04 / 07 Loft conversion types
Mansard Loft Conversions
A mansard loft conversion rebuilds the rear slope of your roof at a very steep angle, typically around 70 degrees, with a flat roof on top. The result looks like a traditional roof from the street, but inside it gives almost vertical walls and full standing height across nearly the whole loft. That is more usable space than any other type of loft conversion.
Mansard roof extensions are common on London's Victorian and Edwardian terraces. They are often preferred by councils in conservation areas, where a large box dormer would not be allowed. Because a mansard almost always needs planning permission, we plan the design around your council's rules from the start.
- Part ofLoft Conversions
- Our process7 clear steps
- Answered here6 common questions
- FreeSurvey & itemised quote
01How it works
What a Mansard Conversion Involves
- Rebuilding the rear roof slope: The existing rear slope is removed and replaced with a new, steep slope, usually around 70 degrees, finished in slate or tiles.
- A flat roof on top: The top of the mansard is a shallow, almost flat roof, finished with a durable membrane and set behind the ridge line.
- Windows as small dormers: Windows are usually set into the steep slope as small dormers with lead or zinc cheeks, which gives a traditional appearance.
- Raised party walls: On terraced houses, the party walls on each side are built up to the height of the new roof.
- A new floor and staircase: As with any loft conversion, the floor is strengthened and a permanent staircase is added.
02Types
Types of Mansard
- Rear mansard: The most common type. The rear roof slope is rebuilt as a mansard and the front slope stays as it is, so the street view hardly changes.
- Front and rear (full) mansard: Both slopes are rebuilt, creating a complete extra storey. This usually happens only where the street already has mansard roofs and the council's design guidance allows it.
- L-shaped mansard: The mansard continues over the rear outrigger of a Victorian terrace. See our L-shaped loft conversion page.
03Mansard vs dormer
What Is the Difference Between a Mansard and a Dormer Loft Conversion?
A dormer loft conversion adds a box with vertical walls onto the existing rear roof slope, which stays in place around it. A mansard replaces the whole rear slope with a new, steep roof. The mansard gives more space and a more traditional appearance, but it involves more construction and usually needs planning permission.
| Mansard | Rear dormer | |
|---|---|---|
| Space | The most usable space of any loft type | Large, but less than a mansard |
| Appearance | Traditional, steep roof with small dormer windows | Box shape on the rear slope |
| Planning | Usually needs planning permission | Often permitted development |
| Relative cost | Higher | Lower |
| Relative build time | Longer | Shorter |
| Often chosen for | Terraces and conservation areas | Most houses outside conservation areas |
04Planning
Do You Need Planning Permission for a Mansard Loft Conversion?
In most cases, yes. A mansard changes the shape of the roof and usually raises the party walls, so it goes beyond what permitted development allows for roof extensions. Flats, listed buildings and properties in conservation areas always need permission for roof alterations of this kind.
Many London councils publish design guidance for mansards. It covers the angle of the slope, the height of the flat roof, how far the mansard sits behind the parapet or gutter, the size and spacing of the windows, and the materials. Following that guidance closely gives an application the best chance of approval. We check your council's guidance and look at existing mansards in your street. Then we prepare the drawings for the application with that in mind.
05Party wall and regulations
Party Wall and Building Regulations
Because a mansard usually raises the party walls on both sides of a terraced house and bears new steel beams onto them, notices are needed for both neighbours under the Party Wall etc. Act 1996. The conversion also needs building regulations approval, which covers:
- the structure of the new roof and floor
- a protected escape route and interlinked smoke alarms
- the staircase
- insulation to the steep slopes and flat roof
- ventilation
We arrange the inspections and hand you the completion certificate.
06Cost
What Affects the Cost of a Mansard Loft Conversion in London?
A mansard is usually the highest-cost type of loft conversion, because the most roof is rebuilt. Every loft is priced after a free survey, and you receive a written, itemised quote. The main factors are:
Every factor is priced line by line after your free survey — no guesswork, staged payments.
Book a free loft survey- 01
Rear, full or L-shaped mansard.
