A loft conversion is one of the best-value ways to add space to a house. You're building on top of structure that already exists — no foundations, no groundworks, no loss of garden — and it typically adds 10–15% to property value. But the costs vary dramatically depending on the type of conversion and what's already up there.
Loft Conversion Costs at a Glance
| Conversion Type | Typical Cost (excl. VAT) |
|---|---|
| Velux / Rooflight | £22,000 – £35,000 |
| Rear Dormer | £35,000 – £55,000 |
| L-Shaped Dormer | £45,000 – £65,000 |
| Hip-to-Gable | £45,000 – £60,000 |
| Hip-to-Gable + Rear Dormer | £50,000 – £70,000 |
| Mansard | £55,000 – £75,000+ |
Velux / Rooflight Conversion
The simplest and cheapest option. No changes to the roof structure — you're adding roof windows into the existing slope. Works well when you already have enough headroom (2.2m minimum at the ridge).
The build is relatively straightforward: strengthen the floor joists, install a staircase, add windows, insulate, and finish. Most Velux conversions complete in 4–6 weeks.
The main limitation is space. With sloping walls on both sides, usable floor area is restricted. You'll get a good bedroom but probably not enough headroom for a large en-suite along the eaves.
Rear Dormer Conversion
The most popular choice in the UK. A dormer extends the rear roof vertically, giving you full headroom across most of the floor area. You get a flat-ceiling room rather than sloped walls — significantly more usable space than a Velux conversion.
The structural work involves removing the rear roof slope, building a new flat-roof dormer box with vertical walls, and supporting everything with new steel beams. A typical rear dormer adds 15–25m² of usable space.
Most rear dormers fall under Permitted Development, meaning you won't need planning permission — but always check with your local authority first, especially in conservation areas.
Hip-to-Gable Conversion
If your house has a hipped (sloped) roof on the side, a hip-to-gable conversion extends it to a vertical gable wall — reclaiming the space lost to the slope. It's commonly combined with a rear dormer for maximum floor area.
The structural work is more involved — you're rebuilding one side of the roof entirely. This typically requires steel beams at ridge and purlin level, new brickwork or blockwork for the gable wall, and new roof covering to match the existing.
Mansard Conversion
The most expensive but most space-efficient option. A mansard replaces one or both roof slopes with near-vertical walls (at 72 degrees typically) and a flat top. It's essentially a new storey rather than a converted loft.
Mansard conversions almost always require full planning permission because they significantly alter the roofline. They're most common in London and dense urban areas where every square metre counts. The structural work is substantial — expect a build time of 10–14 weeks.
Costs Most People Forget
Staircase — the new staircase needs to comply with Building Regulations, which means minimum headroom, maximum pitch, and specific tread dimensions. If your existing landing doesn't have space, you may need to steal a box room or adjust the first-floor layout. Budget £2,500 to £5,000 for the staircase including any structural adjustments.
Fire safety — loft conversions create a three-storey dwelling, which triggers additional fire safety requirements. You'll need fire doors on all habitable rooms, an interlinked smoke/heat alarm system, and potentially a protected escape route. This adds £1,500 to £3,000.
Party wall — if your loft conversion involves work on or near a shared wall (semi-detached or terraced), you'll need a party wall agreement. Budget £1,000 to £2,500.
Scaffolding — required for any external work. A full scaffold on a semi-detached house for 8–12 weeks costs £1,500 to £3,000.
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