Cost calculator

Dormer loft conversion cost calculator

A dormer adds head height and floor area by rebuilding part of the roof. That means structural work, a new roof section, cladding and weathering — which is why it costs substantially more than a rooflight conversion.

New loft floor area

Usable area created, including the dormer.

28 m²

Region

Broad trade availability keeps day rates near the national average.

Estimated range, ex VAT

£53,500 – £88,000

Labour (typical): £41,000
Materials (typical): £29,500

Guidance only. A real quotation depends on site conditions, specification and access — which is exactly what an independent review of your own quote will tell you.

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What affects the cost

Dormer size and type

A full-width rear dormer costs considerably more than a single box dormer, but delivers far more usable floor area per pound.

Structural steels

New floor beams and ridge support are almost always required, plus a crane or manual handling arrangements.

Cladding and weathering

Tile hanging, GRP, zinc or render on the dormer cheeks and roof, plus flashings at every junction.

Staircase and layout

The staircase must land legally and eats space on the floor below.

Ensuite

Pumped waste, extended heating and ventilation, typically adding £6,000–£12,000.

Regional pricing differences

Materials cost roughly the same everywhere in the UK; labour does not. Use the region selector above to apply a local index — London and the South East typically run 15–30% above the national average, while the North East, Northern Ireland and Yorkshire sit below it.

Labour vs materials

Roughly 58% of this type of work is labour and 42% materials. Structural carpentry, steel installation and roofing dominate. Fixed costs — staircase, fire compliance, building control — apply regardless of size.

Common hidden costs

Party wall awards, usually needed on semi-detached and terraced homes.
Fire door upgrades throughout the escape route.
Scaffolding for the full duration, which on a dormer is longer than on a rooflight conversion.
Redecoration of the floor below after structural work.

How to save money

A full-width rear dormer usually costs less per m² than a smaller one — the fixed costs are the same.
Keep the ensuite above existing plumbing.
Use tile hanging matched to the main roof rather than zinc or standing seam.
Confirm whether planning permission or a lawful development certificate is included in the price.

Frequently asked questions

How much does a dormer loft conversion cost?

£48,000–£78,000 ex VAT for a typical rear dormer with ensuite in 2026, higher in London.

Do I need planning permission for a rear dormer?

Usually it is permitted development within volume limits, but front dormers, flats and conservation areas are not. Get a lawful development certificate.

How long does a dormer take?

Ten to fourteen weeks typically, with the roof open for a portion of that.

Does a dormer add value?

A well-executed loft bedroom with ensuite usually adds more value than it costs in most UK markets — but not in every one.

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