How the loft conversion cost estimate works
Loft conversion prices are usually quoted per m² of new floor area, and vary mostly with how much of the roof is rebuilt. The calculator multiplies your floor area by a low and high rate for your conversion type, adjusts for region and finish, then adds the extras most quotes leave out:
Typical build rates (outside London, standard finish)
| Conversion | £ per m² | ~30m² loft | Planning? |
|---|---|---|---|
| Rooflight / Velux | £700–£1,000 | £21k–£30k | Usually not needed |
| Rear dormer | £1,350–£2,000 | £40k–£60k | Often permitted development |
| Hip-to-gable | £1,600–£2,300 | £48k–£69k | Often needed |
| Mansard | £1,950–£2,800 | £58k–£84k | Almost always |
Rates are based on published 2026 UK cost guides and trade price surveys, which vary between sources, so treat them as a starting point. London typically adds 10–25%. Quotes often exclude VAT, bathrooms and decoration.
Fees and extras to budget for
- Design / drawings: about £2,000–£4,500.
- Structural engineer: about £800–£1,500 for calculations for steels and joists.
- Building control: about £800–£1,200. Every loft conversion needs building regulations approval, including fire-safe escape routes.
- Party wall agreements: about £1,000–£2,000 per neighbour, if your home is semi-detached or terraced.
- Planning application: about £300–£600 including drawings, if needed.
This is a budgeting tool, not a quote. Prices depend on your roof structure, access, stairs position and local labour costs. Always get at least three written quotes from experienced loft conversion specialists.
Worked example: a 30m² rear dormer on a semi
A rear dormer adding 30m² of floor, standard finish, outside London and the South East:
- Build: 30m² × £1,350–£2,000 = £40,500–£60,000.
- En-suite shower room: £6,000–£10,000.
- Fees and party wall: drawings, structural engineer, building control and one party wall agreement add £4,600–£9,200.
- Contingency: 10% of build and en-suite, £4,650–£7,000.
- VAT at 20% brings it to about £67,000–£103,000, a mid-point of around £85,000.
A rooflight conversion of the same size comes to roughly £41,000–£64,000, if the loft already has the head height. Switch the type above to compare.
Is your loft suitable?
- Head height: measure from the top of a ceiling joist to the underside of the ridge. Around 2.2m or more makes most types possible.
- Roof structure: older houses usually have traditional cut rafters with open space. Many homes built since the 1960s have trussed rafters (W-shaped timbers) that need more structural work, and so more cost, to remove.
- Stairs: a new staircase needs enough headroom and space on the floor below. Its position often decides the layout of the whole conversion.
- What's in the loft now: water tanks, flues and chimney breasts may need moving or removing.
Fire safety and Building Regulations
Turning a loft into a room makes the house three storeys, which brings stricter fire safety rules. Typically that means a protected escape route down the stairs, fire-resisting doors onto it, and mains-powered, interlinked smoke alarms. Building control will set the exact requirements for your house, and they affect the cost, so raise them early with your designer. Insulation, structure, stairs and ventilation are all covered by the regulations too.
Common mistakes
- Comparing quotes that cover different things. One may include the en-suite, decorating and VAT, another not. List exactly what each covers.
- Forgetting the party wall. On a semi or terrace, notices must be served before work starts. Allow time and money for it.
- No contingency. Roofs hide surprises, such as rotten timbers or old wiring.
Compare with building out using the extension cost calculator. If you're keeping the loft for storage, top up the insulation with the loft insulation calculator. More in project cost calculators.