Better HomesFree calculator · Updated for 2026
Loft conversion cost calculator
Price a Velux, dormer, L-shaped, hip-to-gable or mansard loft in under a minute. You get the build cost, every fee, a contingency and VAT, plus how long the work takes.
- ✓ All five loft types
- ✓ London zones and UK regions
- ✓ Free PDF and shareable link
Which type of loft?
2026 benchmarks
Loft conversion costs in London, 2026
Typical all-in budgets for an outer-London terrace or semi at a standard finish, including drawings, engineer, building control, party wall for one neighbour, a 10% contingency and VAT.
| Loft type | Typical new floor area | Likely all-in budget | Time on site |
|---|---|---|---|
| Velux (rooflight) | 22 m² | £43,500 – £56,000 | 6–8 weeks |
| Rear dormer | 25 m² | £70,500 – £91,000 | 8–10 weeks |
| L-shaped dormer | 32 m² | £83,500 – £108,000 | 10–12 weeks |
| Hip-to-gable | 30 m² | £81,000 – £105,000 | 9–11 weeks |
| Mansard | 32 m² | £102,000 – £132,000 | 12–16 weeks |
Build-only prices you see quoted elsewhere are usually 20–30% lower because they leave out fees and contingency.
Which loft is right for your house?
- Velux works when you already have good head height. It’s the cheapest option and rarely needs planning.
- Rear dormer is the London standard on terraces and semis: a full-height room across the back of the roof.
- L-shaped dormer adds a second dormer over the back addition of a Victorian terrace, often making space for two bedrooms or a bedroom and en-suite.
- Hip-to-gable squares off a sloping side roof on semis and end-of-terraces, usually with a rear dormer.
- Mansard rebuilds the roof with near-vertical sides for the most space. It is common in conservation areas but almost always needs planning.
Is my loft suitable?
Measure from the top of a ceiling joist to the underside of the ridge. Around 2.2 m or more is usually enough for a comfortable conversion. Less than that can still work by lowering ceilings below or choosing a mansard, but it adds cost. Also check where the new staircase can land on the floor below. That is often the deciding factor in a terrace.
Planning and permitted development
Most dormer, hip-to-gable and Velux lofts on houses are permitted development, within 40 m³ of extra volume on terraces and 50 m³ on semis and detached houses. Flats, conservation areas and mansards need planning. Either way, you need building regulations approval and, if you share a wall, a party wall agreement. Read more in our loft planning guide.
FAQs
Questions people ask
How much does a loft conversion cost in London in 2026?
Build costs including VAT are typically £25,000–£45,000 for a Velux loft, £45,000–£75,000 for a rear dormer, £55,000–£85,000 for an L-shaped dormer or hip-to-gable and £60,000–£100,000+ for a mansard. Add drawings, engineer, building control, party wall and a contingency for the full budget.
Can you do a loft conversion for £30,000?
In London, only a simple Velux conversion with good existing head height is likely to come in around £30,000. A dormer, which most London lofts need, usually starts nearer £50,000 all-in.
Does a loft conversion add value?
A well-designed loft that adds a bedroom and bathroom typically adds 15–25% to the value of a London house, which often exceeds the cost of the work.
How long does a loft conversion take?
Most London lofts take 8–12 weeks on site; mansards take 12–16. Allow another 6–12 weeks beforehand for drawings, structural design, party wall notices and approvals.
Do I need to move out?
Usually not. Most of the work happens from scaffolding outside until the staircase goes in, which typically means one or two disruptive weeks near the end.
Can I put this calculator on my website?
Yes. Use the 'Add this calculator to your website' link next to the estimate to copy the embed code. It's free and always uses our latest prices.
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