- 02
Roof finish
Natural slate, tiles, and lead or zinc to the dormer windows.
- 03
Raising the party walls and the steelwork.
- 04
Number and size of windows.
- 05
Planning requirements
Conservation areas may call for specific materials or details.
- 06
Staircase and en-suite
Including a Saniflo installation where needed.
- 07
Scaffolding and access on terraced streets.
- 08
Fees
Planning, structural calculations, building control and party wall surveyors for both neighbours.
Timescale · 07
How Long Does a Mansard Loft Conversion Take?
A mansard usually takes longer than any other type of loft conversion. The whole rear roof is rebuilt and the party walls are raised, and planning permission and party wall agreements have to be in place before work starts. The roof is opened in stages under temporary protection, so the house stays weathertight. Your quote includes a programme for each stage.
One team · 08
One Team From Roof to Finish
We handle every trade the conversion needs:
- electrics through our domestic electrical installation service
- heating through our radiator installation service
- the en-suite through our bathroom plumbing and kitchen and bathroom tiling services
- plastering and decorating
If you would rather finish the inside yourself, see our shell loft conversion option.
09Process
Our Mansard Loft Conversion Process
Planned before we start, managed by one project manager, signed off at the end.
- 01
Free loft survey
We measure the loft, check the roof and party walls, and look at your council's mansard guidance.
- 02
Design and quote
Planning drawings, structural calculations and an itemised quote with a programme.
- 03
Planning and approvals
Planning application, building regulations approval and party wall notices to both neighbours.
- 04
Scaffolding and structure
The party walls are raised, steel beams are installed, and the mansard frame and flat roof are built.
- 05
Roofing and windows
Slate or tiles go on the slope, lead or zinc to the dormers, and the windows are fitted.
- 06
Staircase, first fix and finishing
Wiring, pipework, insulation, the staircase, plastering, the en-suite and decorating.
- 07
Sign-off and handover
Building control completion, certificates and a final walk-round.
10Areas
Mansard Loft Conversions Across London
We build mansard loft conversions across London, with a focus on east and south-east London, including the Victorian and Edwardian terraces of East Ham and Ilford. Find out more on our pages for mansard loft conversion in London, mansard loft conversion in East Ham, mansard loft conversion in Ilford, mansard loft conversion in Barking, mansard loft conversion in Dagenham and mansard loft conversion in Orpington.
11FAQs
Mansard Loft Conversion FAQs
How much does a mansard loft conversion cost in London?
A mansard is usually the highest-cost loft type because the whole rear roof is rebuilt and the party walls are raised. The price depends on the size, the roof materials, the windows, the steelwork and planning requirements. We give you a fixed, itemised quote after a free survey.
Do I need planning permission for a mansard loft conversion?
In most cases, yes. A mansard changes the roof shape and usually raises the party walls, which goes beyond permitted development. Following your council's mansard design guidance gives the best chance of approval.
What is the difference between a mansard and a dormer loft conversion?
A dormer adds a box onto the existing rear roof slope. A mansard replaces the whole rear slope with a new, steep roof. A mansard gives more space and a more traditional look but usually needs planning permission and costs more.
How long does a mansard loft conversion take?
It usually takes longer than any other type of loft conversion, because the rear roof is rebuilt and party walls are raised. Planning and party wall agreements come first. Your quote includes a programme.
Can I have a mansard in a conservation area?
Often, yes. Councils frequently prefer a well-designed rear mansard to a box dormer in conservation areas, especially where other houses in the street already have one. Planning permission is needed, and the design must follow local guidance.
Do both neighbours need to agree?
Both neighbours must be given party wall notices, because the party walls are usually raised. If they do not agree, a party wall surveyor prepares an award so the work can go ahead.
01Loft Conversions
Explore other loft conversion types
Next step
Book Your Free Loft Survey
Tell us about your house and how you would like to use the loft. We will check your council's guidance, show you what a mansard could fit and send you a written, itemised quote.
Send us photos of your loft and the back of your house on WhatsApp to get started, or use our contact form.
- Free survey
- Itemised written quote
- One project manager