Mansard Loft Conversion Cost London 2026

Mansard conversion costs at a glance

These are our published London figures for 2026. The build figures include VAT at 20% and a standard allowance for the staircase, insulation, plastering and basic decoration. Where your mansard lands in the range depends on the roof you start with, what the planning permission allows and the finishes you choose, all covered further down this page.

For every loft type side by side, see our loft conversion cost guide. If a mansard is more than the house needs, our dormer loft conversion cost guide covers rear, L-shaped and hip-to-gable dormers.

What a mansard loft conversion is

A mansard rebuilds the rear slope of the roof almost vertically, typically at 70 degrees or steeper, and caps it with a shallow-pitch flat roof. The new rear wall carries the windows, so the room has near-full height and feels much more like a conventional floor than a traditional loft.

That is the real difference from a dormer. A dormer is a box built out from the rear slope, so part of the slope remains around it. A mansard replaces the slope, which is why it gives the greatest volume of any loft conversion type. Mansards are particularly prevalent on central London's period stucco and brick terraces, where they have been part of the roofscape for over a century.

Rear mansard

The mansard described in our guides is a rear mansard: only the rear slope is rebuilt. The front of the house keeps its existing roof to the street, and the new near-vertical wall faces the garden. The brief is usually the same as for any loft conversion: a properly built new floor, often a bedroom with an en-suite, finished to the standard of the rest of the house.

Front and double mansards

A mansard can also be built on the front slope, or on both slopes, sometimes called a double mansard. That changes the elevation facing the street. Permitted development does not cover any roof extension beyond the plane of the front slope facing the road, and our planning study found that the loft schemes needing a planning application tend to be the larger or more visible designs, such as mansards and front dormers. We have no separate published figure for a double mansard, so it is priced from its own drawings at quotation.

Why a mansard almost always needs planning permission

Most loft conversions in England are permitted development under Schedule 2, Part 1, Class B of the Town and Country Planning (General Permitted Development) Order 2015. A mansard is the exception. Its near-vertical rear wall and the substantial change to the roof profile cannot be delivered within the height and projection limits of permitted development, whether or not the house is in a conservation area.

So plan on a householder planning application from the start. The council fee is £548. A householder decision typically takes 8–10 weeks, and the typical (median) London householder decision between September 2025 and August 2026 took 8.0 weeks from validation, with Westminster the slowest borough at 11.5 weeks. For a mansard, budget 10–16 weeks for the planning stage as a whole.

Loft applications are usually approved. London councils approved 82.0% of loft conversion planning applications decided between September 2025 and August 2026, according to our 2026 London planning approvals study. That figure covers all loft conversion planning applications in the study, not mansards alone.

Conservation areas and Article 4 directions

In a conservation area, planners look closely at a mansard's materials, window proportions and parapet detailing. The quality of the architect's drawings and the heritage statement can have a significant bearing on the outcome, so this is not the place to save on design fees.

Article 4 directions remove some or all permitted development rights in specific areas, and Camden, Islington, Westminster, Hackney and Southwark all have extensive coverage. Because a mansard needs an application in almost every case anyway, Article 4 matters most when you are weighing a mansard against a dormer that might otherwise have been permitted development. Our in-house architectural team prepares the drawings and manages the planning application; the wider rules are in our planning permission guide.

What decides where a mansard sits in the range

The £60,000–£100,000+ range is wide because the cost reflects the structural complexity of rebuilding a roof slope, and because the specification can vary a great deal from one mansard to the next. These are the main levers.

• Structure. A steel ridge beam, valley rafters, new floor joists and temporary propping while the roof is open. The size of the roof sets how much of each is needed.
• Glazing. Full-height glazing to the rear wall and timber windows to match the house. Juliet balconies or French doors add £2,500–£6,000.
• Roof finish. A GRP, zinc or copper finish to the roof adds £2,000–£6,000 over a standard felt-and-batten roof.
• Bathroom and joinery. A quality en-suite and fitted joinery. Bathroom sanitary ware, tiling and fitted wardrobes are typically priced separately unless you ask us to specify them, so those choices stay yours.
• Roof terrace. A well-designed mansard with full-height glazing, a roof terrace and a quality en-suite adds exceptional value in London's top postcodes, and each of those items adds to the build cost.
• Location and quality expected. In prime central London, schemes specified to match the quality expected in Kensington, Notting Hill and Chelsea regularly reach £120,000–£150,000.
• Access. External scaffold surcharges where access is restricted are typically outside a standard quote, so check how any quote you compare treats them.

If a quote you receive is well below these ranges, check line by line that it includes everything listed further down this page.

Structure: steel, the new floor and party walls

A mansard is structural work from the first day. The rear slope is rebuilt, so the job needs structural steelwork (ridge beam, valley rafters and any temporary propping) and a new floor of engineered joists to Building Regulations specification, integrated with the existing ceiling structure. A structural engineer calculates the floor, beam and connection specifications before work starts.

