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 Westminster?
In Westminster, a mansard loft conversion typically costs £84,500–£123,500 for a typical rear mansard, including VAT at 20% (standard rate). Prices across London are typically 30% above the UK average for this kind of work, which is reflected in the range for Westminster. 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 |
What moves the price most:
- Roof covering: natural slate costs more
- Scaffolding on busy streets
- Number of dormers and lead work
- Bathrooms and finishes
Your exact price depends on the survey and drawings. The guide price tool gives a quick range in the meantime.
Do you need planning permission for a mansard loft conversion in Westminster?
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
The work we manage on a mansard loft conversion:
- Dormer windows
- Planning drawings and application
- Insulation, electrics, plumbing and plastering
- Party wall notices to both neighbours
- Staircase and fire-safe escape route
- Building control sign-off
How long does it take?
Allow 9 to 12 weeks on site, after planning approval. Design and approvals come first and typically take 3 to 4 months, depending on the approval route.
Building in Westminster
Victorian and Edwardian terraces dominate many London streets, which is why side return, wraparound and mansard projects are so common here.
Westminster sits in Greater London, in central London. We cover the whole of Westminster, including postcodes SW1.
How it works
- 01
Survey and brief
We visit, discuss your ideas and budget, and survey the property or site at no cost.
- 02
Drawings and written quote
We prepare the design and give you a written, itemised quote with inclusions and exclusions.
- 03
Approvals in place
We handle planning, building control and neighbour notices before work starts.
- 04
Build and handover
Work follows the agreed programme, with inspections along the way and a completion certificate at the end.
Questions about mansard loft conversions in Westminster
What roof covering is used?
Natural slate is traditional; some councils accept other materials. We follow local guidance.
Do you build mansard loft conversions in Westminster?
Yes. We build mansard loft conversions across Westminster, including postcodes SW1. Every project starts with a free home visit.
Do I need party wall agreements?
Yes, mansards on terraced houses almost always involve both party walls.
How much does a mansard loft conversion cost in Westminster?
In Westminster, 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.
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.