Loft conversion cost calculator
Loft conversions are quoted as a single number and delivered as about fifteen. Set the type, what is going up there and where you are, and see the whole bill.
Last updated July 2026 · built by working UK builders
Assumes the existing roof structure is sound and there is at least 2.2m of head height at the ridge before work starts.
A conversion only counts as a bedroom if it meets Part B fire escape rules — a “loft room” without them is not a bedroom and estate agents get this wrong constantly.
The rates behind this calculator
All figures are 2026 UK prices for a competent, insured, VAT-registered contractor. Costs are shown as ranges because building work is delivered in ranges. A “×” value multiplies the whole build cost.
What kind of conversion?
The head height you already have decides most of this for you.
| Option | Cost | Effect on total |
|---|---|---|
| Rooflight (Velux) conversion No structural change to the roof — cheapest by a distance | £26,000–£42,000 | Added |
| Rear dormer The standard UK loft conversion — adds real headroom and floor area | £44,000–£68,000 | Added |
| L-shaped dormer Extends over the back addition on a Victorian terrace | £58,000–£88,000 | Added |
| Hip-to-gable + rear dormer For semis and ends of terrace with a hipped roof | £62,000–£92,000 | Added |
| Mansard Rebuilds the whole roof — most space, most money, usually needs permission | £78,000–£125,000 | Added |
What are you putting up there?
| Option | Cost | Effect on total |
|---|---|---|
| One bedroom, no bathroom | — | ×0.9 |
| Bedroom + en-suite shower room | — | ×1 |
| Two bedrooms + bathroom | — | ×1.18 |
| Office / studio, no plumbing | — | ×0.85 |
Staircase
Regulations demand a proper fixed stair, protected route and fire doors — this is not optional.
| Option | Cost | Effect on total |
|---|---|---|
| Standard stair off the existing landing | £2,800–£5,000 | Added |
| Landing has to be reconfigured / a bedroom loses space | £5,000–£11,000 | Added |
| Bespoke or feature staircase | £9,000–£22,000 | Added |
Anything else?
| Option | Cost | Effect on total |
|---|---|---|
| Fire doors and mains-linked alarms throughout Required on any loft conversion — often quoted separately | £1,600–£3,800 | Added |
| Juliet balcony or full-height glazing | £1,800–£5,500 | Added |
| Roof terrace | £6,000–£18,000 | Added |
| Party wall surveyors (terraced or semi) Almost always needed if you are cutting into a shared wall | £1,200–£3,500 | Added |
| Boiler / hot water upgrade for the extra bathroom | £2,400–£4,500 | Added |
| Removing the old cold water tank and re-plumbing | £900–£2,600 | Added |
| Consumer unit upgrade | £700–£1,400 | Added |
| The main roof covering needs replacing at the same time | £7,000–£18,000 | Added |
Access for scaffold and materials
| Option | Cost | Effect on total |
|---|---|---|
| Easy — driveway or wide access | — | ×0.96 |
| Normal side passage | — | ×1 |
| Terraced, through the house | — | ×1.1 |
Where is the property?
Labour rates and builder demand vary more than materials do.
| Option | Cost | Effect on total |
|---|---|---|
| London | — | ×1.25 |
| South East | — | ×1.12 |
| South West | — | ×1.03 |
| East of England | — | ×1.03 |
| Midlands | — | ×0.97 |
| North of England | — | ×0.92 |
| Wales | — | ×0.92 |
| Scotland | — | ×0.95 |
| Northern Ireland | — | ×0.85 |
Applied to every total
| Item | Amount |
|---|---|
| Design, engineer & building control | 8–12% of build cost |
| Contingency | 10% of build cost |
| VAT | Included at 20% unless stated |
The five types, and which one your roof will allow
Rooflight (Velux) conversion
No change to the roof shape. New floor joists, insulation between and under the rafters, plasterboard, a staircase and rooflights. Cheapest by a distance and the least disruptive, but only works if you already have real head height and are happy with sloping walls eating into the usable floor area.
Rear dormer
A flat-roofed box built out from the rear roof slope. This is the workhorse of British loft conversions because it turns a triangular space into a room with vertical walls. Nobody sees it from the street, which is also why planners generally tolerate it.
L-shaped dormer
A rear dormer that continues over the back addition of a Victorian or Edwardian terrace. It buys a proper second room or a decent bathroom, and costs roughly a third more than a plain dormer.
Hip-to-gable
For semis and end-terraces with a hipped (sloping) side roof. The hip is rebuilt as a vertical gable wall, which creates the volume, and a rear dormer is usually added at the same time. Structurally the biggest job short of a mansard.
Mansard
The entire roof is rebuilt with near-vertical rear walls and a shallow top. It gives the most space and the most conventional room shape, and it almost always needs full planning permission. Common in London, rare elsewhere, expensive everywhere.
What the quotes usually leave out
- Fire doors and alarms throughout the house. Not just the loft — the whole escape route. Budget £1,600–£3,800.
- Party wall surveyors. Terraced and semi-detached conversions cut into a shared wall to seat steels. Notice is a legal requirement; a surveyor each side is common.
- Hot water. Adding a shower to a combi that was already marginal will make the whole house worse. Price the boiler or cylinder upgrade at the same time.
- Redecorating the floor below. Steels get craned or carried through bedrooms. Ceilings crack. It is normal, and it is not usually in the quote.
- The roof itself. If the covering is near the end of its life, do it while the scaffold is up. Doing it separately later means paying for scaffold twice.
Is a loft conversion worth it?
A properly signed-off loft bedroom with an en-suite is one of the more reliable value-adding projects in UK housing, particularly where it takes a house from two bedrooms to three or three to four. But value is added by the certified bedroom, not the space. Skipping building control to save a few thousand can cost far more at sale, when a buyer’s solicitor asks for the completion certificate and does not get one.
Frequently asked questions
How much does a loft conversion cost in 2026?
A rooflight conversion starts around £28,000. A rear dormer with an en-suite — the standard UK conversion — typically runs £55,000 to £85,000 all-in. A mansard on a London terrace can pass £150,000. Region, staircase complexity and the bathroom are the three biggest swing factors.
Do I have enough head height for a loft conversion?
Measure from the top of the ceiling joists to the underside of the ridge. Below 2.2m and you will need to alter the roof or drop the ceilings below, which changes the economics completely. Between 2.2m and 2.4m a dormer usually works. Above 2.4m you may get away with rooflights only. Modern trussed roofs need the trusses cutting and steel installing, which adds cost whatever the height.
Does a loft conversion need planning permission?
Rear dormers on houses are often permitted development, subject to volume limits of 40m³ for terraced and 50m³ for detached and semi-detached homes, and no dormer forward of the roof plane facing a highway. Flats have no permitted development rights at all. Conservation areas and Article 4 directions remove them too. Check with our planning permission checker.
Why does the staircase cost so much?
Because it is rarely just a staircase. Building regulations demand a fixed stair with adequate headroom, a protected escape route to the front door, fire doors on every room off that route and mains-linked alarms. Getting the stair to land somewhere legal often means losing part of a bedroom or rebuilding the landing.
Will the conversion count as a bedroom?
Only if it complies with Part B fire safety and Part K stair rules and was signed off by building control. Rooms sold as "loft rooms" with a fixed ladder or a non-compliant stair do not count, and a buyer’s solicitor will ask for the completion certificate. If you are buying a house with a converted loft, ask for that certificate before you exchange.