol might come in at £55,000–£65,000.
Getting Accurate Quotes
A few practical points when approaching contractors:
- Get at least three quotes from specialists, not general builders who occasionally do lofts. The difference in knowledge of Building Regs matters.
- Ask what is included: VAT, scaffolding, skip hire, and Building Regulations fees are common omissions from headline prices.
- Agree a payment schedule tied to milestones — deposit (10–15%), first fix, second fix, final completion. Avoid paying more than 25% upfront.
- Check insurance: public liability (minimum £1 million) and employer’s liability. Confirm the contractor will notify Building Control or use an Approved Inspector.
- Budget a 10–15% contingency: rotten timber, undersized joists, and concealed chimney stacks are common discoveries once ceiling boards come down.
A well-executed loft conversion typically adds 15–25% to a home’s value in most UK markets, making it one of the most reliable home improvements — particularly where an extra bedroom moves a property into a higher price bracket.
