Loft conversion stages, costs and timings
A rear dormer loft conversion runs through six stages, from scaffold to building control sign-off, over 8 to 12 weeks on site. A dormer conversion costs about £40,000 to £75,000 in the UK. The dormer stage carries the most risk, because the roof is open.
Updated 9 November 2026
What are the stages of a loft conversion?
Each stage, its usual share of the cost, what that is on a £45,000 job, and how long it takes on site. Stages often overlap.
| Stage | Share | On £45,000 | Time on site |
|---|---|---|---|
| 1. Scaffold, clearance and survey check | 8% | £3,600 | 1 week |
| 2. Steels and new floor | 20% | £9,000 | 1 to 2 weeks |
| 3. Dormer and weathertight shell | 25% | £11,250 | 2 to 3 weeks |
| 4. Stairs, partitions, first fix and insulation | 22% | £9,900 | 2 to 3 weeks |
| 5. Plastering, second fix and en-suite | 20% | £9,000 | 2 to 3 weeks |
| 6. Fire safety, sign-off and handover | 5% | £2,250 | 1 week |
What should you check before you approve each stage?
Approve a stage when it is done and you have seen these. Ask for photos of anything that will be covered up.
1. Scaffold, clearance and survey check
Done when: Scaffold up, loft cleared, and the head height and roof structure checked against the survey.
- A dated, signed survey confirming the head height.
- A signed party wall award covering every adjoining owner.
- A Lawful Development Certificate or a planning decision.
2. Steels and new floor
Done when: Steel beams set on the walls, and the new floor joists bearing on the steel.
- The structural engineer’s calculations for the steel actually delivered.
- Building control’s record of its inspection of the steel.
- Photos of every joist bearing, before the floor deck goes down.
3. Dormer and weathertight shell
Done when: Dormer framed and covered, windows and flashings in, and the loft dry in rain.
- A hose test, and photos of every upstand and flashing.
- Written confirmation that the builder’s insurance covers rain getting in during the works.
- The roof covering guarantee registered in your name.
4. Stairs, partitions, first fix and insulation
Done when: Staircase and walls in, cables and pipes run, and insulation fitted.
- A stair drawing checked by building control before the stair was ordered.
- The fire plan in writing: every fire door and alarm position.
- Photos of the insulation with the product label in shot.
5. Plastering, second fix and en-suite
Done when: Plastered, the en-suite waterproofed and tiled, and sockets, radiators and doors fitted.
- Dated photos of the waterproof membrane in the en-suite, before tiling.
- A photographed leak test before any pipework was boxed in.
- The electrician’s registration checked on the scheme’s own register.
6. Fire safety, sign-off and handover
Done when: Alarms and fire doors tested, the final inspection passed, and the snag list closed.
- Trigger one smoke alarm and hear every alarm in the house sound.
- The Building Regulations completion certificate in your hands.
- A handover pack: electrical certificate, engineer’s sign-off, window certificates and roof guarantee.
How much does a loft conversion cost?
| Type | Typical cost |
|---|---|
| Rooflight, no change to the roof shape | £22,500 to £45,000 |
| Dormer, the most common type | £40,000 to £75,000 |
| Hip to gable, semis and end terraces | £50,000 to £85,000 |
| Mansard | £50,000 to £120,000 |
Not every source says whether VAT is included, so ask each builder. London and the South East can cost 15 to 30% more.
These ranges come from:
- Checkatrade, “Loft conversion cost: 2026 UK price guide & breakdown” (opens in a new tab), read November 2026.
- Homebuilding & Renovating, “Loft conversion costs: how much to budget when extending up” (opens in a new tab), read November 2026.
- Federation of Master Builders, “Loft conversion cost: a breakdown of costs” (opens in a new tab), read November 2026.
How long does a loft conversion take?
A rear dormer takes 8 to 12 weeks on site, or 9 to 15 weeks with the usual delays. A rooflight conversion can take 4 to 6 weeks, and a hip to gable or mansard 10 to 14.
Do you need planning permission for a loft conversion?
Often not. A loft conversion is usually permitted development if it adds under 40 cubic metres to a terraced house, or 50 to a semi or detached house. A mansard almost always needs planning permission.
How should you pay for a loft conversion?
Agree a price for each stage before work starts, and release it once you have checked the work.
Keep 5% back from the last stage until the Building Regulations completion certificate is in your hands. Without it, the new room is not legally a bedroom.
With Renopay, the money for each stage is held securely until you approve the work.
Read next: Loft conversion payment schedule · How to protect yourself from scope creep