Cost Guide · Updated June 2026 · UK Loft Conversions

How Much Does a Flat Roof Dormer Cost Per m² in 2026?

A flat roof dormer is the most cost-effective way to turn a cramped loft into a usable bedroom or office, adding full-height headroom and floor space without the expense of a hip-to-gable or mansard roof. In 2026 UK expect roughly GBP 1,500 to GBP 2,500 per square metre of new floor area - about GBP 4,000 to GBP 10,000 for a single box dormer and GBP 12,000 to GBP 22,000 for a full-width rear dormer. This 2026 guide breaks down the cost ladder by size, what drives the price, and a worked GBP 13,200 example.

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GBP 1,500-2,500 per m²
Updated June 2026
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Cost Ladder · 2026

Flat Roof Dormer Cost by Size

Prices include the dormer structure, flat roof covering (GRP or single-ply membrane), insulation to Part L, window, internal plastering and making good. They exclude the wider loft conversion fit-out (stairs, en-suite, electrics) unless stated.

Dormer typeApprox added floorTypical 2026 cost
Single box dormer (window)3-5 m²GBP 4,000-7,000
Large single dormer5-8 m²GBP 7,000-12,000
Double / twin dormer8-12 m²GBP 11,000-16,000
Full-width rear dormer12-20 m²GBP 12,000-22,000

London and the South East run roughly 10 to 20 percent higher. These figures are for the dormer alone - a full loft conversion incorporating the dormer typically runs GBP 35,000 to GBP 70,000.

Price Drivers

What Drives the Cost Per m²

  • Size and width: a full-width dormer costs more in total but less per square metre than a small box dormer, because fixed costs like scaffolding are spread over more floor.
  • Roof covering: a GRP fibreglass or single-ply EPDM membrane flat roof costs more than felt but lasts far longer and is the 2026 standard.
  • Steelwork: wide openings need a steel beam to carry the roof, adding GBP 800 to GBP 2,000.
  • Windows and doors: standard casements are cheaper than large aluminium sliders or a Juliet balcony.
  • Access and scaffolding: restricted rear access and tall properties raise scaffold costs.
Worked Example · 2026

A Real GBP 13,200 Full-Width Dormer

A 16 m² full-width rear dormer on a 1930s semi in the Midlands, built as part of a loft conversion, with a GRP flat roof and two casement windows:

  • Scaffolding: GBP 1,400
  • Structural timber + steel beam: GBP 2,600
  • GRP flat roof + insulation: GBP 2,900
  • Windows + cladding: GBP 2,100
  • Plastering + making good: GBP 1,800
  • Labour + finishing: GBP 2,400

Total: GBP 13,200 inc. VAT - about GBP 825 per m² for the dormer element, mid-range for a full-width build with GRP and steel.

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FAQ · 2026

Flat Roof Dormer Cost FAQs · 2026

In 2026 UK a flat roof dormer costs roughly GBP 1,500 to GBP 2,500 per square metre of added floor area. A small single box dormer adding 3 to 5 square metres runs GBP 4,000 to GBP 7,000, while a full-width rear dormer adding 12 to 20 square metres runs GBP 12,000 to GBP 22,000. Per square metre costs fall as the dormer gets larger because fixed costs like scaffolding are spread over more floor.
A single box dormer with one window typically costs GBP 4,000 to GBP 7,000 in 2026, and a larger single dormer GBP 7,000 to GBP 12,000. The price covers the structure, a GRP or membrane flat roof, insulation, the window, plastering and making good, but not the wider loft fit-out such as stairs or an en-suite.
Usually no. A rear flat roof dormer is generally Permitted Development if it stays within the volume allowance (40 cubic metres for a terrace, 50 for a semi or detached), does not extend beyond the existing roof plane on the principal elevation, and is set back at least 200mm from the eaves. Planning is required in conservation areas, on listed buildings, and for front-facing dormers. Building Regulations always apply.
A single dormer typically takes 1 to 2 weeks once scaffolding is up, and a full-width rear dormer 2 to 4 weeks. As part of a complete loft conversion the dormer stage sits within an overall 6 to 10 week programme covering steelwork, floor, dormer, fit-out and certification.
Yes. A flat roof dormer is the cheapest dormer type because the structure is simpler and it maximises internal head height for a given roof. A pitched roof dormer looks more traditional and suits the front elevation but costs 20 to 40 percent more and yields less usable space.
Often yes. By converting unused loft space into a bedroom or office, a dormer loft conversion can add 10 to 20 percent to a property value in many UK areas, frequently returning more than its cost. An extra double bedroom with head height is one of the most reliable value-adding improvements.
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