Loft conversion costs range from £20,000 to £70,000 depending on the type, size, and specification. A basic Velux conversion costs £20,000 to £30,000. A rear dormer costs £35,000 to £50,000. An L-shaped dormer costs £45,000 to £60,000. A mansard conversion costs £55,000 to £70,000 or more. London and the South East are typically 20% to 30% more expensive than other regions.

A Velux or rooflight conversion is the cheapest option at £20,000 to £30,000. It involves adding windows to the existing roof slope without changing the roof structure. However, it only works if your loft already has enough headroom. Most lofts need at least 2.2 metres from floor joist to ridge to make a Velux conversion practical. If headroom is limited, you'll need a dormer which costs more.

A rear dormer conversion typically costs £35,000 to £50,000. This adds a box-shaped extension to the rear roof slope, creating full-height floor space with vertical walls. An L-shaped dormer wrapping around two sides costs £45,000 to £60,000. Dormer size, window choices, and bathroom inclusion all affect the final price. Dormers offer the best balance of cost and usable space for most homes.

Mansard conversions cost £55,000 to £70,000 or more. A mansard replaces one or both roof slopes with a near-vertical wall and flat top, maximising internal space. They create the most headroom but require the most structural work. Mansards almost always need planning permission due to the significant change in roof profile. They're most common on Victorian terraces in London.

The main factors are conversion type, loft size, structural complexity, and specification level. Adding a bathroom increases costs by £5,000 to £10,000. Steel beams for structural support add £1,500 to £3,000. High-end finishes, underfloor heating, and premium windows push costs higher. Difficult access, chimney removal, and roof condition issues can all add unexpected costs.

A typical loft conversion quote includes structural work, stairs, floor strengthening, insulation, plastering, electrics, windows, building regulations fees, and basic decoration. It usually excludes bathroom fittings beyond basic suite, flooring, fitted furniture, any planning permission fees, and decoration to the rest of the house affected by the stairs. Always clarify inclusions before comparing quotes.

Most loft conversions fall under permitted development and don't need planning permission if they stay within volume limits. Terraced houses can add up to 40 cubic metres. Detached and semi-detached houses can add up to 50 cubic metres. However, you'll need planning permission if you extend beyond the existing roof slope facing a highway, raise the overall roof height, or live in a conservation area.

Common hidden costs include building regulations fees (£500 to £1,000), structural engineer calculations (£400 to £800), architectural drawings if not included (£1,000 to £2,500), Party Wall agreements if applicable (£700 to £1,500 per neighbour), scaffolding (often £1,500 to £3,000), and skip hire. Budget an extra 10% to 15% contingency for unexpected issues like roof repairs or asbestos removal.

A loft conversion typically adds 15% to 20% to your property value. On a £400,000 home, that's £60,000 to £80,000 of added value against a cost of £40,000 to £55,000. Adding a bedroom with en-suite adds more value than a bedroom alone. Quality of finish matters significantly. However, returns vary by area, so check comparable sold prices for converted properties on your street before committing.

Choose a Velux conversion if headroom allows, as it avoids expensive structural changes. Keep the bathroom directly above the existing bathroom to simplify plumbing. Use standard window sizes rather than custom. Avoid moving the boiler or water tank if possible. Get three to five quotes and compare like for like. Consider managing decoration yourself after the main build. Phase the fit-out if budget is tight.