Average Cost of a Rear Dormer Loft Conversion 2026 (UK)
A rear dormer loft conversion costs around £40,000–£60,000 in 2026 for a typical double-bedroom build, rising to £55,000–£75,000 for a large full-width dormer or £52,000–£72,000 with an ensuite. A rear dormer is the best-value way to add a full-height room in the loft: it creates far more usable floor space than a rooflight (Velux) conversion, usually sits within permitted development, and costs less per m² than a hip-to-gable or mansard. Here is what drives the price, how it compares with the other loft types, and the planning and building rules.
2026 rear dormer loft conversion costs
| Build | Typical cost (2026) | Notes |
|---|---|---|
| Small single-room rear dormer | £30,000–£45,000 | One bedroom or home office, no ensuite |
| Standard rear dormer (double bedroom) | £40,000–£60,000 | The most common project |
| Large / full-width rear dormer | £55,000–£75,000 | Bigger footprint, more glazing |
| Rear dormer with ensuite | £52,000–£72,000 | Adds plumbing and drainage |
| Guide rate per m² | £1,500–£2,000 per m² | Finished, standard specification |
Ranges assume a standard staircase, one or two dormer windows and a mid-range finish. London and the South East typically sit 20–30% above these figures.
What drives the price
The headline figure covers the structural work to open up the roof, the dormer box itself (walls, flat roof and windows), a new staircase, insulation to current Building Regulations, and a plastered, wired and decorated room. What moves a quote up is the size of the dormer (a full-width dormer needs more steel and covering), an ensuite (drainage runs and waterproofing), the staircase position (a tricky landing can cost more than the loft itself), fire safety upgrades to the floors and doors below, and your region. Rooflights versus a full glazed gable, and standard versus premium joinery, account for most of the rest.
Rear dormer vs the other loft types
| Loft type | Typical cost (2026) | Best for |
|---|---|---|
| Rooflight / Velux | £20,000–£35,000 | Lofts that already have the head height |
| Rear dormer | £40,000–£60,000 | Adding a full-height double room |
| Hip-to-gable | £45,000–£70,000 | Semi or detached homes with a hipped roof |
| L-shaped dormer | £55,000–£80,000 | Victorian terraces with a back addition |
| Mansard | £60,000–£95,000 | Maximum space, conservation areas |
A rear dormer is the sweet spot for most semi-detached and terraced homes: much more space than a rooflight conversion for a modest step up in cost, and cheaper than a hip-to-gable because it does not rebuild the sloping hip. If your roof is hipped, compare both in our hip-to-gable cost guide and the hip-to-gable comparison.
Planning and building rules
Many rear dormers are permitted development if they stay within the added-volume limits — 40m³ for terraced houses and 50m³ for semis and detached homes — sit below the ridge line and are set back from the eaves. They are not permitted development on flats, in most conservation areas, or where the home has already used its allowance. A lawful development certificate (£150–£300 council fee) is cheap certainty before you build. Either way, every loft conversion needs Building Regulations approval covering structure, fire escape (a protected stair route), insulation and the new staircase, and a Party Wall agreement is usually needed with attached neighbours.
Rear dormer loft conversion costs at a glance — 2026
Typical finished totals at standard spec, plus the London uplift.
Standard specification, finished. Source: BestBuilders 2026 quote data · 519 UK towns
Full 2026 rate card
| Item | Typical cost (2026) | Notes |
|---|---|---|
| Small single-room rear dormer | £30,000–£45,000 | Bedroom or office, no ensuite |
| Standard rear dormer, finished | £40,000–£60,000 | Double bedroom, plastered and decorated |
| Large / full-width rear dormer | £55,000–£75,000 | More steel, roof covering and glazing |
| Ensuite shower room within loft | +£5,000–£9,000 | Drainage, tanking and fittings |
| New staircase (standard) | £1,500–£4,000 | Softwood to hardwood; tricky landings cost more |
| Steel beams and structural work | £2,000–£5,000 | Ridge and floor beams, calculated by an engineer |
| Dormer windows or French doors + Juliet | £600–£2,500 each | Glazing spec drives this |
| Rooflights to the front slope (each) | £600–£1,400 | Cheaper daylight than extra dormers |
| Fire safety upgrade (doors, alarms, stair) | £1,200–£3,500 | Required for a habitable loft room |
| Architect / technician drawings | £750–£2,500 | See our architect cost guide |
Regional price differences
| Region | Against UK average |
|---|---|
| London | +20–30% (labour, access, parking) |
| South East and commuter towns | +10–20% |
| Southern and Eastern England | +5–15% |
| Midlands | Around the average |
| North West, Yorkshire | −5–10% |
| North East, Wales, Scotland | −10–15% |
The same standard rear dormer priced at £45,000 in Yorkshire can pass £58,000 in outer London before any change in specification. Always judge quotes against local rates — our extension and loft cost guides break the bands down by region.
How the build runs (6–10 weeks on site)
- Design and structural calcs — measured survey, layout and engineer's beam calculations (2–4 weeks before site).
- Planning check — confirm permitted development or obtain a lawful development certificate; serve any Party Wall notices.
- Building Regulations — full plans or building notice covering structure, fire escape, insulation and the staircase.
- Structure and dormer shell — scaffold up, roof opened, steels and floor joists in, dormer framed, roof covered: watertight by week 3–5.
- First fix and finish — staircase, electrics, plumbing, insulation, plasterboard, glazing, decoration and Building Control sign-off (weeks 5–10).
Choosing the builder: checklist
- Two recent rear dormer conversions you can visit or phone about — not just general building work.
- Structural engineer's calculations supplied for the steels and floor, and a named Building Control route.
- £2m+ public liability insurance, verified, plus who arranges the Party Wall agreement.
- A written stage-payment schedule — never more than 10–15% up front.
- Three itemised quotes on identical drawings so the numbers are genuinely comparable.
Red flags
- A price well below £30,000 for a finished dormer — something is missing from the scope.
- No structural engineer's calculations offered for the steels and floor.
- "You won't need planning" with no lawful development certificate on the table.
- Fire safety (protected stair, doors, alarms) left off the quote for a habitable room.
- Large cash deposit requests or VAT-free offers — no protection if it goes wrong.
FAQs
Related guides
- How much does a hip-to-gable loft conversion cost?
- How to plan a rear dormer, step by step
- Do I need planning permission for a rear dormer?
- Is a loft conversion worth it?
Get 3 Free Loft Conversion Quotes — FMB-Vetted Builders
BestBuilders matches you with up to 3 FMB-vetted UK loft specialists. Compare rear dormer quotes on the same drawings — no cost, no obligation.