What is a mansard loft conversion?
A mansard loft conversion rebuilds the roof with a nearly vertical slope (about 72 degrees) and a flat top, effectively adding a new storey. Small dormer windows sit within the steep slope. Mansards are common on period terraces in towns and cities.
Because a mansard changes the shape of the roof, it almost always needs planning permission. Many councils have design guidance for mansards on terraced streets.
What we handle
- Planning drawings and application
- Structural design and calculations
- Party wall notices to both neighbours
- New roof structure, slate or tile covering and lead work
- Dormer windows
- Staircase and fire-safe escape route
- Insulation, electrics, plumbing and plastering
- Building control sign-off
Options to consider
Rear mansard
A rear mansard keeps the front roof unchanged.
Front and rear mansard
Allowed on some streets where neighbours already have one.
Lead or zinc dormers
Traditional detailing that councils often prefer.
Two rooms and a bathroom
A mansard often fits two bedrooms and a shower room.
How much does a mansard loft conversion cost?
A mansard loft conversion typically costs £65,000–£95,000 for a typical rear mansard across the UK, including VAT at 20% (standard rate). Build time is usually 9 to 12 weeks on site, after planning approval. Design, structural calculations and council fees are extra.
| Region | Guide range (for a typical rear mansard) |
|---|---|
| London | £84,500–£123,500 |
| South East | £73,000–£106,500 |
| East of England | £69,000–£100,500 |
| South West | £66,500–£97,000 |
| West Midlands | £63,000–£92,000 |
| East Midlands | £62,000–£90,000 |
| North West | £61,000–£89,500 |
| Yorkshire and the Humber | £60,500–£88,500 |
| North East | £58,500–£85,500 |
| Wales | £60,000–£87,500 |
| Scotland | £63,000–£92,000 |
| Northern Ireland | £56,000–£81,500 |
What changes the price:
- Front, rear or both
- Roof covering: natural slate costs more
- Number of dormers and lead work
- Party wall surveyor fees
- Scaffolding on busy streets
- Bathrooms and finishes
Use the guide price tool for a figure based on your measurements, or see all guide prices.
Do I need planning permission?
England
In England a mansard almost always needs planning permission because it changes the roof shape. Many councils publish design guidance for mansards on terraced streets, and some streets are more likely to be approved where neighbours already have one.
Wales
In Wales a mansard roof needs planning permission and is judged against local design guidance.
Scotland
In Scotland a mansard needs planning permission and a building warrant.
Northern Ireland
In Northern Ireland a mansard needs planning permission from the council.
Even when planning permission is not needed, building regulations approval (a building warrant in Scotland) usually is. We make the applications and book the inspections.
How long does it take?
Typically 9 to 12 weeks on site, after planning approval. Surveys, design and approvals usually take 3 to 4 months beforehand.
How it works
- 01
Free visit
We look at the property, discuss what you want and measure up.
- 02
Design and fixed quote
Drawings, calculations and an itemised price for the agreed specification.
- 03
Approvals
Planning or permitted development, building regulations and neighbour matters.
- 04
Build and sign-off
Built to a written programme, with a completion certificate at the end.
Mansard loft conversions: questions and answers
Does a mansard need planning permission?
Almost always, because it changes the roof shape. Councils assess it against local design guidance.
How long does it take?
Around 9 to 12 weeks on site, plus 2 to 3 months for planning and party wall matters beforehand.
Why choose a mansard over a dormer?
A mansard gives more usable floor space and a neater appearance from the street, but costs more and needs planning permission.
Do I need party wall agreements?
Yes, mansards on terraced houses almost always involve both party walls.
Can I stay at home?
Usually. The new roof is built under a temporary cover so the house stays dry.
What roof covering is used?
Natural slate is traditional; some councils accept other materials. We follow local guidance.