Loft conversions, house extensions, renovations, basements and new builds across London and Hertfordshire.
Services
Loft conversions, house extensions, renovations, basements and new builds across London and Hertfordshire.
Services
Loft conversions, house extensions, renovations, basements and new builds across London and Hertfordshire.
Services
Loft conversions, house extensions, renovations, basements and new builds across London and Hertfordshire.
Services
Call 020 8037 7332 or get an instant estimate online.
Mansard loft conversions · London & Hertfordshire
Mansard loft conversion London: cost, planning and build time — from £76,800 inc. VAT (£64,000 + VAT)
By Anderson & the LCCL Construction Team
Finchley N3 · Design & build, LABC award winners 2024 and 2026
A mansard replaces the existing roof with a new one: a near-vertical rear slope at about 72 degrees, a flat top, and full-height rooms across almost the whole footprint. It gives more space than any other loft conversion and it is the traditional form on London's Georgian and Victorian terraces — which is why it almost always needs a planning application, and why many councils approve it where they would refuse a box dormer. This page gives you the 2026 numbers, the planning picture, the house types it suits and how the job runs with one team.
Quick answer
£76,800 – £148,800 inc. VAT (£64,000 – £124,000 + VAT) for a mansard loft conversion, design and build inclusive. Postcode dependent — most mansards are in Inner and Central London.
Almost always a planning application: a mansard changes the roof shape and usually exceeds permitted development. Approved where it matches the street pattern.
12–16 weeks on site; 12–16 weeks of design and approvals before that.
A free 30-minute phone call with our team, no obligation.
Guide price
£76,800 – £148,800 inc. VAT (£64,000 – £124,000 + VAT)
Typical build
12–16 weeks on site
Guarantee
12-month workmanship guarantee; 10-year LABC structural warranty
Awards
LABC 2024 & 2026 winners
LABC Building Excellence Award 2024
Best Residential Extension in London — Bohun Grove, East Barnet EN4: rear extension, hip-to-gable loft conversion and whole-house renovation
LABC Building Excellence Awards 2026
London Regional Winner, Best Conversion or Alteration — Weech Road, Hampstead NW6: penthouse loft conversion and full renovation
One contract
Architecture, structural engineering, planning, construction and project management — all under one contract
Guaranteed & insured
12-month workmanship guarantee; 10-year LABC structural warranty
What a mansard loft conversion from LCCL includes
Design and build inclusive — the price covers the whole job, from the survey to the completion certificate.
Measured survey of the roof space, stairs and existing structure
Architectural design and drawings, with revisions until you're happy
Lawful Development Certificate or planning application, prepared and submitted for you
Building regulations drawings and structural calculations by our chartered engineer
Party wall notices served on your behalf (£300 each, at cost)
New floor structure, steels, the complete new mansard roof structure, slate or tile covering, lead work and windows
Staircase, insulation, fire doors and escape windows to building regulations
Plaster finish, first-fix electrics and plumbing; bathroom or en-suite where specified
Second-fix carpentry, decoration and the items you supply fitted
LABC inspections and completion certificate
A dedicated project manager and a weekly progress update
12-month workmanship guarantee; 10-year LABC structural warranty
How much does a mansard loft conversion cost in London in 2026?
Guide ranges, design and build inclusive. Every project is priced from your postcode and house type.
Mansard
Typical use
Guide range
Rear mansard (front slope kept)
One large bedroom and bathroom, rear wall near-vertical
£76,800 – £102,000 inc. VAT (£64,000 – £85,000 + VAT)
Full mansard (front and rear rebuilt)
Two bedrooms and a bathroom across the whole footprint
£102,000 – £132,000 inc. VAT (£85,000 – £110,000 + VAT)
Full mansard, period detailing
Slate, dormers in the mansard slope, brick parapets to match the terrace
£120,000 – £148,800 inc. VAT (£100,000 – £124,000 + VAT)
Related conversion
Guide range
On site
L-shaped dormer
The permitted development alternative on a terrace with an outrigger
£58,800 – £98,400 inc. VAT (£49,000 – £82,000 + VAT)
10–12 wks
Rear dormer
Box dormer on the rear slope — lower-cost, less space, usually no planning
£47,000 – £67,000
8–10 wks
Whole-flat or whole-house renovation with the loft
The Weech Road NW6 approach — loft and remodel together
See home renovation
Phased
What the price includes: survey, drawings, lawful development certificate or planning, building-regulations drawings, structural calculations, new floor structure, dormer or gable structure, roof and windows, stairs, insulation, fire doors and escape windows, first and second fix, plastering, decoration, a bathroom where specified, LABC certificate and guarantees. Not included unless specified: sanitaryware above the allowance, bespoke joinery, party wall surveyor fees if a neighbour dissents (from £750). The figure on your itemised estimate is the figure you pay — nothing is added afterwards. Postcode dependent — the instant estimator adjusts the range for your area in thirty seconds, and a detailed itemised estimate follows one survey.
