Dormer Installation Cost Calculator
Estimate UK 2026 dormer addition cost by style (flat / shed, gable, hipped, eyebrow), size, retrofit vs new-build, property height, and access — with rooflight, plasterboard finish, planning fee, and crane line items.
Dormer Installation Cost Calculator
Estimate UK 2026 dormer addition cost by style (flat / shed, gable, hipped, eyebrow), size, retrofit vs new-build, property height, and access — covering Building Regulations Part A, L, and B compliance with optional rooflight, plasterboard finish, planning fee, and crane line items.
Estimated installed cost
£17,898
Range: £15,213 – £21,478
framing + window + interior + planning + crane
Framing + roofing
£10,958
Window unit
£950
Plasterboard finish
£5,610
Planning
£380
Crane
£0
What this calculator estimates
This calculator quotes the all-in installed price for a UK roof dormer addition in 2026 pounds sterling. It covers the framing, exterior sheathing and roofing, weather-tight envelope, optional rooflight or window, optional plasterboard interior finish, Building Regulations / planning fee, and an optional crane day for large or eyebrow dormers.
The bill is split into the line items real loft-conversion contractors invoice:
- Framing and roofing — the bulk of the job. Cut the existing roof, install steels or doubled rafters, sheathe with breathable membrane, install the new roofing (flat single-ply or tiled to match), clad the cheek walls, and lead-flash the intersection with the main roof.
- Rooflight or window unit — typically one rooflight per dormer. Adjust by toggling the window line off and pricing oversized units separately.
- Interior finish — plasterboard, skim, skirting, electrical first-fix (one circuit, two double sockets, one switch, one fitting), and decoration ready for paint.
- Building Regulations fee — local council fee. Planning permission adds £200 to £450 on top.
- Crane day — required for large or eyebrow dormers where pre-built wall and roof sections are lifted in panels. Optional on small and medium dormers.
A minimum job fee of £2,800 applies in most UK metro markets.
How to use it
- Number of dormers — usually one rear dormer; some L-shaped conversions add a side dormer too.
- Style — flat / shed for budget and maximum headroom, gable for kerb appeal, hipped for premium detached homes, eyebrow for bespoke heritage joinery.
- Size — small (0.9 m x 1.2 m), medium (1.5 m x 1.8 m), large (2.4 m x 3 m).
- Construction type — new build (during initial roof construction) saves about 15 percent versus retrofit.
- Property height — single storey, two-storey, or three-storey-plus. Three-storey adds scaffold and roof-edge protection.
- Site access — clear ground vs terraced (no rear access, all materials through the house).
- Window — defaults ON. Toggle OFF for a decorative dormer.
- Interior finish — plasterboard, skim, electrical first-fix.
- Planning / Building Regs fee — defaults ON.
- Crane day — toggle ON for large or eyebrow dormers, or where rear access is so tight that craning over the house is faster than carrying materials through.
Typical 2026 UK dormer cost ranges
| Scope (single dormer, retrofit, two-storey terraced, moderate access, rooflight + plasterboard + Building Regs) | 2026 installed price |
|---|---|
| Small flat / shed (~0.9 m x 1.2 m) | £8,500 – £12,500 |
| Small gable | £10,500 – £15,500 |
| Medium flat / shed | £14,000 – £21,500 |
| Medium gable | £17,500 – £27,500 |
| Medium hipped | £21,500 – £34,000 |
| Large gable (~2.4 m x 3 m) | £30,000 – £48,000 |
| Large hipped | £36,000 – £58,000 |
| Eyebrow (medium) | £33,000 – £55,000 |
| Three-storey adder | +25% |
| Difficult access (terraced rear, narrow side return) adder | +30% |
Add 8 to 15 percent in central London zones 1-3 and 5 to 10 percent in Conservation Areas requiring matching slate or clay tile.
Cost drivers
Style. Flat-roof dormers are by far the cheapest because the roof is one plane and the cheek walls are simple boxes. Gable dormers add two pitched roof planes, vertical end walls, and a more complex tile-to-tile valley. Hipped dormers add four roof planes and far more lead-flashing perimeter. Eyebrow dormers require custom-bent rafters and laminated plywood sheathing — figure 60 to 110 percent more than gable.
