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Dormer Cost Calculator (UK)

A dormer loft conversion instantly adds massive living space and value to your home. Calculate the estimated costs based on your roof type and requirements.

Conversion Specs

Outline Cost Breakdown

Core Conversion Labour & Material£35,000
Total VAT (20%)+ £7,000
Total Expected Range:
£42,000 - £48,300

Dormer Guide

  • Flat Roof Dormers

    The most common style. It gives the maximum amount of headroom and floor space internally and is generally the cheapest to build.

  • Planning Permission

    Most dormer conversions fall under permitted development if they are at the rear of the property (and you aren't in a conservation area).

  • Party Wall Agreements

    If you live in a semi-detached or terraced house, you will need to serve a Party Wall Notice to your neighbors before structural timber works begin.

Everything You Need to Know About Dormer Loft Conversions (UK)

A dormer loft conversion is one of the most popular ways to extend a house in the UK. Unlike a Velux or "rooflight" conversion—where the original roofline remains completely untouched and windows are simply added—a dormer actually alters the shape of the roof to create vertical walls and a horizontal ceiling. It drastically increases the usable floor space and headroom.

Types of Dormers

The final cost is heavily dictated by the shape and style of the dormer you install:

  • Flat Roof Dormer: This is a box-like structure built directly out of the pitched roof. While not always the most aesthetically pleasing from the outside, it provides the maximum amount of internal space.
  • Gabled / Pitched Roof Dormer: Also known as a "dog-house" dormer, these feature a traditional triangular roof. They look fantastic from the outside and blend seamlessly with period properties, but they offer less internal headroom and are significantly more expensive to build due to the complex roofing angles.
  • L-Shaped Dormer: Highly common on Victorian terraced properties. It involves building two connecting dormers: one on the main roof, and one on the rear outrigger roof. This essentially creates an entire top floor containing multiple bedrooms and a bathroom.

What's Included in the Build Cost?

The calculation ranges provided above are estimates for "builder-finished" conversions. This typically includes:

  • Scaffolding and structural supports: Including steel beams (RSJs).
  • Carpentry: Building the dormer timber frame, roof, and floor joists.
  • Insulation & Plasterboarding: Fully insulated to modern Building Regulations.
  • Stairs: A custom-built, regulation-compliant staircase from the first floor.
  • Electrics: Wiring, sockets, and recessed spot lighting.

What isn't usually included: Carpet/hardwood finishing, painting and decorating, and premium fixtures.

Do you need Planning Permission?

A massive advantage of dormer conversions in the UK is that they often fall under Permitted Development Rights (PD). This means you do not need formal planning permission, provided your design meets the following criteria:

  1. The dormer is not facing the highway (i.e. it is built at the back of the property).
  2. You don't exceed the volume limit. (Your total roof space allowance is 40 cubic metres for terraced houses, and 50 cubic metres for detached/semi-detached).
  3. You are not altering the building higher than the highest part of the existing roof.

Even if it falls under PD, you must still comply with Building Regulations, which ensures the floor can hold weight, fire safety rules are met, and the stairs are structurally sound.