Dormer Costs UK 2026 — What to Charge to Build a Dormer Loft Extension

Types of Dormer and What to Charge

The word "dormer" covers a wide range of structures, and the price gap between the smallest and largest is enormous. A single front cheek dormer over a stairwell is a different job from a full-width rear box dormer that adds a whole bedroom. Pricing accurately starts with classifying the dormer correctly. Here are the main types with current UK price bands.

Flat-Roof Box Dormer

The flat-roof box dormer is the workhorse of UK loft conversions. It projects vertically from the existing roof slope with flat sides (cheeks), a flat or very low-pitch roof, and a face that holds windows. It maximises internal head height and floor area for the money, which is why it is by far the most common dormer specification on terraced and semi-detached homes.

Construction is timber-framed cheeks and roof, clad in tile, slate, render, zinc, lead or — most commonly now — a fibreglass (GRP) or EPDM flat roof finish. A single small box dormer added to an existing loft is the entry point to dormer pricing.

• Small single box dormer (one window): £8,000–£20,000
• Medium box dormer (two windows): £15,000–£28,000
• Per m² of dormer footprint: £1,800–£2,800/m²

Full-Width Rear Dormer

A full-width rear dormer runs the entire width of the rear roof slope, turning a cramped loft into a full-height room with usable floor area across the whole footprint. This is the single most popular conversion type for Victorian and Edwardian terraces because it delivers a double bedroom with an en-suite under permitted development on many properties.

Because it spans the full width, the structural demands are greater: you typically need steel beams to carry the existing ridge and the new dormer load, full floor-strengthening joists, and substantially more cladding and roofing area. This is where quotes diverge sharply between firms — the cheap ones are usually missing the steels.

• Full-width rear dormer (structure + finish only): £20,000–£45,000+
• Per m² of new room area: £2,000–£3,200/m²

Note this band is for the dormer build itself — staircase, internal fit-out, plastering and decoration push the all-in figure higher (see the full conversion band below).

Pitched / Gable-Fronted Dormer

A pitched dormer has a small gabled or hipped roof rather than a flat top. It is more traditional in appearance and is often required by planning in conservation areas or on front-facing elevations where a flat box would be refused. The trade-off is cost: a pitched roof, gable wall and additional flashing detail add labour and materials for less internal volume than a box dormer of the same footprint.

• Single pitched-roof dormer: £10,000–£24,000
• Twin pitched dormers (matched pair): £18,000–£38,000

Price these 15–25% above an equivalent box dormer to reflect the extra roof carpentry, tiling and the slower, more detailed finishing they demand.

Hip-to-Gable Conversion

On a semi-detached or end-of-terrace house with a hipped roof, converting the sloping hip end into a vertical gable wall creates the volume needed for a usable loft — and is very often paired with a rear dormer. Strictly this is a roof remodelling rather than a dormer, but loft firms quote it alongside the dormer and customers treat it as one job, so you must price it as part of the package.

The work involves removing the hip rafters, building up a new gable wall in blockwork or timber frame, and re-roofing the new section. It is structurally significant and usually needs steels and a Party Wall award on the gable boundary.

• Hip-to-gable element (on top of dormer): £10,000–£20,000
• Hip-to-gable + full-width rear dormer combined: £35,000–£60,000+

Front Cheek Dormers and Roof Lights

Small front cheek dormers — single windows on the front slope, often over a stairwell or to light a landing — are the lowest-cost dormer work. They add light and a little head height without the structural scope of a full box. On a tight budget or where planning restricts the front elevation, a roof light (Velux-style) dormer or simple flat-cheek dormer is the answer.

• Single front cheek dormer: £6,000–£12,000
• Conservation-style front dormer (matched detail): £10,000–£18,000

What Goes Into the Price — Breaking Down the Build

Structural Work — Steels and Floor Joists

The structural element is the part inexperienced quoters miss, and it is where the money is. Almost every dormer needs new floor joists to carry the loft as habitable space — existing ceiling joists are rarely rated for floor loads. A full-width or load-bearing dormer also needs steel beams (RSJs) to carry the ridge, the dormer cheeks and sometimes the floor.

• Structural engineer's calculations and drawings: £500–£1,200
• Steel beams supplied and installed (typical dormer): £1,500–£4,000
• New floor joists and strengthening: £1,200–£3,500

Roof Alterations and Cutting In

Forming the opening means cutting and trimming the existing rafters, propping the roof while you work, and tying the new dormer structure into the sound roof around it. This is skilled carpentry that carries weather risk — the roof is open while you are working, so you need to programme it tightly and keep the structure watertight at the end of each day.

Cladding and Roof Finish

The dormer's exterior is a major cost variable. Cheeks and face can be tile-hung, slate-hung, rendered, or clad in lead, zinc or composite. The flat roof is most commonly finished in fibreglass (GRP) or EPDM rubber, both of which give a long warranty and a clean detail.

• Fibreglass (GRP) flat roof: £90–£130/m²
• EPDM rubber flat roof: £80–£120/m²
• Tile or slate hanging to cheeks: £70–£110/m²
• Render finish to cheeks: £45–£80/m²
• Zinc or lead cladding: £120–£200/m²

Windows, Insulation and Labour

Windows are usually double-glazed casements set into the dormer face, or roof lights in the slope. Insulation must hit current building regulations — typically a U-value around 0.18 W/m²K for the roof and walls — which on a thin dormer cheek often means high-performance PIR board or a warm-roof build-up. Labour is the largest single line: a two- or three-person team on a full-width dormer will be on site for three to five weeks.