On a terraced or semi-detached house the structural work to the roof and floor typically involves the party wall: cutting into or raising it, installing steel beams, or other structural work on or next to it. A Party Structure Notice must be served at least 2 months before the start, and the neighbour has 14 days to respond. Surveyor costs in London are typically £800–£1,500 with a single agreed surveyor, or £800–£3,500 per neighbour where each side appoints its own. Under the Party Wall etc. Act 1996 the surveyors decide who pays; in practice the building owner usually pays all reasonable costs.

Start the party wall process as early as possible, ideally while the planning application is being processed, so it does not delay the start on site. We manage the process for you where it applies; our party wall agreement cost guide has the detail.

Building regulations approval is required for every loft conversion, whatever its planning status. It covers a compliant staircase (typically with a 42-degree maximum pitch), fire protection to escape routes with smoke alarms integrated with the existing system, insulation to Part L, structural calculations and windows that meet the energy and fire-safety requirements. The work is signed off by building control.

How long a mansard takes

Our guides allow 2–4 months before the start on site for design, structural calculations and any planning permission; for a mansard, budget 10–16 weeks for the planning process itself. Time on site is then typically 14–20 weeks, against 10–14 weeks for a rear dormer.

1. Consultation and technical assessment. We measure the existing loft volume, ridge height, rafter spacing and the ceiling height below, review the party wall position and the planning constraints, and check the staircase can be routed without an unacceptable loss of space on the floor below.
2. Design and planning. Our in-house architectural team prepares the drawings, or we work with your architect and structural engineer. The design and structural package is complete before work starts on site.
3. Structure and roof. Structural steelwork and the new floor structure go in first. The mansard is then built, the roof covered, leadwork and flashings fitted at every junction, and the windows installed.
4. Stairs, services and finishes. A compliant staircase connects the new floor to the one below. Insulation, first and second fix electrics, plumbing for any en-suite, plastering and decoration follow.
5. Sign-off and handover. Building control signs off the work. Our 10-year workmanship warranty starts at practical completion and passes to the new owner if you sell.

Homeowners can usually stay in the house during a loft conversion, because most of the work takes place above the existing living space. We seal off the work area and maintain safe access to your home throughout.

What a mansard quote should include

A complete loft conversion quotation from a London contractor should cover all of the following.

• Structural steelwork: ridge beam, valley rafters, floor joists and any temporary propping
• New floor structure of engineered joists, integrated with the existing ceiling structure
• A compliant staircase from the floor below
• Roof works: the mansard structure, felt, battens, tiles or slate to match, and leadwork and flashings at all junctions
• Insulation to Part L of the Building Regulations
• Windows and rooflights that meet the energy and fire-safety requirements
• Plasterboard and plastering to all walls and ceilings
• First and second fix electrics, with smoke alarms linked to the existing system
• Plumbing for an en-suite, where included
• Decoration: mist coat and two full coats of emulsion throughout, gloss to woodwork
• Building regulations compliance: fire protection to escape routes, structural calculations and energy compliance

Typically not included in a standard quote: fitted wardrobes and storage, bathroom sanitary ware and tiling (unless specified), flooring beyond a standard option, and external scaffold surcharges where access is restricted.

Fees and other costs to budget for

Beyond the build contract, these costs apply to almost every London mansard. They are easy to leave out of an early budget.

• Architect or designer: £2,000–£6,000 for drawings, the planning application and the building regulations submission.
• Structural engineer: £600–£1,500 for the floor, beam and connection calculations.
• Householder planning application: £548.
• Building regulations: £700–£2,000 in plan check and inspection fees.
• Party wall surveyors: £800–£1,500 with a single agreed surveyor, or £800–£3,500 per neighbour where each side appoints its own.
• VAT: loft conversions are standard-rated at 20%; there is no reduced rate for residential conversions of this type.

Value, and how we price a mansard

A quality loft conversion typically adds 15–20% to a London property's value. That premium depends on the work: a poorly executed loft with a non-compliant staircase, inadequate fire protection or visible water ingress can reduce a property's saleability.

Our quotations are fully itemised and based on a complete specification, with no provisional sums for items we have not properly scoped. We use JCT Minor Works contracts on all projects: fixed pricing, a defined programme and a transparent variation process. We are CHAS Elite, Constructionline Gold and Federation of Master Builders members, with £10M public liability insurance, and every job carries our 10-year workmanship warranty.

If you already have planning drawings, send them through our contact form, which accepts PDF, JPG, PNG, HEIC and DWG files, or read how we price from a drawing pack. You can also call 020 8080 8240, message us on WhatsApp on 07300 037353 or email [email protected]. More on our approach is on our loft conversions service page.

Frequently Asked Questions

Prices shown are indicative ranges based on London market data for 2026. Actual costs depend on the existing structure, specification and programme. All figures include VAT at 20% unless otherwise stated. Your own price is fixed in a written, itemised quote.