What Is a Mansard Loft Conversion?
A mansard conversion alters the entire rear roof slope — and often the party walls — to create a near-vertical rear wall with a flat or very shallow pitch at the top. The result is a large, square room at the top of the house, maximising usable floor area with full-height walls.
Mansard conversions are particularly associated with London Victorian and Edwardian terraced properties, where the original rear slope is often steeply pitched and the available loft space is limited or awkward.
Most mansard conversions in London require planning permission (unlike dormer loft conversions, which can often be built under permitted development for rear-facing dormers). This is because a mansard changes the roof line visible from the street in conservation areas, and extends the rear elevation significantly.
Why Are Mansards More Expensive?
A mansard conversion is more expensive than a Velux or simple dormer conversion because:
- The entire rear roof structure must be removed and rebuilt
- The new rear wall is typically built to masonry or timber frame with a facing material to match or complement the existing
- Party wall work on both sides (shared roof/party wall) is almost always required
- The structural implications are more complex — the new roof load is redistributed differently
- Build time is longer, and scaffolding requirements are greater
Cost Breakdown — London Mansard Loft Conversion
Structural and Building Works
| Item | Typical Cost Range |
|---|---|
| Strip-out existing rear roof structure | £2,000–£5,000 |
| New rear mansard wall (masonry/timber frame) | £8,000–£18,000 |
| New flat roof over mansard section | £4,000–£8,000 |
| New joisted floor (loft floor) | £3,000–£7,000 |
| Party wall work (cutting in / toothing) | £1,500–£4,000 |
| Scaffold (typically more extensive for mansard) | £3,000–£6,000 |
Dormer Windows and Roof Lights
| Item | Typical Cost |
|---|---|
| Rear dormer window (aluminium-framed) | £1,500–£3,500 each |
| Front Velux or roof light | £600–£1,200 each |
Internal Fit-Out
| Item | Typical Cost Range |
|---|---|
| Staircase (standard straight flight to loft) | £2,500–£6,000 |
| First-fix electrics and plumbing | £3,000–£8,000 |
| Plastering and drylining | £3,000–£7,000 |
| Bathroom installation (en-suite) | £5,000–£12,000 |
| Second-fix and decoration | £4,000–£9,000 |
| Flooring (client-supplied) | — |
Professional Fees and Approvals
| Item | Typical Cost |
|---|---|
| Architect (planning and building regs drawings) | £3,000–£7,000 |
| Planning application (including London Local Authority fee) | £800–£2,000 |
| Structural engineer | £800–£2,000 |
| Party wall surveyor (both sides) | £1,500–£4,000 |
| Building Control (local authority) | £600–£1,500 |
Total Cost Range
| Specification | Estimated Total |
|---|---|
| Standard mansard (1 bed + 1 bathroom, standard finishes) | £80,000–£130,000 |
| High-spec mansard (large room, premium finishes, full bathroom) | £130,000–£180,000+ |
These figures are for a standard London Victorian or Edwardian mid-terraced property in Greater London (2026 prices). Prices vary by specific location, access, structural complexity, and finish specification.
What Adds to the Cost?
- Bathroom or en-suite: Adding a bathroom to the loft level adds £5,000–£15,000 to the cost
- Structural complexity: If the existing party walls require significant rebuilding or if the roof structure is non-standard, structural costs increase
- Conservation area: Stricter planning requirements may require more expensive facing materials (stock brick, slate) to match the existing
- Access constraints: Scaffold that must bridge a rear alleyway or cannot be placed on the public highway adds to costs
- Heritage features: Maintaining or restoring original details adds cost
RCB Design & Build
RCB manages mansard loft conversions from planning through delivery, coordinating architect, structural engineer, party wall matters, and build contract. We provide detailed estimates and scope documents before any site commitment.