How much does a loft conversion actually cost?
Loft conversion costs vary hugely depending on the type of conversion and how much of the roof structure changes. A simple loft conversion inside your existing roof space costs far less than one needing a hip-to-gable extension or a rear dormer, because those add new roof structure rather than just fitting out an existing void. Costs have also risen sharply since the pandemic — a hip-to-gable conversion that might have been priced at around £50,000 a few years ago is a very different number today.
Permitted Development usually gets you the most usable loft space and the largest dormer size you can build without applying for full planning permission — for a semi-detached or detached house that’s up to 50 cubic metres of added roof volume, or 40 cubic metres for a terraced house. Go through full planning instead, and your council can apply stricter limits than the PD default.
To show you exactly what today’s money buys, we’ve broken down a real Fluent project: a hip-to-gable and rear dormer loft conversion on a typical 3-bed semi-detached house, completed in 2026. The total build cost for the loft-related works came to **£96,125 excluding VAT**. Here’s exactly where that money went.
Why this quote is split into three zones
Most loft conversion quotes aren’t priced as one lump sum. When the roof needs more than one type of structural change, a builder prices each zone separately, because each one involves different materials and different labour. On this project, the roof broke down into:
The hip-to-gable extension — the side of the roof, where a sloped hip end was rebuilt as a full vertical gable wall
The rear dormer — a box-shaped structure added to the back of the roof, with its own walls, flat roof, and waterproofing
The loft conversion itself — the middle section of roof, running from the front of the house back to where the dormer starts, converted within the existing roofline
Most projects only need one or two of these zones, not all three. Treat this as an upper-end example of what a bigger, more involved conversion costs, not a like-for-like comparison to a simpler job.
The real cost breakdown, zone by zone
Zone – Cost (ex VAT)
- Rear dormer – £33,001
- Loft conversion (front-of-house to start of dormer) – £23,123
- Hip-to-gable (side of roof) – £14,559
- Fit-out for the two new loft bedrooms (plastering, paint, doors, windows, flooring) – £10,632
- Shower room fit-out – £10,499
- Landing and staircase – £3,783
- Materials lift hire – £528
Total – £96,125
Three numbers jump out. The rear dormer alone cost more than the hip-to-gable and landing combined, because a dormer adds a full new external wall, roof structure, and waterproofing detail, whereas a hip-to-gable roof largely reuses the existing pitch. The middle zone — the loft conversion section between the front of the house and the start of the dormer — cost £23,123 on its own, covering floor joists, steelwork, insulation, and plasterboard for that portion of the roof. And fitting out the two new loft bedrooms plus the shower room added a further £21,131 combined, covering the finishes — plastering, painting, flooring, doors, windows, radiators and electrics — once the shell was built.
There’s no work to the existing first-floor bedrooms in this project — the whole build stays within the roof and loft space. The cost document prices one Velux rooflight, lighting one of the new bedrooms
What’s inside the £23,123 loft conversion zone
This is the part most homeowners underestimate, and it’s where quote comparisons go wrong. This zone — the section of roof from the front of the house to where the rear dormer begins — included:
– Steel beams and columns: £4,033, plus intumescent fire paint at £667
– Floor structure, insulation and floorboard – £2,176
– Internal stud walls, insulation and plasterboard: £3,043
– External wall insulation and plasterboard: £3,790
– Ceiling structure including fire-rated plasterboard: £2,433
– Preliminaries (skip hire, acrow propping): £2,242
– Electrics, fuseboard and certification: £1,114
None of this is visible once the room is finished and decorated. If a quote you’re comparing doesn’t itemise steel, insulation, and fire-rated plasterboard separately, ask where those costs have gone — they haven’t disappeared, they’ve been folded into a number you can’t check.
What’s inside the £33,001 rear dormer zone
The rear dormer is the single biggest line item, and it breaks down like this. The dormer itself is built primarily in timber — stud walls, timber roof structure, timber floor — with steel used only where structural support is needed to carry the load. Under Permitted Development, the external finish has to be similar in appearance to the existing house, so the dormer walls are tile-hung to match the existing roof tiles, built onto a metal subframe:
– Tile hanging and metal subframe to the dormer walls (to match the existing roof under PD): £6,505
– Scaffolding across three lifts of the job: £4,340
– Timber external wall structure, insulation and sheathing: £3,222
– Flat roof structure, insulation and GRP covering: £4,914
– Timber internal stud walls and plasterboard: £1,976
– Floor structure beneath the dormer: £2,057
– Structural steelwork and connections (support only, not the main frame): £2,269
– Skips, strip-out and demolition: £2,904
Notice that scaffolding alone cost £4,340 on this zone — and that’s before the hip-to-gable and general elevation scaffolding is added on top. Scaffolding gets used across the whole build, so a quote that shows one small scaffolding line for the entire project, rather than per-zone, is worth questioning.
Why this project sat right at its Permitted Development limit
The rear dormer (30.67m³) and hip-to-gable extension (18.9m³) together added 49.57m³ of new roof volume, against a Permitted Development allowance of 50m³ for this semi-detached house. That’s cutting it close by design: Permitted Development normally gets you the most usable space and the largest dormer size you can build without applying for full planning permission, and this homeowner wanted to make the most of that allowance. The middle loft conversion zone doesn’t count towards this volume limit, because it stays within the existing roofline rather than adding new structure.
If your loft conversion is creeping close to that 50m³ limit for a semi-detached or detached house, or 40m³ for a terraced house, get your volume calculations checked before your builder starts pricing. Going even slightly over collapses the whole project back into a full planning application — and going through planning can be more restrictive than the PD default, since your council can apply its own tighter limits on top. It also typically adds around 8 weeks and a fee to the timeline.
The three cost drivers behind the price
- How much of the roof is new structure versus a converted void. The zone that stays within the existing roofline is cheapest, because it’s converting space that already exists. A hip-to-gable extension adds a new wall where there wasn’t one. A rear dormer adds a box-shaped structure with its own walls, roof, and waterproofing, making it the most expensive per square metre. This project needed all three zones, which is why it sits at the higher end of loft conversion costs.
- Fitting out the new rooms is a real cost on top of the shell. Once the structure is built, the two new bedrooms and the shower room still need plastering, painting, flooring, doors, windows, radiators and electrics before they’re usable. On this project that added £21,131 combined on top of the shell cost.
- Fire regulations add hidden cost. Fire-rated doors, fire-rated plasterboard to ceilings, and mains-wired interlinked smoke alarms are all mandatory under Building Regulations for a habitable loft, not optional extras. If a competing quote is significantly cheaper, check these are actually included.
What to do with this before you get your own quotes
Use this breakdown as a checklist, not a price guarantee. Costs vary by region, by builder, and by exactly how many zones your own conversion needs. But if a quote you’re holding doesn’t let you see each zone priced separately, with steelwork, insulation, fire-rated materials, and scaffolding broken out, ask for that detail. That’s usually where the gap between two competing quotes actually lives.
