Typical UK cost breakdown
Indicative UK ranges for a completed, building-control-signed conversion including insulation, plastering and basic decoration. Fittings and furniture are extra.
Indicative figures · reviewed August 2026
Published UK conversion cost ranges plus Estimo modelling by conversion type. Figures are indicative guidance, not a quotation, and actual quotes from tradespeople vary by job, access and specification.
| Cost element | Typical range | What it depends on |
|---|---|---|
| Velux / rooflight conversion | £22,000 – £35,000 | No change to the roof shape. Cheapest option, but only works where head height is already sufficient. |
| Rear dormer conversion | £35,000 – £55,000 | The most common UK conversion. Adds usable floor area and full head height across the room. |
| Hip-to-gable (plus rear dormer) | £45,000 – £65,000 | For semis and end-terraces with a hipped roof. Rebuilds the side roof slope into a vertical gable. |
| Mansard conversion | £55,000 – £80,000 | Largest gain in space, biggest structural works, and almost always needs planning permission. |
| En-suite shower room | £4,000 – £9,000 | Plumbing runs, waste, extraction and tiling. Often quoted as an optional extra. |
| Structural steels and calculations | £2,000 – £6,000 | Beams to carry the new floor, plus a structural engineer's design fee. |
| Planning, building control and party wall | £1,500 – £4,000 | Application fees, plans, inspections and a party wall surveyor for terraces and semis. |
Do you need planning permission?
- Many rear dormer and rooflight conversions fall under permitted development, subject to volume limits and no changes to the front roof slope.
- Mansards, front-facing dormers, flats, listed buildings and conservation areas almost always need a full application.
- Building control approval is always required, permitted development or not. Keep the completion certificate — buyers' solicitors will ask for it.
What makes the price move
Head height in the existing loft
You need roughly 2.2–2.4m from the joists to the ridge. Less than that means raising the roof or lowering ceilings below — both add tens of thousands.
Roof structure
A traditional cut roof converts easily. A modern trussed roof has to be structurally rebuilt, which is a major cost jump.
Where the stairs go
Fitting a compliant staircase often means reworking a bedroom or landing below. The knock-on redecoration is a real cost, not a detail.
Escape routes and fire regulations
A new storey usually triggers fire doors throughout, mains-linked alarms and a protected stairwell — work that happens on the floors below.
Finish level
Bathroom specification, joinery, glazing and built-in storage can swing the final bill by £10,000 or more on the same shell.
Region
London and the South East commonly run 20–35% above the national average for the same conversion type.
Hidden costs people miss
- Upgrading doors and alarms on the floors below to meet fire regulations.
- Party wall awards for terraced and semi-detached homes — typically £700–£2,000 per neighbour.
- Scaffolding, sometimes charged weekly and easy to underestimate if the build overruns.
- Redecorating the landing and rooms disturbed by the new staircase.
- Boiler or heating upgrades if the existing system cannot serve extra radiators and an en-suite.
- Building control completion certificate — without it, the extra bedroom may not count when you sell.
How to check a quote for this job
- Does the quote name the conversion type (Velux, dormer, hip-to-gable, mansard) and the finished floor area?
- Are structural engineer fees, steels and building control inspections included or excluded?
- Is the staircase — and making good the room it comes out of — inside the price?
- Are fire safety works to the floors below itemised?
- Is scaffolding included for the whole programme, or charged per week?
- Is there a written programme of works with a start and finish date?
- What is the payment schedule, and is it tied to stages completed rather than dates?
- Is VAT included, and does the builder hold public liability insurance?
Not sure if your quote is fair?
Upload the quote or describe the job in plain English. Estimo gives you an indicative price range, flags missing line items and explains what's driving the total.
Estimo gives indicative, AI-generated guidance. Prices vary by region, specification and date — always confirm with a qualified trade before committing.
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