How Much Does a Loft Conversion Cost?
Written by James Walker · Last reviewed 15 August 2026
James Walker has 2 degrees in construction (BSc (Hons) Building Surveying · MSc Environmental Design of Buildings) and an extensive background in issue resolution, resolving over £4 billion of consumer issues.
The short answer
- A rooflight conversion typically costs £20,000–£40,000; a dormer £35,000–£65,000.
- Head height decides feasibility before cost is even relevant.
- The staircase is often the hidden cost, because of what it takes from the floor below.
- A conversion that does not meet Building Regulations is not legally a bedroom, which affects value.
- Prices last updated August 2026.
On this page
Cost by conversion type
| Scope | Typical range |
|---|---|
| Rooflight / Velux | £20,000–£40,000 |
| Dormer | £35,000–£65,000 |
| Hip-to-gable | £42,000–£68,000 |
| Hip-to-gable with rear dormer | £55,000–£75,000 |
| Mansard | £55,000–£90,000 |
What pushes the price up
Conversion type. Rooflight, dormer and hip-to-gable are three different levels of structural intervention. The jump between them is large.
Floor structure. Existing ceiling joists almost never carry a habitable floor. New structure is standard, not an extra.
The staircase. Fitting a compliant stair often means losing space or reconfiguring a bedroom below. That secondary work is frequently outside the quoted figure.
Fire requirements. Compliant doors, alarms and sometimes changes to the escape route through the whole house.
Ensuite. Adding one means plumbing to the top of the house and often a pump.
Roof alterations. Raising or altering the roof to gain height changes the project fundamentally.
How long it takes
4–14 weeks.
How to stage the payments
Completion payment set aside, kick-off of around 10%, then steels and new floor, roof alterations watertight, stairs, first fix, plastered, second fix. Hold the final 10% until the building control completion certificate is issued, that certificate is what makes it a bedroom.
Keeping the cost down
- A rooflight conversion, where height allows, is far cheaper than a dormer
- Skip the ensuite if budget is tight
- Position the stair to minimise disruption below
How it works
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