Loft Conversion Types
Mansard Loft Conversions
The biggest thing you can do to a loft — a new roof form with near-vertical rear walls, giving you a full extra storey rather than a converted roof space.
What Makes a Mansard Different
Every other loft conversion works within the roof you already have. A mansard replaces it.
The rear roof slope is rebuilt at a very shallow angle — close to vertical, typically around 70 degrees — with a flat or near-flat roof across the top. What you end up with reads less like a converted loft and more like an extra floor on the house, with full-height walls, standard-height windows and none of the sloping ceilings that define the other types.
It's the most expensive conversion, the most disruptive, and the one most likely to need a planning application. It's also the only one that reliably delivers two rooms and a bathroom on a narrow terrace. In parts of West London where the houses are tall and thin — Hammersmith, Shepherd's Bush, the older parts of Chiswick and Teddington — it's often the only conversion that solves the problem people actually have.
Is a Mansard Right for You?
Consider One if You Need Two Rooms
Or a bedroom plus a proper bathroom, and a dormer or L-shaped dormer won't get you there.
Consider One if Your Ridge Is Low
A mansard rebuilds the roof form, so it isn't limited by what's up there now in the same way the other types are.
Look Elsewhere if Permitted Development Matters
A mansard rarely qualifies, so you should be prepared for an application from the outset.
Check the Street First
Mansards succeed at planning far more often on streets where they already exist. If several houses on your terrace have one, you are in a very different position from someone proposing the first.
What the Work Involves
Removing the Existing Rear Roof
This is real demolition, not an opening — the house is exposed and scaffolded with temporary weather protection for a period. It is the most disruptive stage of any loft conversion.
New Walls and Roof
The near-vertical rear walls are built in masonry or framed and clad, with a new flat or shallow-pitched roof over.
Structure Throughout
Steels for the new floor and to carry the rebuilt roof. On a terrace this involves both party walls, and the party wall process is effectively a certainty rather than a possibility.
The Stairs
Because a mansard usually creates two rooms, the staircase design is more involved — and the two-metre headroom requirement over the whole flight still applies.
Fire Safety
Rising to a third storey changes the fire strategy. Escape windows are not accepted, the stairway must be protected throughout, and building control will be closely involved from the design stage.
Full Fit-Out
Insulation, plastering, heating, electrics, bathroom, decoration.
What a Mansard Costs
| Scope | Typical West London range |
|---|---|
| Mansard conversion | £65,000 – £105,000 |
| With two rooms and a bathroom | £78,000 – £105,000 |
These are estimates for West London, not quotes. What your loft actually costs depends on your roof, your access and what you put in the room — which is exactly what a survey is for.
Alongside the build cost, budget for the things a mansard brings that a dormer doesn't: an architect or designer, a planning application, a structural engineer, a party wall surveyor for each affected neighbour, and building control fees. On a mansard those professional costs are a meaningful line rather than a rounding error.
Mansards and Planning Permission
Assume you will need it.
A mansard changes the roof form substantially, and under permitted development a roof enlargement must not exceed the height of the existing roof, must keep the original eaves with a 0.2 metre set-back, and must use materials similar in appearance to the house. A mansard typically struggles against at least one of those, and the volume it uses — against a 40 cubic metre allowance on a terrace or 50 cubic metres otherwise — is the largest of any conversion type.
If your house is in a conservation area the question doesn't arise: Roof enlargements are not permitted development on designated land, which includes conservation areas. Ealing has 30 of them, and six carry Article 4 directions that remove householder rights further. If you're in one, this conversion needs full planning permission — which doesn't mean refusal, just an application.
None of which means refusal. Councils weigh mansards on the character of the street more than on the principle, and on streets where mansards are already the norm they are routinely approved. It does mean building the application into your timetable from the start — realistically adding two to four months before anyone puts up scaffolding.
Building regulations approval is required either way. It's a separate consent from planning and it applies even when no planning permission is needed at all.
Mansard Conversions Across West London
Mansards suit the taller, denser period terraces:
Mansard Questions
How much more space than a dormer?
Substantially more, and better shaped. A mansard commonly gives two usable rooms where a dormer gives one, because the walls are full height rather than sloping.
How long does a mansard take?
Longer than any other type — commonly three to four months on site, plus the design and planning period before it. The planning stage is usually the bigger part of the wait.
Is the house liveable during the work?
Many people stay, but a mansard is the type where moving out for part of it is worth costing. The rear roof comes off, and that stage is genuinely disruptive.