A loft conversion in 2026 typically costs £22,000–£45,000 for a straightforward Velux or rooflight conversion that just opens up the existing roof space; £30,000–£65,000 for a dormer, the most common route on an ordinary semi or terrace; £42,000–£68,000 for a hip-to-gable conversion on a house with a sloped roof end; and £55,000–£75,000 or more for a mansard, which rebuilds most of the roof and gives the most usable space of any option. Most homes land somewhere in the £35,000–£60,000 range once stairs, an en-suite and a reasonable finish are included.
But those bands only tell you what a given type costs — not which type your house actually needs, which is the more useful question. A roof with generous headroom and a steep pitch might do everything you want with rooflights alone; a lower, shallower roof might need a full mansard just to clear building regulations, whatever the budget says. Getting that judgement wrong is how people end up pricing a £30,000 job that was never going to give them a usable room.
I'm a building surveyor, not a loft conversion contractor, so what follows is 2026 pricing cross-checked against current UK cost guides, what actually drives the number, and the headroom, structural, planning and party wall checks I'd want settled before anyone commits to a type or signs a contract.
The short answer
Figures below were checked on 10 August 2026 against 2026 cost guides from Checkatrade, HomeOwners Alliance and specialist loft conversion guides, cross-referenced against each other. As with most trades in this series, London and the South East — Dom’s patch included — commonly run 20–30% above the national figures quoted here, more than most trades because of the added scaffolding, party wall and structural costs that come with denser housing.
| Type | Typical cost (2026) | What's included |
|---|---|---|
| Velux / rooflight | £22,000–£45,000 | Rooflights let into the existing roof slope, no structural alteration to the roof shape, cheapest and fastest route where headroom already works |
| Dormer (single or double) | £30,000–£65,000 | Box-shaped extension built into the roof slope, adds genuine headroom and floor area, the most common route on a semi or terrace |
| Hip-to-gable | £42,000–£68,000 | Sloped hip end squared off into a vertical gable wall, common on 1930s–60s semis and detached houses, often combined with a rear dormer |
| Mansard | £55,000–£75,000+ | Most of the roof slope rebuilt near-vertical, the most floor area and headroom of any option, also the most disruptive and expensive |
Per square metre, that works out at roughly £1,250–£2,500/m² for the build itself, nearer £1,500–£3,000/m² once stairs, a bathroom and decoration are folded in — a wider range than most trades in this series, because so much depends on whether the roof needs structural alteration at all or just insulating and lining.
What drives the price
Size and floor area. A larger loft plate means more roof to alter, more floor to insulate and board, and usually a bigger staircase run below — the per-m² rate holds roughly steady, but fixed costs (staircase, structural design, building control) don't scale down much for a small loft, so tiny conversions rarely look proportionally cheap.
Structural work. Almost every loft conversion needs new floor joists, because the ceiling joists already up there were sized to carry a ceiling, not a floor and furniture, and dormer or hip-to-gable conversions usually need a steel beam to carry the new roof structure where a wall or chimney breast is altered. That's a job for a structural engineer, not a guess — I've covered what that costs separately in my structural engineer cost guide.
Staircase. A new fixed staircase, typically £1,500–£4,500 depending on design and how much of the first floor it needs to eat into, is one of the more disruptive parts of the job because it usually reduces a bedroom or landing below — and it's a genuine building regulations requirement, not an optional extra.
Plumbing and an en-suite. Adding a bathroom pushes the price up by roughly £5,000–£10,000 depending on how far the new fittings are from existing soil pipe runs, and whether a pump is needed to get waste down to the stack.
Roof type and pitch. A steep, deep roof needs less structural alteration to gain useable headroom than a shallow, modern truss roof, which sometimes needs a full re-roof as part of the job — a cost a quote based on a neighbour's similar-looking house can miss entirely.
Finish level. The bands above assume a decent standard finish. A high-end finish — bespoke joinery, underfloor heating, a fitted bathroom suite rather than a budget one — can add 20–40% on top without changing the structural scope.
Region. London and the South East commonly run 20–30% above the national figures quoted here, more than most trades because denser housing means more scaffolding, more party wall involvement, and less room to store materials on site.
Headroom and structural feasibility — check this before you commit
The single most useful thing to establish before pricing anything is how much usable headroom the existing roof actually gives you, and how much of the floor plate clears it. As a rough working figure, a loft room needs something in the region of 2.2–2.3 metres of headroom over at least half its floor area to read as a proper habitable room rather than a cramped one — and that threshold matters for more than comfort. A room that falls short of it is routinely valued by surveyors and mortgage valuers as a box room rather than a genuine bedroom, which caps what the conversion actually adds to the property regardless of what was spent building it.
That headroom figure, measured against your ridge height and roof pitch, is what actually decides which type of conversion you need — not preference or what a neighbour did. A steep, deep Victorian or Edwardian roof might clear the threshold with rooflights alone. A shallower 1960s–80s truss roof commonly needs a dormer just to get there, and a low, wide bungalow sometimes needs a full mansard or hip-to-gable-plus-dormer combination before the numbers work at all. Pricing a Velux conversion on a roof that will never clear building regulations headroom is a common way to waste a design fee.