Size. Cost-per-cubic-metre falls as dormer size increases because mobilisation, scaffold hire, weather-tight envelope, and Building Regs fee are roughly fixed. A medium dormer is rarely twice the cost of a small one.
Retrofit vs new-build. Retrofit adds 12 to 18 percent because the existing roof must be cut, weather-tight tarping must be staged, and any tile or slate batch-match must be sourced (or the entire roof slope must be re-tiled to avoid a colour mismatch).
Interior finish. Plasterboard, skim, electrical, and decoration add 25 to 40 percent to a basic dormer shell.
Site access. Mid-terrace houses are the most expensive site condition in UK construction. All materials carry through the house, scaffold is split-level, and waste skips need parking suspensions (£150 to £400 in London).
Code references and standards (UK)
- Approved Document A (Structure) — Structural loading and openings, Building Regulations 2010 (as amended).
- Approved Document B (Fire Safety) — Means of escape from a loft conversion (mandatory fire door programme on the stair).
- Approved Document C (Site preparation and resistance to moisture) — Damp-proofing and roof weather resistance.
- Approved Document F (Ventilation) — Whole-house and room-by-room ventilation rates after a loft conversion.
- Approved Document L1B (Conservation of fuel and power) — Insulation U-value for the new dormer (typical roof U-value 0.16 W/m²K or better).
- Approved Document Q (Security) — Security requirements for windows and doors to habitable rooms.
- BS 5534 — Code of practice for slating and tiling.
- BS 5250 — Management of moisture in buildings (ventilation calc behind plasterboard).
- The Party Wall etc Act 1996 — Notices and awards on shared-wall works.
- Work at Height Regulations 2005 — Mandatory fall protection during dormer framing.
When to add a dormer versus when to expand differently
Add a dormer when:
- The loft has 35 degrees or steeper pitch and at least 2.4 m of ridge-to-joist height.
- You need 6 to 20 m² of additional living area for the lowest cost-per-sqm.
- The existing roof has 5+ years of remaining life.
- Permitted Development applies or planning permission is granted.
Expand differently when:
- The loft pitch is under 30 degrees — usable headroom is minimal and the cost-per-usable-sqm is poor.
- The roof is engineered trusses without engineer’s sign-off.
- You need 30+ m² of new space — a rear extension or side return is usually cheaper per usable sqm.
- The existing roof is 15+ years old — re-roof first or bundle the dormer into a re-roof project.
Diagnostic checklist before signing a contract
- Permitted Development assessment or full planning application status confirmed.
- Building Regulations application made and reference number quoted on the contract.
- Party Wall Notice served and awards completed before tear-out.
- Structural engineer’s calculations stamped and provided.
- Existing roof tile / slate batch identified for matching (or full-slope re-roof in the scope).
- Egress requirements confirmed (means of escape, fire door to the stair, smoke alarm).
- Ventilation calc included in the contract.
- Weather-tight plan in writing.
- Scaffold and skip permits priced separately if they apply.
- Warranty terms specified for framing, roofing, and rooflight separately.
Related calculators
- Roof replacement cost calculator — bundle the dormer with a re-roof for tile or slate match.
- Skylight installation cost calculator — cheaper alternative when you only need light, not floor area.
- Roof flashing cost calculator — lead-flashing is critical at the dormer-to-main-roof intersection.
Sources: 2026 NFRC, Checkatrade, MyBuilder installed-quote data; Approved Documents A, B, C, F, L1B, Q; BS 5534; BS 5250; the Party Wall etc Act 1996; Work at Height Regulations 2005; London, Manchester, Bristol, Leeds dormer-specialist contractor benchmarks.
Frequently asked questions
How much does a dormer loft conversion cost in the UK in 2026?
A typical rear gable dormer on a Victorian terrace with one window, plasterboard finish, and Building Regulations sign-off runs £18,000 to £32,000 installed in 2026. A small flat-roof dormer (~0.9 m x 1.2 m, single rooflight, no interior) starts around £8,500 to £12,500. A large hipped-roof dormer (~2.4 m x 3 m) for a primary bedroom can reach £42,000 to £70,000. Eyebrow dormers are bespoke joinery — figure 2 to 3 times the cost of a gable of equivalent footprint. Source: 2026 NFRC, Checkatrade, MyBuilder, and London / Manchester / Bristol dormer-specialist installed-quote data.
