loft conversion costs: what it actually takes
Loft conversions are one of the most profitable jobs in residential building. They are also one of the easiest to underquote. The structure looks sound from the hatch, then you get up there and find the ridge is sagging, the purlins are undersized, and the party wall needs a surveyor. By the time building control has signed off, you have eaten your margin.
This guide covers real loft conversion costs in 2026 so you can quote them properly.
types of loft conversion
The type dictates the cost. Here is the hierarchy from cheapest to most expensive:
- Velux (rooflight). No change to the roof shape. Just rooflights, insulation, floor strengthening, and a staircase. Cheapest option at £20,000 - £35,000. Only works if you have enough head height already — minimum 2.2m at the ridge.
- Dormer. A box structure built out from the roof slope to create more floor area and headroom. The most common type. Costs £35,000 - £55,000 for a standard rear dormer.
- Hip-to-gable. Converts a hipped roof end into a vertical gable wall. Common on semi-detached houses. Usually combined with a rear dormer. Expect £45,000 - £65,000.
- Mansard. The entire roof slope is rebuilt at a near-vertical angle. Maximum space, maximum cost, and almost always needs full planning permission. Budget £55,000 - £80,000+.
where loft conversions go wrong
Three things catch contractors out on lofts:
- Structural surprises. The existing roof structure is rarely what you expect. Timbers that looked fine from below are split, undersized, or bodged from a previous job. You will not know until you open it up. If you have not done a proper structural survey, price the steelwork and timber as High uncertainty.
- Party wall. On terraced and semi-detached properties, you will almost certainly trigger the Party Wall Act. The neighbour might agree quickly or they might appoint their own surveyor and drag it out for months. Budget £1,000 - £2,000 per neighbour and allow for delays.
- Building regs. Fire separation, escape windows, sound insulation between the new floor and the floor below, staircase headroom — building control will check all of it. If the staircase does not fit within the existing footprint, you are into structural alterations on the floor below. That is a different job entirely.
worked example: rear dormer loft conversion
| Item | Cost Range | Risk Level | Notes |
|---|---|---|---|
| Scaffolding | £1,500 - £2,500 | Low | Fixed price. Get it quoted up front. |
| Structural steelwork | £2,500 - £5,000 | High | Depends on existing structure. Engineer designs vary. |
| Dormer construction | £5,000 - £8,000 | Medium | Timber frame, flat roof, cladding |
| Roofing & leadwork | £2,000 - £3,500 | Low - Medium | GRP or felt flat roof. Lead flashings to existing. |
| Staircase | £2,500 - £4,500 | Medium | Standard flight = Low. Tight layout = High. |
| Floor strengthening & insulation | £1,500 - £2,500 | Medium | Existing joists rarely meet current regs |
| Electrics (first fix + second fix) | £1,500 - £2,500 | Low | Lighting, sockets, smoke detection |
| Plumbing & heating | £1,200 - £2,500 | Low - Medium | Radiators easy. En-suite adds £3k+. |
| Plastering & decoration | £2,000 - £3,500 | Low | Drylining throughout. Predictable. |
| Windows & fire escape | £1,500 - £3,000 | Low | Must meet Part B escape requirements |
Total range for a rear dormer loft conversion: £21,700 - £37,500. Add an en-suite bathroom and you are looking at another £4,000 - £8,000 on top. A hip-to-gable with rear dormer will sit closer to £45,000 - £65,000 once you factor in the additional structural work.
quoting loft conversions properly
The scaffolding is a known cost. The electrics are predictable. But the structural steel depends on what the engineer finds, and the staircase depends on the layout of the floor below. Stop adding a blanket contingency and start rating each line individually. A simulation built on honest risk ratings will give you a quote range that actually reflects reality — and protects your margin when the surprises hit.