Mansard Loft Conversion Cost 2026: Prices, Planning + London Guide
A mansard loft conversion costs £38,000–£90,000 in the UK in 2026, and £45,000–£108,000 in London where prices run 15–20% higher. It replaces the rear roof slope with a near-vertical 72° wall and a flat top, giving the maximum floor area and full head height of any loft conversion — and because it changes the roofline, it always needs planning permission.
The mansard sits at the upper end of our loft conversion cost guide and is the London terrace specialist: on streets where councils reject box dormers, the mansard's traditional profile is the design that gets approved.
What is a mansard loft conversion?
A mansard loft conversion rebuilds the rear slope of the roof as a wall pitched at 72°, clad in slate or tile, with a flat roof on top and windows set into the steep face. The party walls are raised in brick to form the new profile, and the whole rear roof structure is removed and rebuilt rather than added to. A double mansard does the same to the front slope as well, on streets where the roofline already carries them.
The design dates from 17th-century Paris and appears across London's Georgian and Victorian terraces, which is why planning officers in conservation areas treat it as traditional rather than modern. Structurally it is the heaviest loft conversion: a steel frame carries the new floor and the 72° wall, and the party walls take the extra load. Our Victorian terrace loft conversion guide covers why those party walls usually cope.
How much does a mansard loft conversion cost?
A mansard loft conversion costs £38,000–£90,000 in the UK in 2026, per Checkatrade, MyJobQuote and MyBuilder's 2026 guides. A rear-only mansard on a terrace sits at £38,000–£75,000; a rear-and-front (double) mansard at £55,000–£90,000. The band is wide because the mansard rebuilds the roof: terrace width, party wall height increase and cladding material move the price more than the finish does.
| Scope | UK cost (2026) | London cost (+15–20%) | What you get |
|---|---|---|---|
| Rear mansard | £38,000 – £75,000 | £45,000 – £90,000 | Rear slope rebuilt at 72°, party walls raised, flat roof, 2–3 windows; double bedroom + en-suite |
| Rear + front mansard | £55,000 – £90,000 | £65,000 – £108,000 | Both slopes rebuilt; full second floor with two rooms + bathroom; front windows to match the street |
Per m² the mansard runs £1,300–£2,000, at the upper end of the £1,000–£2,500/m² loft range, because of the brickwork and full roof rebuild. A mansard on a 5m-wide terrace produces 35–45m² of floor with full head height to the rear wall — no sloping ceilings, no eaves storage — so the per-m² figure ends up close to a dormer's £1,150–£1,700 (MyJobQuote 2026) despite the higher total.
How much does a mansard loft conversion cost in London?
A mansard loft conversion costs £45,000–£108,000 in London in 2026: £45,000–£90,000 for a rear mansard and £65,000–£108,000 for a rear-and-front mansard, 15–20% above the UK bands. London is also where most mansards are built, because inner-borough conservation-area guidance steers terrace owners towards the mansard profile and away from box dormers.
The premium is specific items, not vague "London prices": labour at 55–65% of the build, at rates £80–£120 a day above the national average; party wall surveyors on both sides of a mid-terrace at the top of the £700–£1,500 range; a pavement scaffold licence and parking suspension at £300–£900; and conservation-area conditions — natural slate on the 72° face, timber sash windows, lead flashings — adding £4,000–£8,000. Camden, Islington, Hackney, Kensington & Chelsea and Westminster publish roof-alteration guidance that accepts a rear mansard where neighbours already have one and refuses a box dormer on the same street; Home Renovation Guides sees the same pattern in Lambeth and Wandsworth.
Mansard Loft Conversion Cost Calculator
The calculator applies the two scope bands from the cost table to your house type, finish and region; end-of-terrace and semis run slightly higher because the exposed flank wall is raised as well as the party wall. Results round to the nearest £500 and exclude the items under the result.
House type, scope and finish for a 2026 range.
Do I need planning permission for a mansard roof?
