What Does a Dormer Cost?
Quick Answer
A dormer loft conversion costs £30,000–£60,000, with around £45,000 typical
Guide prices from published 2026 UK cost guides, not quotes. Small 15–20m² conversions from about £20,000; London-focused guides run considerably higher
Dormer Conversion Cost by Size (2026 UK)
MyJobQuote’s April 2026 dormer guide prices a finished dormer loft conversion (structure, windows, insulation, stairs and finishing) by floor area like this:
| Conversion size | Guide price | Notes |
|---|---|---|
| Small (15–20m²) | £20,000–£30,000 | Single bedroom without en-suite |
| Medium (20–30m²) | £30,000–£45,000 | Double bedroom; add an en-suite and a £35,000 job becomes about £40,000 |
| Large (30–40m²+) | £45,000–£60,000 | Full-width rear dormers and twin-dormer layouts sit here; a twin dormer with bedroom and en-suite prices at about £47,000 |
PriceYourJob’s average for a dormer conversion is lower at around £30,000, and Checkatrade’s general loft-conversion band is £27,500–£75,000, so the honest national picture is a £30,000–£60,000 band with real spread either side. Check the dimensions work before pricing anything with our dormer sizing calculator and head height calculator.
Dormer Types and Variations
| Type | Guide price | Notes |
|---|---|---|
| Dormer window only (into an existing dormer) | from ~£1,000 | Checkatrade’s 2026 figure for supplying and fitting one small uPVC dormer window, including access; the structure itself is not included |
| Dormer added to an existing loft space | £4,000–£15,000 | Structure only, no room conversion: around £9,500 for a standard 2–4m² dormer, with a basic rear dormer from about £6,000 |
| Full-width rear box dormer | £45,000–£60,000 | MyJobQuote’s large-dormer band; London builder guides publish £52,000–£78,000 |
| L-shaped dormer | £55,000–£90,000 | Published almost entirely by London-focused firms, where it suits Victorian terraces with rear additions: Alstruct quotes £55,000–£85,000 and houseUP £55,000–£90,000 nationally, rising to £70,000–£120,000 in London. National data is thin, so treat as indicative |
| Hip-to-gable plus rear dormer | £50,000–£80,000 | See the loft conversion cost page for the full type-by-type comparison |
| Second (double) dormer | up to +£15,000 | MyJobQuote’s premium for a double dormer over a single |
What’s Included in a Conversion Quote
- Structural steelwork and new floor joists (check with the loft steel beam calculator)
- Dormer construction and roof covering (GRP, felt, tiles or lead)
- Windows and glazing
- Insulation to current Part L standards
- Staircase, plastering, electrics and decoration
- Fire safety upgrades: fire doors and interlinked smoke alarms
- Scaffolding
Costs on Top
- En-suite: £5,000–£15,000 across guides; plan it with the loft bathroom calculator
- Planning application, if needed: £548 householder fee (England, from 1 April 2026; index-linked annually)
- Party wall agreement: £700–£2,700 per agreement across guides (about £1,000 with a single agreed surveyor); check whether notice is needed with the loft party wall calculator
- Building regulations: always required; council full-plans fees for a loft conversion are typically a few hundred pounds plus inspections, set by each authority
- Architect or designer: from about £250 for concept drawings to £1,500–£2,000+ for a full submission package
Planning Permission
Many rear dormers fall under permitted development: the dormer must not rise above the existing ridge, must sit back from the eaves, cannot face the highway, and the added roof volume is capped at 40m³ on a terrace or 50m³ on a semi or detached house. Side and front dormers usually need planning permission. Check your allowance with the loft PD volume calculator and always confirm with your local planning authority.
London and the South East
MyJobQuote’s dormer guide adds 20–30% to national figures for London and the South East, and the L-shaped and full-width figures published by London loft firms sit well above the national bands in these tables. In London, budget from the top of each range upwards.
Timescale
A typical dormer conversion takes around 6 weeks on site, with larger and L-shaped layouts running to 8–10 weeks. Allow several weeks beforehand for design, structural calculations, building regulations and any planning application.
Important Notes
- All figures are guide prices from published 2025–2026 UK cost guides, not quotes: get at least three itemised quotes
- Quotes differ in what they include; confirm scaffolding, glazing spec and making good are all itemised
- Every habitable dormer conversion needs building regulations approval including protected fire escape routes
Figures cross-checked against published UK cost guides July 2026. Last updated: July 2026