The four common types, side by side
There are four common types of UK loft conversion — Velux, dormer, hip-to-gable, and mansard. Each has a tight 2026 price band, a different structural scope, and a different fit with the permitted development (PD) rules. The cheapest type is not always the right type, because the cheapest type is the one that fits the existing roof with the least structural work.
| Type | 2026 UK price band (3-bed semi) | Structural work | PD usually OK? | |------|--------------------------------:|-----------------|---------------| | Velux (rooflight-only) | £20,000–£35,000 | Minimal | Yes | | Dormer | £30,000–£55,000 | New dormer walls + roof | Yes (rear) | | Hip-to-gable | £35,000–£60,000 | Replace hipped end with gable + dormer | Yes | | Mansard | £45,000–£75,000 | Rebuild rear slope | No — full planning |
For the full national price guide — including regional multipliers, cost-per-m² bands, and what's included in a quote — see Pillar 2: How Much Does a Loft Conversion Cost in 2026? UK Price Guide by Type.
Velux (rooflight-only)
A Velux conversion is the lightest-touch option. The existing roof shape is unchanged, and rooflights are installed between the rafters, typically on the rear slope. There is no new wall, no new roof structure, and no extension of the building envelope.
Best for:
- Properties with at least 2.2 m of usable ridge headroom.
- Houses with an existing roof shape that does not need extending to be useful.
- Budget-led projects where £20k–£35k is the realistic ceiling.
Limitations:
- Floor area is constrained by the existing roof slope — you cannot extend the loft horizontally.
- Headroom falls away sharply at the eaves, so the usable floor area is usually the central 60–70% of the loft footprint.
- Less suitable for a generous en-suite: a dormer usually gives 30–50% more usable floor area for the same ridge height.
PD position: Almost always PD under GPDO 2015 Schedule 2 Part 1 Class C, which permits rooflight installation subject to a 150 mm projection cap and the obscure-glazing rule for side-facing rooflights. No planning application required in the vast majority of cases.
Dormer
A dormer is a box-shaped extension that projects vertically from a sloping roof, typically to the rear. It is by far the most popular type of loft conversion in the UK because it gives the most usable floor area and headroom for the money, and is almost always PD if it sits on the rear slope.
2026 cost: £30,000–£55,000 (national, 3-bed semi). Most popular choice across all 2026 UK pricing guides.
Best for:
- Most UK 3-bed semis where the existing ridge headroom is marginal (2.0 m or less).
- Homeowners who want a generous bedroom plus a proper en-suite.
- Properties where the rear garden has enough depth to absorb a 3–4 m dormer projection.
Variations:
- Single dormer (rear): the standard 3-bed semi choice. £30k–£45k.
- L-shaped dormer (rear + side return): adds a side wing to the dormer, doubling floor area. £45k–£65k, depending on the side return depth.
- Full-width dormer: spans the entire rear of the house, often used on detached properties. £45k–£70k.
PD position: A rear dormer is usually PD under GPDO 2015 Schedule 2 Part 1 Class B, subject to the 40/50 m³ volume cap, the "must not extend beyond the plane of the original roof slope fronting the highway" rule, and the standard Class B conditions on materials and eaves set-back. A front dormer needs a full planning application.
The full PD rules, including when a dormer needs full planning, are covered in Pillar 1: The Complete UK Guide to Planning Permission for a Loft Conversion.
Hip-to-gable
A hip-to-gable conversion replaces the sloping hipped end of a roof with a vertical gable wall, and usually adds a rear dormer to the new vertical face. It is the standard answer for semi-detached and detached houses with a hipped roof — typically 1930s semis in the South East and many detached houses across the UK.
2026 cost: £35,000–£60,000 (national, 3-bed semi).
Best for:
- Semi-detached and detached houses with a hipped end.
- Properties where the hipped end is so shallow that a Velux or a rear dormer would not give usable headroom.
- Homeowners who want the maximum floor area without paying for a mansard.
Structural notes:
- The hipped end is rebuilt as a vertical gable wall, supported by a new steel beam at the ridge.
- The rear dormer is then a standard Class B dormer against the new gable.
- The work involves more steel and more structural carpentry than a straight dormer, which is why the band is £5,000–£10,000 above the dormer baseline.
PD position: Usually PD under GPDO 2015 Schedule 2 Part 1 Class B, with the same 40/50 m³ volume cap and the same position rules as a dormer. Article 2(3) land and listed buildings force a full planning application.
Mansard
A mansard is a near-vertical (typically 70°) rear roof plane that effectively creates a full new storey. It is the most expensive type per m² because it requires rebuilding the entire rear slope, almost always needs a full planning application, and frequently forces a redesign of the staircase below to meet Part K headroom.
2026 cost: £45,000–£75,000 (national, 3-bed semi), with London and high-spec projects frequently running £80,000–£100,000+.
Best for:
- Properties where the homeowner wants the maximum additional floor area.
- Detached houses in conservation areas where a full planning application is unavoidable anyway.
- Projects with a clear budget headroom (£75k+) and a willingness to commit to a 10–14 week on-site build.
PD position: A mansard always needs a full householder planning application. The 70° rear slope, the volume of the rebuild, and the materials-matching condition almost always breach the Class B limits, so PD is not available. Add the £548 planning application fee (from 1 April 2026), the architect’s full planning drawings pack, and the 8-week determination period to the project timeline.
Which type is right for your house?
A practical decision tree for 2026:
- Ridge headroom 2.2 m or more, no need for an en-suite? → Velux (£20k–£35k).
- Ridge headroom 2.0–2.2 m, want a proper en-suite, 3-bed semi with rear garden depth 4 m+? → Rear dormer (£30k–£55k).
- Semi-detached or detached with a hipped end, ridge headroom under 2.0 m? → Hip-to-gable + rear dormer (£35k–£60k).
- Detached with a large rear garden, want maximum floor area, willing to commit to a full planning application? → Mansard (£45k–£75k+).
Headroom and the Part K rule
The single most common reason a Velux is not feasible is headroom. Approved Document Part K (Protection from falling, collision and impact) requires 2.0 m of headroom above the pitch line of the stair where reasonably practicable, with a possible relaxation to 1.9 m centrally and 1.8 m at the worst point where space is very constrained.
In practice, a Velux conversion only works if the existing ridge height gives 2.2 m+ of usable headroom at the centre of the loft. Below 2.0 m, even with a Velux, the room will not satisfy Part K and Building Control will require a redesign — usually to a dormer or a hip-to-gable.
This is why the measured building survey matters: the architect cannot price a Velux conversion without knowing the existing ridge height. A £1,500 survey is the difference between a Velux quote that works and a forced redesign to a dormer at £30,000+ more.
icelabz provides RICS-compliant measured building surveys across London and the South East, with deliverables in 2D CAD and (optionally) Revit BIM and point cloud. Contact us for a fixed-fee quote and a typical 10–15 working-day turnaround.