Yes, always. A mansard changes the roofline and exceeds the 40m³ permitted development cap on a terraced house, so it falls outside Class B of the Planning Portal's householder rules and needs a full planning application: £528 householder fee in England, eight-week decision, drawings by an architect or architectural technologist (£1,500–£4,000 including structural design).
| Conversion type | Permitted development? | Why |
|---|---|---|
| Mansard | No — planning permission every time | Changes the roofline; volume exceeds 40m³ on terraces; party walls raised |
| Rear dormer | Usually yes | Rear-facing, within 40m³ (terrace) or 50m³ (semi/detached), below the ridge |
| L-shaped dormer | Rarely on a terrace | Two dormers total 45–60m³, over the 40m³ cap |
| Hip to gable | Usually yes | Within 50m³ on semis; side elevation change caught in conservation areas |
| Velux / rooflight | Yes | No roof enlargement; windows project under 150mm |
Planning permission is not the obstacle it sounds. On a terrace where the neighbours already have mansards, approval is routine and the design rules are known in advance: 72° pitch, set back behind the party wall parapet, windows aligned with those below, slate or tile cladding. Refusals come from front mansards on streets without them, and from designs that rise above the neighbours' flat roofs. Listed buildings need listed building consent as well. The terraced house loft conversion guide explains how the 40m³ cap pushes terrace owners to a mansard.
What are the key differences between a dormer and a mansard loft conversion?
A dormer adds a box onto the existing roof slope and keeps the ridge; a mansard removes the rear slope entirely and rebuilds it as a 72° wall with a flat top. The mansard produces more floor area at full head height, at a higher cost, a longer build and a compulsory planning application; the dormer is cheaper, faster and usually permitted development, but leaves sloping ceilings.
| Rear dormer | Mansard | |
|---|---|---|
| UK cost (2026) | £30,000 – £65,000 | £38,000 – £90,000 |
| What changes | Box added to the rear slope; ridge kept | Whole rear slope rebuilt at 72°; party walls raised |
| Floor gained (5m terrace) | 20–30m², sloping ceilings remain at sides | 35–45m², full head height to the rear wall |
| Planning | Usually permitted development | Planning permission always |
| Weeks on site | 8–14 | 12–18 |
| Works with ridge under 2.1m? | No | Yes — the roof is rebuilt higher |
| Conservation-area outcome | Often refused on terraces | Approved where neighbours have one |
The head-height row is the decisive one for many terraces. A dormer needs 2.2m from joist top to ridge before the build starts; a mansard does not, because the new flat roof sits at or just below the old ridge and the 72° wall creates the height. Terraces with 1.9–2.1m at the ridge — common on late-Victorian two-up-two-downs — have a mansard or nothing. The L-shaped dormer is the third option where the terrace has a rear outrigger and the council accepts dormers.
What is included in a mansard loft conversion quote?
A full-service mansard quote includes the roof strip and rebuild, the party wall brickwork, steel frame and floor, the 72° cladding and flat roof, windows, staircase, insulation to Part L, first and second fix, plastering and building control sign-off. It excludes the items below, which add £6,000–£14,000 on a London mid-terrace before the bathroom fit-out.
| Item | Included in a mansard quote? | Typical cost if excluded (2026) |
|---|---|---|
| Roof strip, steel frame, 72° wall, flat roof, cladding | Yes | — |
| Party wall brickwork raised to the new profile | Yes | — |
| Staircase, insulation, plasterboard, electrics, plumbing first fix | Yes | — |
| Planning drawings + structural engineer | No | £1,500 – £4,000 (engineer alone £400–£1,200) |
| Planning application fee | No | £528 (householder, England) |
| Party wall surveyor | No | £700 – £1,500 per neighbour — two on a mid-terrace |
| Building control | Sometimes | £500 – £1,000 |
| Chimney breast removal | No | £3,000 – £8,000, with the neighbour's agreement for a shared stack |
| En-suite fit-out, decorating, flooring | No | £4,000 – £8,000 bathroom; £1,500 – £4,000 finishes |
How long does a mansard loft conversion take?
