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.
How much does a mansard loft conversion cost in Clapham?
In Clapham, a mansard loft conversion typically costs £84,500–£123,500 for a typical rear mansard, including VAT at 20% (standard rate). Building costs in London run about 30% above the UK average, and the figures for Clapham are adjusted to reflect that. Design, structural calculations and council fees are extra.
| Size or type | Guide range |
|---|---|
| A typical rear mansard | £84,500–£123,500 |
| Higher specification | £123,500–£154,500 |
The main things that change the cost:
- Party wall surveyor fees
- Bathrooms and finishes
- Number of dormers and lead work
- Front, rear or both
Use the guide price tool to see how size and specification change the figure, then ask for a written, itemised quote.
Do you need planning permission for a mansard loft conversion in Clapham?
Here is how the rules work for a mansard loft conversion in 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.
Building regulations approval comes from the council’s building control service or a registered building control approver. The work is inspected at key stages, such as foundations, drainage and insulation, and a completion certificate is issued at the end.
We check this for your address before design work starts, so there are no surprises later.
What we handle
On a typical mansard loft conversion we handle:
- Staircase and fire-safe escape route
- Party wall notices to both neighbours
- Structural design and calculations
- New roof structure, slate or tile covering and lead work
- Planning drawings and application
- Building control sign-off
How long does it take?
Allow 9 to 12 weeks on site, after planning approval. Allow time beforehand for drawings and approvals, usually 3 to 4 months.
Building in Clapham
Many London boroughs have conservation areas and detailed design guides, so early design advice saves time at the planning stage.
Clapham sits in Greater London, about 3 miles south of central London. We cover the whole of Clapham, including postcodes SW4.
How it works
- 01
Survey and brief
We visit, discuss your ideas and budget, and survey the property or site at no cost.
- 02
Design and fixed quote
Drawings, structural calculations and an itemised, fixed price for the agreed specification.
- 03
Approvals
Planning or permitted development confirmation, building regulations and any party wall matters.
- 04
Construction and completion
We build, put right any snags and hand over the building control certificate.
Questions about mansard loft conversions in Clapham
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.
What roof covering is used?
Natural slate is traditional; some councils accept other materials. We follow local guidance.
Who approves a mansard loft conversion in Clapham?
Planning matters in Clapham go to the council covering your address, and the work needs building control approval. We prepare and submit the applications for you.
Can I stay at home?
Usually. The new roof is built under a temporary cover so the house stays dry.
How much does a mansard loft conversion cost in Clapham?
In Clapham, a mansard loft conversion typically costs £84,500–£123,500 for a typical rear mansard, including VAT at 20% (standard rate). Your fixed price depends on the survey and drawings.