The second check is structural, and it's one a lot of homeowners underestimate: the ceiling joists visible from below were sized to carry a ceiling and some loose storage, not a floor, a bed and a wardrobe. Almost every conversion needs new floor joists on new or strengthened support, and where a dormer, hip-to-gable or mansard alters the roof structure, a steel beam designed by a structural engineer is usually needed to carry the load the roof timbers used to. Get that scoped and costed before you commit to a headline figure from a contractor's website — it's routinely what turns a quote into a much larger invoice partway through the job.
On value: research from Nationwide puts the average uplift at around 20–25% of a property's value where the conversion adds a genuine extra bedroom and bathroom, rising toward 24% in London. That's a real number worth having in mind against a £40,000–£60,000 spend — but it assumes the room clears the headroom threshold above. Convert a loft that will only ever pass as a box room and you'll have spent proper-conversion money for box-room value.
Planning permission, Building Regs and party wall — the paperwork before you commit
Most loft conversions on a house (flats and maisonettes have no permitted development rights here and always need planning permission) can proceed without a planning application, provided the added roof volume stays within 40 cubic metres for a terrace or 50 cubic metres for a semi-detached or detached house. That allowance is cumulative for the life of the building — a previous rear extension or dormer eats into what's left — and hip-to-gable conversions count toward it too. Design conditions apply too: materials broadly matching the house, no verandas or balconies, side windows obscure-glazed, and set back from the original eaves. A conservation area or an Article 4 direction can remove permitted development rights entirely, in which case a full application is needed regardless of size.
Whichever route gets you there, Building Regulations approval is always required — permitted development covers planning, not building control. Building control has to sign off structural stability (the joists and any steel, properly calculated); fire safety, typically a protected escape route in 30-minute fire-resistant construction, upgraded FD30S fire doors on the route including existing first-floor doors, mains-linked smoke alarms, and usually an escape window; insulation; and the staircase, which gets a modest headroom relaxation for loft stairs where the roof genuinely won't allow the usual 2 metres. Skipping this sign-off is a common finding on survey: an unregistered loft conversion routinely holds up a sale, because a buyer's solicitor will ask for the completion certificate and a lender may insist it's regularised first.
On a semi or terrace, a loft conversion very often triggers the Party Wall Act, because the work touches the shared wall, roof structure or a party chimney — a notice has to go to each neighbour, and if they don't consent, a party wall surveyor is needed to agree the works, typically £700–£2,000 per neighbouring property. On a mid-terrace with two neighbours, that alone can add £1,400–£4,000 to the true cost of the project before a brick is touched, and it's worth building into the budget from the start rather than discovering it once notices are served.
What I'd watch for in a quote
- Structural engineer's calculations and steel priced in, not treated as a variation once the design is finalised — this is one of the more common causes of a mid-project cost increase.
- Staircase design and any relocated services (a water tank, boiler flue or soil stack that needs moving) itemised, not glossed over.
- Fire door and alarm upgrades to the rest of the house included, not just the new room — building control will expect the whole escape route addressed.
- Party wall costs budgeted separately, since they're a genuine cost of the project even though the builder doesn't collect them.
- Building control fees and route stated — full plans or building notice — and confirmation a completion certificate will be provided at the end.
Questions I get asked
How much does a loft conversion cost for a 3-bed semi in 2026? A single dormer on an ordinary 3-bed semi typically costs £35,000–£50,000 in 2026, supplied and fitted, giving a proper double bedroom and en-suite. A basic Velux/rooflight conversion on the same house can come in at £25,000–£35,000 if headroom already works, while hip-to-gable — common on 1930s semis with a sloped roof end — typically runs £45,000–£60,000. London and the South East commonly trend 20–30% above these figures.
Do I need planning permission for a loft conversion? Often not, provided the added roof volume stays within permitted development limits — 40 cubic metres for a terraced house, 50 cubic metres for a semi-detached or detached house — and the design meets conditions on materials, set-back from the eaves and not exceeding the height of the existing roof. That allowance is cumulative over the life of the building, so a previous rear dormer or extension eats into it. Flats and maisonettes have no permitted development rights for loft conversions and always need planning permission, and a conservation area or an Article 4 direction can remove the rights entirely.
Does a loft conversion still need building regulations approval under permitted development? Yes, always, regardless of whether planning permission was needed. Building control has to sign off structural stability (new floor joists or a steel beam, properly designed), fire safety (a protected escape route, upgraded fire doors, mains-linked smoke alarms and usually an escape window), insulation, and the staircase. Skipping this is a common finding on survey — an unregistered loft conversion routinely holds up a sale, because a buyer's solicitor will ask for the completion certificate and a lender may want it regularised before releasing funds.
How much value does a loft conversion add? Research from Nationwide puts the average uplift at around 20–25% of a property's value where the conversion adds a genuine bedroom and bathroom, rising toward 24% in London. That figure assumes a proper room, though — a loft space that doesn't clear roughly 2.3 metres of headroom over at least half its floor area is often valued by surveyors as a box room rather than a real bedroom, which caps what it actually adds regardless of what was spent building it.
If you're weighing up a loft conversion and want an honest read on headroom and roof structure before you commission drawings, that's exactly what my building survey work is for. Call 07946 618203 or get in touch for a straight answer on feasibility before you pay a contractor to tell you the same thing.
Small print. This guide is general information, not advice on a specific property, and it doesn’t replace a proper inspection and quote for your own home. Prices were researched and correct to the best of my knowledge on 10 August 2026; costs move and vary by region and property. Always get at least three written quotes.