A mansard loft conversion rebuilds the rear slope of your roof as a full new storey. The new rear wall rises at a steep 70 degree pitch, hung in slate, and the walls inside are vertical, so you gain the biggest single room any loft conversion can offer.
A mansard always needs planning permission, which is exactly why it is usually the answer in conservation areas where rear dormers are refused.
We design and build mansards across London with the planning application, party wall process and build all handled by one in-house team.
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A mansard loft conversion replaces the rear slope of your roof with a full new storey. The rear wall is rebuilt at a steep 70 degree angle and clad in slate, the inner walls are vertical, and a flat roof sits over the top, so almost every square metre of the new floor is usable.
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Around 12–16 weeks on site. The full process runs roughly 26–30 weeks from your first call to handover, because every mansard goes through a full planning application first.
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Always required. There is no permitted development route for a mansard, so we prepare, submit and manage the full planning application for you. Councils usually take 8–10 weeks to decide, and the mansard form has a strong track record with planning officers, including in conservation areas.
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A site manager you know by name. Weekly photo updates by WhatsApp. The same trade teams we've worked with for over seven years. No rotating subcontractors, no agency labour walking in and out.
Each project is labelled with the time on site, the final price and the council that covered it.
Slate-hung mansard adding a full master suite to a Victorian terrace in a conservation area.
Mansard storey behind the original parapet on a Georgian terrace, with bedroom and walk-in shower room.
Rear mansard with master bedroom, en-suite and fitted wardrobes on a Victorian mid-terrace.
Conservation area mansard in Clapham Old Town, approved after a rear dormer had been refused.
Mansard storey with large double bedroom and study corner on an Edwardian terrace.
Full-width mansard creating a bright top-floor living space on a Victorian terrace.
Still unsure? Daniel will tell you straight at the free survey. book one here →
| Type | Build time | Floor area added | Planning | Best for |
|---|---|---|---|---|
| Velux | 6–8 weeks | Smallest | Usually permitted development | Homes with enough existing headroom and a leaner budget |
| Dormer | 8–10 weeks | Large | Usually permitted development | Victorian and Edwardian terraces, 1930s semis |
| L-Shaped Dormer | 10–12 weeks | Largest (two rooms) | Often permitted development | Victorian terraces with a rear back addition |
| Hip-to-Gable | 10–12 weeks | Large | Usually permitted development | Semi-detached and end-of-terrace with hipped roofs |
| Mansard | 12–16 weeks | Largest single room | Planning permission required | Conservation areas, period properties needing maximum space |
From your first call through to the day you move back in: a clear, predictable timeline that protects your home and your schedule.
Step 01
Daniel measures the roof and parapets, checks the party walls and your street's planning history, and tells you on the visit whether a mansard is realistic.
Step 02
Design drawings, structural approach and a fully itemised fixed-price proposal, including the planning application as its own line. A 15% deposit secures your build slot.
Step 03
Every mansard needs planning permission. We submit and manage the application through the council's 8–10 week determination, with structural calculations, Building Control and party wall notices running alongside.
Step 04
A temporary roof keeps the house dry while the rear slope comes off. The new storey is weathertight around week 8 and finished over weeks 12–16. Weekly photo updates by WhatsApp.
Step 05
We walk the finished storey together, Building Control certifies completion, and 5% stays in retention until the snagging is signed off.
A mansard loft conversion in London typically costs £72,000 to £95,000, VAT included. At the lower end you get one large double bedroom with standard finishes.
The mid-range adds an en-suite shower room. The upper end buys a full master suite with a dressing area, premium en-suite, engineered wood floors and fitted joinery.
It is the most expensive loft conversion type because the rear roof is rebuilt as a complete new storey, and every quote we issue is itemised line by line with no provisional sums.
Yes, always. A mansard changes the shape and height of the roof, so it never falls under permitted development.
We prepare the drawings, submit the application and deal with the planning officer on your behalf, so you never have to chase the council yourself.
Determination usually takes 8 to 10 weeks, and because the mansard follows the traditional London roof form, well-designed applications have a strong approval record, including in conservation areas.
Around 12 to 16 weeks on site.
Add roughly three weeks for the survey and proposal stage and 8 to 10 weeks for the planning application, and the full journey from first call to handover usually comes out at 26 to 30 weeks.
It is a longer road than a dormer, but at the end of it you have an entire extra storey rather than an extension to the existing roof.
Because the mansard is a roof form London conservation officers recognise.
It has been part of the London streetscape since the Georgian era, the steep slate face sits back behind the parapet, and from the street the terrace keeps its original outline.
Councils that refuse flat-roofed box dormers on protected rear elevations will often approve a well-detailed mansard, which is why so many of the ones we build are in conservation areas in Kensington, Islington, Fulham and Clapham Old Town.
A mansard raises the party walls on both sides of the house in brickwork, so notices must be served on both neighbours under the Party Wall etc. Act 1996 before the build starts.
We draft and serve the notices for you, and if a neighbour dissents we work with the appointed party wall surveyors until the award is agreed.
Surveyor fees sit outside the fixed build price, but they appear as their own line on your proposal, so there are no surprises later.
A dormer is a box built out from the existing rear slope, while a mansard removes that slope entirely and rebuilds it as a near-vertical slate wall with a flat roof over the top.
The mansard gives you more floor area, full headroom right across the room and a far more traditional appearance, and it is the only realistic route where dormers are restricted.
The trade-off is cost and time: a dormer runs £52,000 to £68,000 and is usually permitted development, while a mansard runs £72,000 to £95,000 and always needs planning permission.
Adds the most usable headroom and floor space. The most popular choice for Victorian and Edwardian terraces across South London.
About Dormer conversionsTwo dormers joined in an L over the rear and side return. Adds two rooms, typically a bedroom and a generous bathroom.
About L-Shaped Dormer conversionsSquares off a sloping side roof to create the volume needed for a proper room, perfect for semi-detached and end-of-terrace homes.
About Hip-to-Gable conversionsThe simplest, fastest, lowest-cost option. It works inside the existing roof with skylight windows, often without planning permission.
About Velux / Rooflight conversionsMansards come into their own in London's conservation-heavy neighbourhoods: the Georgian squares of Islington, the garden squares of Kensington, and the period terraces of Fulham, Clapham Old Town and Chiswick, where planning policy steers lofts towards the traditional roof form.
We build them across the whole of London. Tap any area to see what a mansard could do on a street like yours.
Send us your postcode and a few details about the house.
Daniel will arrange a free home survey, confirm whether a mansard is the right route, and give you an honest read on planning prospects for your street.
If it stacks up, a fully itemised fixed-price proposal follows within 7 days of the visit. If another conversion type suits your house better, he'll recommend that instead.
Prefer to talk? Call Daniel on 07414 648 263.
