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 Chiswick?
In Chiswick, 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 Chiswick. 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 biggest cost drivers are:
- Bathrooms and finishes
- Number of dormers and lead work
- Party wall surveyor fees
- Scaffolding on busy streets
A guide range is only a starting point: the guide price tool gives an instant figure for your size, and a free survey turns it into a fixed quote.
Do you need planning permission for a mansard loft conversion in Chiswick?
The approval route for Chiswick is set by planning law 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.
Where work involves a shared wall, building on the boundary, or digging foundations within 3 metres of a neighbour’s building and deeper than its foundations (or within 6 metres for deeper work), the Party Wall etc. Act 1996 applies and notices must be served before work starts.
Our first visit includes a check of your planning history, so you know the route before paying for drawings.
What we handle
On a typical mansard loft conversion we handle:
- Planning drawings and application
- New roof structure, slate or tile covering and lead work
- Party wall notices to both neighbours
- Insulation, electrics, plumbing and plastering
- Structural design and calculations
- Staircase and fire-safe escape route
How long does it take?
Build time is typically 9 to 12 weeks on site, after planning approval. Allow time beforehand for drawings and approvals, usually 3 to 4 months.
Building in Chiswick
Victorian and Edwardian terraces dominate many London streets, which is why side return, wraparound and mansard projects are so common here.
If your home is in Chiswick or close by in Hammersmith, Acton and Barnes, a free home visit is the first step.
How it works
- 01
Free visit
We look at the property, talk through what you want and measure up. There is no charge and no obligation.
- 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 sign-off
We run the build to a written programme and hand over the completion certificate.
Questions about mansard loft conversions in Chiswick
Does a mansard need planning permission?
Almost always, because it changes the roof shape. Councils assess it against local design guidance.
How much does a mansard loft conversion cost in Chiswick?
In Chiswick, 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.
Do you build mansard loft conversions in Chiswick?
Yes. We build mansard loft conversions across Chiswick, including postcodes W4. Every project starts with a free home visit.
Can I stay at home?
Usually. The new roof is built under a temporary cover so the house stays dry.
Do I need party wall agreements?
Yes, mansards on terraced houses almost always involve both party walls.