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Mansard loft conversion on a Victorian terraced house
Maximum Space — Premium Finish

Mansard Loft Conversion Specialists

A mansard loft conversion costs between £35,000 and £60,000+ in the UK — the most expensive type, but it delivers the largest usable floor space.

A mansard conversion replaces the rear roof slope with a near-vertical wall and a flat roof. No other conversion type creates more usable floor space. That makes it the premium choice for terraced homes and urban properties across the UK.

From £35,000
Timescale: 10–14 weeks

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What is a Mansard Loft Conversion?

A mansard loft conversion replaces the existing sloped rear roof with a near-vertical wall — typically angled at 72 degrees — and a flat or very low-pitch roof section. The style takes its name from François Mansart, the 17th-century French architect who popularised it. Crucially, that near-vertical wall removes the sloped ceilings that cut headroom in a conventional loft conversion. As a result, you get the largest usable floor area of any conversion type.

Unlike a dormer, which adds a box extension to an existing roof, a mansard replaces the entire rear slope. The result is a full-width room with vertical walls and standing headroom from wall to wall. In practical terms, that usually means 30–50 m² of extra space — enough for a bedroom plus en-suite. That is why mansards suit terraced and townhouse homes in London and other urban areas, where every square metre counts.

Because a mansard alters the overall shape and appearance of the roof, it usually needs a full planning application rather than Permitted Development rights. That sounds daunting, but it rarely needs to be. Our experienced planning team handles every step, from pre-application discussions with your local authority through to final approval. Where neighbours have already converted, there is often strong planning precedent on your side.

Key Benefits of a Mansard Conversion

Most Floor Space

30–50 m² of usable floor area with full standing headroom from wall to wall. The most generous of all conversion types.

Premium Finish

Near-vertical walls and a flat roof give a clean, contemporary look that lifts the whole property.

Works on Most Properties

Suits terraced, semi-detached, and detached homes. Particularly popular for Victorian and Edwardian terraces.

Ideal for Terraced Homes

Builds upward and rearward without extending sideways — the perfect solution for closely bounded terraced properties.

Is a Mansard Conversion Right for My Home?

A mansard is the right choice when you want the maximum possible loft space and are happy to go through planning. In practice, most urban homes are strong candidates. Run through this quick checklist before you commit:

  • Terraced or townhouse property — ideal application for a mansard
  • Semi-detached or detached home where maximum space is the priority
  • Urban or London property — mansards are commonplace and planning approval rates are high
  • Conservation area property — our planning team has strong experience with heritage applications
  • Property with adjacent neighbours who have already converted (strong planning precedent)
  • Time for the planning process (typically 8–13 weeks) factored into your project timeline
A completed dormer conversion — a more modest alternative to the full mansard
Not sure whether a dormer or mansard is right for your home? Our free survey gives you an honest recommendation.

What's Included in Every Mansard Conversion

Free site survey and planning feasibility assessment
Full architectural drawings and planning documentation
Planning application submission and liaison with your local planning authority
Building regulations submission and approval
Structural engineer calculations (beams, floor, roof)
All scaffold, plant hire, and materials
Demolition of existing rear roof slope
New near-vertical rear wall and flat roof construction
Mansard windows (dormer windows or rooflights in flat section)
Warm flat roof insulation to Part L standards
Internal stud walls, plasterboard, and first-fix electrics
New staircase, fire doors, and building regs sign-off

Typical Cost and Timescale

Cost Guide

From £35,000

All planning, structural, and build costs included. The premium specification and larger floor area make this the most investment-intensive conversion type — and the one that adds the most value.

Overall Programme

10–14 weeks

Build programme once planning is granted. Planning typically takes 8–13 weeks. We begin all design and structural work during the planning period to minimise the overall project timeline.

Fixed-price quotes — no surprises once contracts are signed.

Frequently Asked Questions

Why does a mansard loft conversion usually need planning permission?
A mansard conversion changes the shape and appearance of the roof. It swaps the existing slope for a near-vertical rear wall, typically at 72°, topped by a flat or low-pitch section. On many properties this change is visible from the street and alters the character of the building. As a result, it falls outside Permitted Development rights and needs a full planning application in most cases. However, if your neighbours have already done similar work, there is often strong precedent for approval. We handle every planning submission and liaise directly with your local planning authority.
How much additional space does a mansard loft conversion add?
A mansard conversion typically adds 30–50 m² of usable floor space. That makes it the most generous of all conversion types. Because the rear wall sits near-vertical at 72°, the whole floor has full standing headroom, with no sloped ceilings eating into the space. As a result, a mansard works especially well for fitting two rooms into one conversion — a bedroom plus en-suite, or a bedroom plus home office.
What is the difference between a mansard and a dormer loft conversion?
A dormer adds a box-shaped extension to an existing sloped roof. A mansard goes further: it replaces the entire rear slope with a near-vertical wall and a flat roof section. Because that wall runs the full width of the house, a mansard gives far more floor space and headroom than a dormer, which may only cover part of the roof. Mansards do cost more and usually need planning permission. In return, they deliver the maximum possible usable space — a real advantage on urban and terraced homes where a dormer alone falls short.
Is a mansard loft conversion suitable for my terraced house?
Yes — the mansard is the conversion of choice for terraced and townhouse properties in urban areas, especially London. It builds upward and rearward from the existing roof line without extending sideways. As a result, it suits homes that are tightly bounded on both sides. The near-vertical rear wall and flat roof also sit comfortably with the character of Victorian and Edwardian terraces. We have extensive experience with mansards in conservation areas and on listed buildings, and our planning team can advise on the specific rules that apply to your property.
How long does planning permission take for a mansard conversion?
A standard full planning application in England takes 8 weeks for a decision, counted from the date your local authority validates the application. Some authorities run slower and may ask for an extension of up to 13 weeks. Conservation areas and listed buildings often take longer, since they need extra heritage assessments. To avoid delays, we submit the most complete application possible and rarely get a request for further information. Better still, all planning work is included in your fixed-price quote — there are no extra charges for planning fees or resubmissions.

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