Loft Conversion Cost by Type: Rooflight vs Dormer vs Mansard
Loft conversions typically cost {range:loft-conversion:mid}, but the type and party wall situation can shift that by tens of thousands.
A loft conversion typically costs £52,000, with a realistic range of £40,500 – £67,600 depending on the type of conversion, whether you need a party wall agreement, and whether you're adding a bathroom. A simple rooflight conversion sits near the bottom of that range; a mansard sits well above it. The headline figure on its own hides most of what actually moves your quote, so this guide breaks down the four conversion types and the party wall costs that catch a lot of homeowners out.
The four types, and why the price gap is so wide
All loft conversions start from the same basic shell — insulation, flooring, a staircase, electrics and plastering — but the roof work underneath that shell varies enormously:
- Rooflight (Velux-type): typically £32,300. No change to the roof shape, just windows fitted into the existing slope. This is the cheapest option because there's no steelwork and usually no planning application.
- Dormer: typically £52,000. A box-shaped extension built into the rear roof slope adds full-height headroom and floor space, but needs new roof structure, flashing and often steel beams.
- Hip-to-gable with rear dormer: typically £61,000. On a house with a hipped (sloped) end roof, this squares off the gable end before adding a dormer — more structural work, more roofing, and usually scaffolding on an extra elevation.
- Mansard: typically £70,300. The most invasive option: most of the existing roof slope is rebuilt at a steep near-vertical angle to maximise headroom. This is closer to building a new roof than converting an existing one, which is why it costs roughly as much again as a rooflight conversion.
| Material | Typical | Usual range | Lifespan |
|---|---|---|---|
| Roof windows only (Velux-type) | £32,300 | £25,200 – £42,000 | – |
| Rear dormer | £52,000 | £40,500 – £67,600 | – |
| Hip-to-gable with rear dormer | £61,000 | £47,500 – £79,200 | – |
| Mansard | £70,300 | £54,800 – £91,400 | – |
The jump between rooflight and dormer is mostly roof carpentry and steel. The jump from dormer to mansard is mostly the sheer amount of roofing, structural work and scaffolding time — a mansard can take several extra weeks on site.
Party wall costs most quotes don't show separately
If you share a wall with a neighbour, the Party Wall etc. Act requires you to give written notice between two months and a year before work starts — and if they don't consent, you may need a party wall surveyor (sometimes two, one for each side). This is rarely broken out as a separate line on a builder's quote, but it's a real cost that scales with how many walls you share:
- Detached house, or no shared wall affected: typically £52,000
- Semi-detached (one neighbour): typically £53,500
- Mid-terrace (two neighbours): typically £55,100
The jump isn't the building work itself — it's survey and agreement fees, and the schedule risk if a neighbour is slow to respond or disputes the notice. Start the party wall conversation as early as possible; a notice given too late is a common reason loft projects slip by months rather than weeks.
Adding a bathroom
Most loft conversions used as a bedroom end up wanting an en-suite. Budget for the difference up front rather than as an afterthought:
- No bathroom: typically £45,300
- En-suite shower room: typically £52,000
The extra cost is less about the sanitaryware and more about running new plumbing and waste up through the house, plus the electrics for an extractor fan and shaver socket — which is also why it's worth having your electrician look at the existing circuit while they're up there; see our House rewire guide if the rest of the house's wiring is older.
The hidden cost: planning permission
Most loft conversions in England are permitted development — no planning application needed — but only if you stay within fairly specific limits: the extra roof volume must not exceed 40 cubic metres on a terraced house or 50 cubic metres on other houses (including any previous roof extensions), the new roof must be no higher than the existing one, it can't extend beyond the roof slope on the side facing the street, there can be no balconies or raised platforms, and side-facing windows must be obscure-glazed and non-opening below 1.7 m. None of this applies if the house is on designated land such as a conservation area — that needs a full application regardless of size.
A hip-to-gable or mansard conversion is far more likely to brush up against these limits than a rooflight job, simply because it changes more of the roof. If your builder's drawings are close to the cubic metre limits, get that checked before you commit, since a rejected or retrospective application can add months and legal cost.
Because hip-to-gable work changes the roof's shape and area, it's also worth sense-checking the roofing portion of the quote against current material and labour rates — our New roof guide has typical costs per roof type if you want a second reference point.
Getting a fair quote
With this much variation between conversion types, the only reliable way to compare quotes is to check they're pricing the same type of conversion, the same party wall exposure and the same bathroom spec. If a quote looks unusually high or low for the type of job you're asking for, use our "is my quote fair" tool to check it against current local rates before you sign anything. For full regional pricing and worked examples, see our Loft conversion cost guide.