What moves the price: whether the front slope is rebuilt too; the roof covering — natural slate and lead cost more than tile and GRP; party wall parapets, which some councils require to match the terrace; the steels, which span the full width because the old roof structure goes; the bathroom; scaffolding and access on a tight Central London street; and the finish inside. Inner and Central London cost more than outer London — the instant estimate applies your postcode.
Mansard loft conversions by house type
Georgian or Victorian terrace, Central London
Where the street already has mansards, a matching one is the conversion the council expects. Westminster, Camden and Kensington & Chelsea have detailed mansard guidance — slope angle, parapets, dormer size and slate — and a scheme that follows it is approved; one that does not is refused. Two party wall notices.
Top-floor flat or maisonette
A mansard is often the only way to add a floor to a flat, because a box dormer is rarely acceptable and there is no permitted development for flats. Freeholder consent and a planning application are both needed; we handle the application and the party wall process.
Victorian terrace, outer London
Less common — an L-shaped dormer under permitted development usually gives nearly the same space for less. A mansard makes sense where the roof is too low for a dormer, or where the street pattern supports it.
Planning
Do you need planning permission for a mansard loft conversion?
Almost always yes. The question is not whether you need it but how to design the mansard the council will approve.
Route
When it applies
Time
What we do
1. Full planning
Nearly every mansard — the new roof exceeds permitted development limits and changes the roof form
8 weeks
Drawings to the borough's mansard guidance, design statement, precedent search and application
2. Pre-application advice
Conservation areas and boroughs that engage (Wandsworth is helpful; some are not)
4–6 weeks
We advise whether the fee is worth it for your address
3. Listed building consent
Listed terraces — common in Central London
8 weeks
Heritage statement and application alongside the planning application
Building regulations & party walls
The other two approvals
Every loft conversion needs building-regulations approval whether or not it needs planning: the new floor, the steels, the stairs, insulation, fire doors and escape windows. We do this through LABC with full-plans drawings and our own structural calculations, so the inspector is checking a design he has already approved.
On a semi or terrace the Party Wall Act applies because the new steels bear on the shared wall. Notices go out at least two months before we start; most neighbours consent, and where a surveyor is appointed we manage it.
Councils judge a mansard against the terrace: a 70–72 degree slope, a flat top set back from the parapet, dormers proportioned to the windows below, slate where the street is slate, and party wall parapets carried up where the neighbours have them. Precedent on the same terrace is the strongest argument there is — we search it before we draw.
Mansard loft conversion cost guide · Mansard loft conversion in Hampstead — project story
Recent mansard loft conversion projects
Bohun Grove, East Barnet EN4 — rear extension, hip-to-gable loft and whole-house renovation
LABC 2024 Best Residential Extension in London
Semi-detached family home One programme loft + extension + renovation
A total renovation of a family semi: a rear extension opening into a full-width kitchen and dining space with floor-to-ceiling glazing and skylights, a hip-to-gable loft conversion with a rear dormer, two Lusso Stone bathrooms, enlarged bedrooms, new windows throughout, a regulated outbuilding and landscaping.
Because the loft and extension ran under one contract, the steelwork for the open-plan ground floor and the loft floor was designed together and lifted in on the same day — one crane hire, one scaffold, one building-control file.
See the Bohun Grove project · All projects
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ANDERSON · FOUNDER, LCCL CONSTRUCTION
Anderson's advice: use pre-application advice where the council is helpful
Wandsworth planning is much more helpful on pre-application advice than Islington in our experience. Where a borough engages, a pre-app meeting before the full application saves a refusal and a resubmission; where it does not, a well-argued design statement and precedent search do more.
