A mansard loft conversion rebuilds one or both roof slopes almost vertically, at around 72 degrees, with a nearly flat top. It creates the largest rooms of any loft conversion but is usually the most expensive, and normally needs planning permission because the roof is largely rebuilt. Mansards are common on period terraces and in urban areas.
Last reviewed 22 September 2026. Based on our research into official and published sources, listed at the end of this page.
What is a mansard loft conversion?
A mansard replaces a roof slope, usually at the back, with a much steeper structure and a nearly flat roof on top. The result feels close to a full extra storey.
Because the roof is substantially rebuilt, construction programmes tend to be longer and scaffolding costs higher than for other types.
Which houses suit it
Mansards are often used on period properties and in urban streets where space is tight and a full-height room matters.
How much does a mansard loft conversion cost?
In Berkshire, expect roughly £57,000 to £104,000 to build a typical 30m² mansard conversion, before VAT and fees. Elsewhere in the UK the typical range is £50,000 to £80,000. These are research-based estimates, not quotes.
Mansard: typical build costs in Berkshire
Conversion type
Floor area
Typical build cost
Build time
Mansard
30m²
£57,000 to £104,000
12 to 16 weeks
Ranges, not quotes. Build cost only: excludes VAT, professional fees, planning and building control. Last updated September 2026.
Add VAT, drawings, a structural engineer, building control and any party wall costs to get a total. Use the calculator for your own size and finish, or read the cost guide.
Planning and building regulations
Mansard conversions usually need planning permission, because rebuilding the roof slope normally goes beyond what permitted development allows.
In a conservation area, read the council's guidance and any conservation area appraisal before designing a mansard.
Whatever the planning position, every loft conversion needs building regulations approval, covering structure, fire safety, stairs and insulation. See our planning guide and building regulations guide.
Pros and cons
Pros
The largest loft rooms of any conversion type
Room for more than one bedroom or a bedroom and bathroom
Can suit the look of period streets
Cons
Usually the most expensive type
Normally needs planning permission
Longer, more disruptive build
How long it takes
Checkatrade's typical build time for a mansard conversion is 12 to 16 weeks on site, after design, approvals and any party wall process. See the process and timeline guide.
Mansard conversions usually need full planning permission, because the roof is largely rebuilt.
Where it's common in Berkshire
Based on the local mix of homes, this type is most likely to suit houses in Ascot, Reading, Twyford and Windsor:
From 2021 Census and VOA figures for each town. A tendency, not a guarantee for any one house.
Frequently asked questions
How much does a mansard loft conversion cost?
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In Berkshire, a typical 30m² mansard conversion costs roughly £57,000 to £104,000 to build, before VAT and fees, against £50,000 to £80,000 elsewhere in the UK. These estimates come from published cost guides; your quotes will depend on your roof and the specialist.
Does a mansard loft conversion need planning permission?
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Usually, yes. A mansard largely rebuilds the roof, which normally goes beyond permitted development, so you should expect to make a planning application.
Why choose a mansard over a dormer?
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A mansard gives the most internal space and near-vertical walls, so it feels like a full extra floor. The trade-off is a higher cost, a longer build and a planning application.
This guide is general information to help you plan. It is not legal or professional advice: check your own project with your council or a qualified professional.
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