Do I need planning permission for a dormer in the UK?
Often the dormer falls within Permitted Development if it is on the rear or side roof, does not exceed 50 cubic metres (40 m³ on terraced houses), uses materials matching the existing house, and is on a property not in a Conservation Area or AONB. Front-facing dormers, dormers in Conservation Areas, listed buildings, and properties on Article 4 Direction streets always require full planning permission. Building Regulations approval is always required regardless of planning status — fire safety, insulation (Approved Document L1B), and structural opening (Approved Document A) all apply. The calculator defaults to a £380 Building Regs fee — add £200 to £450 if you need full planning.
Is a flat-roof dormer cheaper than a gable dormer?
Yes, by 20 to 30 percent for the same footprint. A flat-roof (sometimes called shed or pram) dormer has a single roof plane at minimum 1:60 fall, simple cheek walls, and a straight box-gutter intersection with the main roof. A gable dormer has a two-pitch roof with vertical end walls and a more complex valley intersection. Flat-roof dormers are by far the most common choice on terraced houses across London because they maximise headroom and are the cheapest to build. Gable dormers are usually chosen on detached and semi-detached homes for kerb appeal where front-facing planning is required. Hipped and eyebrow dormers cost 30 to 90 percent more than gable.
What is the minimum roof pitch needed for a dormer?
Most pitched UK roofs over 35 degrees can take a dormer because there is enough headroom inside the existing loft. Roofs under 30 degrees usually do not have enough rise to make the dormer worth the cost. Trussed roofs (common since the 1970s) require structural engineering to cut and modify trusses — never cut a truss without a stamped calculation from a structural engineer (typically £600 to £1,800 in 2026). Cut-and-pitch (rafter and purlin) roofs, common in pre-1965 housing, are far easier to modify because rafters can be doubled and headed off without redesigning the entire roof load path.
How long does a dormer loft conversion take?
A typical loft conversion with a single rear dormer takes 6 to 10 weeks from start to handover, with the dormer itself running 2 to 3 weeks of that programme. Critical path is the weather-tight envelope — once the existing roof is cut, the dormer must be framed, sheathed, breathable-membraned, and rooflight-installed before the next rain. Reputable contractors will schedule the cut for a dry weather window and have a temporary tarp plan ready. Plastering, electrical, decoration, and stair installation run in the back half of the programme.
Does a dormer add value to my UK property?
Yes — a properly designed and Building Regs-compliant loft conversion with dormer typically returns 15 to 25 percent of the property value in resale uplift in London, 10 to 18 percent in regional cities. The 2026 Nationwide Home Improvement Report estimates an average £55,000 to £95,000 value uplift on a typical 3-bed semi for a dormer loft conversion costing £35,000 to £55,000 — so the ROI is usually positive provided the work is signed off and the bedroom counts as a sleeping room in the EPC and HM Land Registry filings. Always retain the Building Regs completion certificate — buyers and conveyancers ask for it.
What goes wrong with cheap dormer builds?
Three common failure modes: (1) weather-tight envelope opens too long — temporary tarps leak during framing and ruin the ceilings below; (2) the box-gutter at the back of a flat-roof dormer is undersized or has the wrong fall, causing ponding and eventual leak into the new room; (3) ventilation is forgotten — the dormer interrupts the existing eaves-to-ridge airflow on that section of roof and without restoring eaves vents to the dormer cheeks, condensation builds up. Always insist on (a) a written weather-tight plan, (b) a code-compliant box-gutter with 1:60 fall and proper outlet sizing, and (c) a ventilation calc compliant with Approved Document F.
Do I need party wall consent for a dormer on a terraced or semi-detached house?
Yes — if the dormer is on a roof slope shared with or close to an adjoining property, the Party Wall etc Act 1996 applies. You must serve a Party Wall Notice on adjoining owners at least 2 months before work starts. Most loft conversions involve cutting into or affixing to the party wall, which triggers a Section 2 notice. A party wall surveyor (typical fee £950 to £1,800 in 2026, often charged to the building owner) drafts the award. Skipping the notice exposes you to an injunction during the build and to substantial damages claims if the adjoining property is damaged.