We know which boroughs are which because we apply in them every month — and we tell you before you pay a pre-app fee whether it is worth it for your address.
How a mansard loft conversion runs with LCCL
From the first call to the completion certificate. Because a mansard needs a planning application and often listed building consent, allow 12–16 weeks for design and approvals before work starts.
WEEK 0
Free consultation call
A 30-minute phone call with our team about the house, what you want from the space and a realistic budget. Your instant estimate is the starting point.
WEEKS 1–4
Survey & design
Measured survey, architectural drawings and your detailed itemised estimate — design, planning, structure, construction and warranty priced line by line. The drawings follow the borough's mansard guidance from the first sketch.
WEEKS 4–12
Approvals
Planning application (eight-week decision), listed building consent where needed, building-regulations drawings, structural calculations, party wall notices to both neighbours. Nothing starts on site until it is signed off.
BUILD
On site
12–16 weeks on site for a mansard loft conversion. Scaffold up, roof opened and made weather-tight, then floor, stairs, first fix, plaster, second fix. Weekly updates from your project manager.
HANDOVER
Sign-off
Snagging, LABC final inspection and completion certificate, guarantees in writing. 10-year LABC structural warranty.
“I highly recommend LCCL Construction. They completed our loft conversion to a very high standard, and the quality of workmanship throughout was excellent. The team was professional, reliable, and completed everything within a very reasonable timeframe.…”
Alina A · Google Reviews
“I couldn't be happier with my loft conversion and extension done by LCCL Building Construction. Real craftsmanship, professional management of the entire build from start to finish, seamless integration of new space within the existing one. The work was…”
Plamen Petrov · Trustpilot
“LCCL Construction has beautifully transformed our house with a loft conversion, house extension and renovation including design and build, we are amazed with high quality of the build, professionalism and dedication to successfully complete our highly…”
Leonard Shehu · Google Reviews
Mansard loft conversion questions we get asked most
£76,800 – £148,800 inc. VAT (£64,000 – £124,000 + VAT) in 2026, design and build inclusive — survey, drawings, planning application, structural calculations, the complete new roof structure and covering, stairs, insulation, first and second fix, decoration, a bathroom where specified and LABC sign-off. A rear mansard sits at the bottom of the range; a full mansard with slate and period detailing at the top. Central London costs more — the instant estimate applies your postcode.
Almost always. A mansard replaces the roof and exceeds the permitted development limits, so it needs a full planning application with an eight-week decision, plus listed building consent on a listed terrace. Councils approve mansards that follow their guidance and the pattern of the street.
Twelve to sixteen weeks on site, plus 12–16 weeks for design, planning and building regulations before that. The old roof comes off under a temporary scaffold roof, so the house stays dry while the new structure goes up.
Space and approval. A mansard gives full-height rooms across nearly the whole footprint, and on a period terrace it is the roof form the council expects — where a box dormer would be refused, a mansard is approved. On an outer-London terrace a dormer is usually the better value.
Often, yes — it is the usual way to add a floor to a top-floor flat or maisonette. You need the freeholder's consent and a planning application; there is no permitted development for flats. We handle both and the party wall process. Weech Road NW6 is our LABC award-winning example.
Usually, and it is to your advantage: the slope, the parapets, the dormer proportions and the slate are what the officer checks against the terrace. Matching an approved mansard next door is the strongest planning argument available.
Yes, on both sides — the new roof and steels bear on the party walls and the parapets are often built up on them. We serve the notices at cost (£300 each) at design stage so the two-month period runs during the planning application.
Most clients do. The old roof is removed under a temporary scaffold roof, so the house is never open to the weather, and the stair comes through in the last third of the programme. The noisiest weeks are the first three.
Where we build mansard loft conversions
From Finchley N3 we cover North London, North-West London, Central London, South-West and West London, and Hertfordshire — and we also work across the rest of London, Buckinghamshire, Surrey, Kent and Essex. Recent mansards: Marylebone W1 and Hampstead NW6 — and the period terraces of Central and North-West London where a mansard is the roof the council expects.