Loft Extension Cost Calculator
Enter your loft conversion type, floor area, specification, and UK region — get an instant, detailed cost estimate including structure, interior fit-out, windows, stairs, and all associated fees.
Calculate your loft conversion cost
Fill in your loft details below for an instant cost estimate — including structural work, dormer or roof alterations, interior fit-out, Velux windows, staircase, and all professional fees.
Your loft conversion details
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Your Cost Estimate
Full breakdown including all works & fees
Fill in your loft details and click Calculate to get your personalised cost estimate — including a full breakdown of structural work, interior fit-out, windows, staircase, and professional fees.
Which loft conversion is right for you?
Different conversion types suit different properties, budgets, and planning situations. Here’s a guide to the five main options available in the UK.
Velux / Roof light
The simplest and most affordable option. Velux windows are installed into the existing roof slope without altering the roof structure. Ideal if your loft already has sufficient head height (min. 2.2m at ridge). Usually within permitted development.
£15,000–£25,000Dormer loft
The UK’s most popular loft conversion. A box-shaped extension projects vertically from the existing roof slope, dramatically increasing headroom and floor area. Rear dormers are usually permitted development. Can accommodate a bedroom and en-suite.
£35,000–£55,000Hip-to-gable
Ideal for semi-detached or detached homes with a hipped roof. The sloping hip end is converted into a vertical gable wall, substantially increasing usable floor area. Often combined with a rear dormer for maximum space. Almost always requires planning permission.
£40,000–£65,000Mansard loft
The most complex and expensive option — the entire roof structure is rebuilt with near-vertical (72°) sides and a flat top. Maximises usable space and is common in Victorian terraces in London. Almost always requires full planning permission.
£55,000–£90,000Modular / prefab pod
A factory-built modular unit craned onto the roof structure. Faster to install than traditional methods (typically 1–2 weeks on site vs 8–14 weeks). Growing in popularity for terraced houses. Quality varies — choose an established supplier with strong references.
£25,000–£45,000Bungalow conversion
Converting a bungalow loft is often the most transformative project — effectively adding an entire upper storey. Typically combines elements of hip-to-gable and dormer work. Requires structural engineer input, full planning, and a robust party wall agreement if semi-detached.
£50,000–£100,000+Loft conversion costs by type & size
Typical 2026 UK costs for a fully completed loft conversion including structural work, interior fit-out, 1 en-suite, staircase, and all professional fees. Prices are for Midlands/North England — add 15–30% for London and South East.
| Conversion type | Typical size | Low estimate | Mid estimate | High estimate | Duration |
|---|---|---|---|---|---|
| 🪟 Velux / Roof light | 15–30 m² | £15,000 | £20,000 | £28,000 | 4–6 weeks |
| 🏠 Rear dormer | 25–40 m² | £35,000 | £45,000 | £58,000 | 8–12 weeks |
| 🏘️ Hip-to-gable | 30–50 m² | £40,000 | £52,000 | £68,000 | 10–14 weeks |
| 🏛️ Mansard | 35–55 m² | £55,000 | £72,000 | £95,000 | 12–18 weeks |
| 📦 Modular / pod | 20–35 m² | £28,000 | £38,000 | £50,000 | 5–8 weeks |
How to save money on your loft conversion
Follow these tips from experienced builders and homeowners to keep costs under control and avoid costly mistakes.
Get 3 detailed quotes
Always obtain at least three itemised quotes from specialist loft conversion companies — not general builders. Prices can vary by 40–60% for the same specification. Ensure every quote covers the same scope of works so you can compare like-for-like.
Check your head height first
Measure from the top of the ceiling joist to the underside of the ridge. You need at least 2.2–2.3m for a habitable loft. If you’re below this, a Velux conversion won’t work — you’ll need a dormer or mansard, which costs significantly more. Know this before getting quotes.
Check permitted development rights
Many loft conversions — particularly rear dormers on terraced and semi-detached houses — fall within permitted development and don’t require a planning application. Saving the £2,000–£3,000 planning fee is straightforward if your project qualifies. Check with your local planning authority or via the Planning Portal first.
Plan well in advance
The best loft conversion specialists are booked 3–6 months in advance. Rushing or accepting the first available contractor leads to higher prices and potentially lower quality. Start getting quotes 4–6 months before your target start date, especially if you want summer works when dry weather speeds up the roof-open stage.
Cost estimates you can trust
Our loft conversion cost calculator uses 2026 UK trade pricing data sourced from specialist loft conversion companies, RICS building cost surveys, and contractor rate cards — adjusted for your conversion type, size, finish level, and regional location.
We break costs down into their constituent parts — structural works, interior fit-out, windows, staircases, and professional fees — so you can see exactly where your money goes and challenge any quote that seems off.
- ✓2026 UK labour and material rates built in
- ✓5 conversion types with individual cost models
- ✓Regional price variation across all UK nations
- ✓Finish level (standard / premium / luxury) factored in
- ✓Professional fees, planning & structural costs included
- ✓No ads, no sign-up, no data stored — runs in your browser
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