Mansard Loft Conversion Cost Calculator
A mansard rebuilds the whole roof into a near-vertical rear wall with a shallow top. It gives the most floor area of any loft type and costs the most, and unlike the others it always needs planning permission.
Open the calculator →Price your mansard in 30 seconds
4 steps. Property width, staircase route, en-suite and region.
How this calculator works
A mansard rebuilds the rear roof into a near-vertical wall with a shallow pitch on top, usually at about 72 degrees. It produces more habitable floor than any other loft type and it is the only one that is, in effect, a new storey.
Width drives the price because the whole structure is rebuilt from the wall plate up. The fit-out cost per square metre is not much above a dormer; the structural cost is.
On planning: permission is always required and refusal is a genuine risk, particularly where the street has a uniform roof line. Get the decision first. A refused mansard with a signed builder contract is an expensive place to be.
Mansard loft conversion cost by property (2026)
| Option | Typical 2026 cost | Notes |
|---|---|---|
| Mid-terrace, standard width | £55,000–£68,000 | The most common mansard |
| End-terrace or semi | £65,000–£80,000 | Wider, often a gable too |
| Wide semi or detached | £78,000–£95,000 | Two rooms plus en-suite |
| Cost per m² | £2,000–£3,000 | The highest of any loft type |
| Planning application and drawings | £1,200–£3,500 | Always needed, refusal is real |
| Winder or quarter-turn stair | +£2,000–£4,000 | Where the landing is tight |
| En-suite, stack reachable | +£4,000–£7,000 | Rises sharply if it is not |
Figures match our mansard cost guide. Planning is always required — see the mansard planning guide.
Mansard calculator FAQs
Because it alters the shape and volume of the roof beyond anything permitted development allows. The rear roof becomes a near-vertical wall, which changes the profile of the house from the street and from behind. There is no version of a mansard that slips inside PD.
It depends on how much floor you gain. On a narrow Victorian terrace a mansard can turn an unusable roof into two proper rooms where a dormer would give you one. On a house that already has good ridge height, a dormer gets most of the benefit for £20,000–£30,000 less.
Twelve to sixteen weeks on site, plus eight to twelve weeks for the planning decision before that. Plan on six months from first drawing to finished room.
You can appeal, redesign to a large dormer, or stop. This is why the planning application comes first and why paying for drawings before a decision is the wrong order — the drawings are useful either way, the scaffolding deposit is not.
Related cost guides
- Loft conversion costs — every type priced
- Planning permission — the PD volume limits
- Loft conversion quotes — 3 free quotes