A mansard loft conversion takes 12–18 weeks on site, the longest of any loft type, and 8–12 months end to end once planning, party wall awards (two months' notice) and building control approval are added. The extra weeks over a dormer go into brickwork: raising two party walls and building the 72° frame is wet-trade work that waits on weather.
| Phase | Weeks on site | Work |
|---|---|---|
| Scaffold and temporary roof | 1–2 | Full rear scaffold, often a temporary tin-hat roof so the house stays dry once the slope comes off |
| Strip and party walls | 2–5 | Rear slope removed; party walls raised in matching brick to the new parapet line |
| Steel frame and floor | 4–7 | Steels into raised party walls; floor joists; 72° timber or steel frame |
| Roof and cladding | 7–10 | Flat roof (GRP, EPDM or single-ply), slate or tile hanging on the 72° face, windows, lead flashings |
| Stair, first fix, insulation | 10–14 | Stairwell cut, staircase, electrics, plumbing, Part L insulation, plasterboard |
| Second fix and finish | 14–18 | Bathroom, doors, skirting, decoration, completion certificate |
Can you do a loft conversion for 20k?
Not a mansard, which starts at £38,000 outside London and £45,000 inside it. £20,000 buys a basic Velux loft conversion outside London on a loft with 2.3m+ head height, with nothing left for fees. A mansard quote under £35,000 has left out the party wall brickwork, the roof rebuild, the staircase or building control — Home Renovation Guides recommends asking for the exclusion list in writing before comparing quotes.
What is the 10 year rule for loft conversion?
The 10 year rule makes a planning breach immune from enforcement 10 years after the work was substantially completed. A mansard built without permission more than a decade ago cannot be ordered down — relevant on London terraces where many 1980s–90s mansards went up without paperwork. The gap remains at sale: buyers' solicitors ask for a Lawful Development Certificate for existing use (£129) with dated evidence, or an indemnity policy (£20–£100). Building regulations breaches never time out, so an uncertified mansard floor structure from any year needs a regularisation application and, often, opening-up. Use the rule to fix an inherited problem, never to plan around it.
Related loft conversion guides
- Victorian terrace loft conversion — party walls, chimney breasts and conservation areas on pre-1901 terraces
- Terraced house loft conversion — the 40m³ cap and two-neighbour party wall reality
- L-shaped dormer loft conversion cost — the dormer alternative on outrigger terraces
- Dormer loft conversion cost — cheaper, faster and usually permitted development
- Edwardian house loft conversion — wider terraces where a dormer often wins on value
- End-of-terrace loft conversion — one party wall and the flank-wall options
Frequently asked questions
How much does a mansard loft conversion cost?
£38,000–£90,000 in the UK in 2026: rear mansard £38k–£75k, rear + front £55k–£90k. London adds 15–20%.
How much does a mansard loft conversion cost in London?
£45,000–£108,000: rear mansard £45k–£90k, rear + front £65k–£108k. Labour, two party wall surveyors, scaffold licences and conservation-area materials make the premium.
What are the key differences between a dormer and a mansard loft conversion?
A dormer adds a box and keeps the ridge; a mansard rebuilds the rear slope at 72°. Mansard: more floor, full head height, £38k–£90k, 12–18 weeks, planning always. Dormer: £30k–£65k, 8–14 weeks, usually permitted development.
Do I need planning permission for a mansard roof?
Yes, always — it changes the roofline and exceeds the 40m³ cap. £528 householder application, eight-week decision. Approval is routine where neighbours already have one.
Can you do a loft conversion for 20k?
Not a mansard — it starts at £38,000. £20,000 is a basic Velux budget outside London with no fees included.
What is the 10 year rule for loft conversion?
Planning breaches become immune from enforcement 10 years after completion. Building regulations breaches never do. Fix paperwork with a £129 LDC or indemnity policy.
Which lofts cannot be converted?
No landing space for a compliant stair, or a flat without freeholder consent. Low ridge height is not a barrier for a mansard — the roof is rebuilt higher.
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