• Dormer face windows (double-glazed casement): £500–£1,200 each
• Insulation to building regs (PIR / warm roof): £40–£70/m²
• Labour as a share of total dormer cost: 40–55%

Full Dormer Loft Conversion — The All-In Figure

Customers usually want one number for the finished room, not the dormer in isolation. A full dormer loft conversion bundles the dormer build with the staircase, structural floor, electrics, plumbing for an en-suite, plastering, flooring and decoration. This is the figure most homeowners search for.

• Standard dormer loft conversion (bedroom): £40,000–£60,000
• Dormer conversion with en-suite and high spec: £55,000–£75,000+
• London and the South East: add 15–30% on the figures above

Always make clear in your quote where the dormer build ends and the fit-out begins. Many disputes come from a customer assuming the "dormer price" included a fitted en-suite and fitted wardrobes. Separate the structure, the shell and the fit-out as distinct sections.

Cost Drivers — What Pushes a Dormer Quote Up

Two dormers of identical size can differ by £15,000 because of factors that have nothing to do with the dormer itself. Check every one of these before you commit a price.

• Party Wall agreement: Terraced and semi-detached work on or near the boundary triggers the Party Wall etc. Act 1996. A surveyor's award costs the customer £700–£2,000 per neighbour, and delays of several weeks are common if a neighbour dissents. Flag it early — it is the customer's legal cost but your programme risk.
• Access and scaffolding: A full-width rear dormer needs a scaffold for the whole job, often with a loading platform and rubbish chute. Budget £1,500–£4,000 for scaffold on a typical conversion. Rear gardens with no side access mean carrying every steel and sheet through the house — add labour and protection.
• Planning vs permitted development: Many rear dormers fall under permitted development (subject to volume limits — 40m³ for terraces, 50m³ for semis and detached). Front dormers and conservation areas usually need full planning. A planning application adds £200–£600 in fees plus 8–12 weeks, and a refusal can kill the job.
• Building control: Every dormer needs building regulations approval. Budget £500–£1,000 for building control fees and inspections, and price in the time for staged sign-offs (structure, insulation, final).
• Region: London and the South East run materially higher on labour. The same dormer that is £22,000 in the Midlands can be £30,000+ inside the M25.
• Existing roof condition: Cutting into a tired roof often uncovers rotten timbers, failed felt or undersized rafters. Put a provisional sum in the quote rather than absorbing the surprise.

Worked Examples

Example 1 — Small box dormer, 1930s semi (Midlands)

A homeowner wants a single box dormer to the rear to make an existing loft room usable. Footprint roughly 6m². The job is permitted development, no Party Wall issue (detached side), GRP flat roof, render cheeks, one casement window.

• Structural calcs + floor joists: £2,400
• Carpentry, cutting in, dormer frame: £4,800
• GRP roof, render, window, insulation: £3,600
• Scaffold + building control: £1,900
• Margin and overhead: £3,300
• Total to customer: ~£16,000

Example 2 — Full-width rear dormer, Victorian terrace (Manchester)

A full-width rear box dormer turning the loft into a double bedroom. Steels to carry the ridge, full floor strengthening, tile-hung cheeks, GRP roof, two windows. Party Wall awards needed both sides.

• Steels + structural floor: £6,200
• Carpentry, dormer structure, cutting in: £11,500
• Roof finish, tile hanging, windows, insulation: £8,800
• Scaffold, building control, provisional sums: £4,200
• Margin and overhead: £7,300
• Total dormer build to customer: ~£38,000 (Party Wall surveyor fees paid separately by the homeowner)

Example 3 — Hip-to-gable + rear dormer, full conversion (Surrey)

An end-of-terrace with a hipped roof. Hip rebuilt as a gable, full-width rear dormer, and the whole loft fitted out as a bedroom with en-suite. South East rates apply.

• Hip-to-gable rebuild + steels: £16,000
• Full-width dormer structure and finish: £24,000
• Staircase, en-suite, electrics, plastering, decoration: £18,000
• Scaffold, building control, planning, provisional sums: £6,000
• Margin and overhead: £14,000
• Total all-in to customer: ~£78,000

Quoting Tips — What to Check Before You Price

Dormer quotes go wrong when the builder prices off a description and a photo rather than a proper survey of the roof and structure. Before you commit a price, check the following:

• Head height: Measure the existing ridge height. If you do not have ~2.2m of usable head height after the floor build-up, the dormer may not deliver a habitable room and the customer will be disappointed.
• Structural route: Get the structural engineer involved before quoting a full-width or hip-to-gable job. The steel size and bearing points change the price materially.
• Boundary and Party Wall: Identify every shared wall and neighbour affected. Tell the customer up front that Party Wall awards are their cost and can delay the start.
• Planning status: Confirm whether the dormer is permitted development or needs an application. Never assume — conservation areas, Article 4 directions and previous extensions all eat into the PD allowance.
• Access: Walk the side and rear. No side access means everything goes through the house — quote the extra labour and protection.
• Existing roof condition: Get into the loft. Note rotten timbers, undersized rafters and failed felt. Put a provisional sum in writing for what you cannot see.

Include a short scope document with your quote that separates the structure, the dormer shell, and the internal fit-out. A clear breakdown elevates your quote above competitors who just send a single number, and it protects you when the customer later asks why the wardrobes were not included.

Quick Reference: Dormer Prices UK 2026