Loft conversions range from a simple rooflight scheme that barely changes the roofline to a full mansard that effectively rebuilds it, and the cost, headroom gained and planning implications shift considerably between them. Which type suits a given house is largely dictated by existing roof pitch, ridge height and available headroom rather than preference alone. This guide compares the four common types so you can work out which is realistic before commissioning drawings.
The four main types
A rooflight or Velux conversion adds windows into the existing roof slope without altering its shape, making it the least disruptive and cheapest option, but it depends on already having reasonable head height in the loft. A dormer conversion adds a box-shaped extension to the rear roof slope, creating substantial extra headroom and floor space and remaining the most popular choice for typical UK roofs. A hip-to-gable conversion extends a sloped hip end into a vertical gable wall, usually suiting semi-detached and detached houses with a hipped roof, and is often combined with a rear dormer for maximum space. A mansard conversion replaces most of one roof slope with a near-vertical wall and a shallow-pitched roof, delivering the most usable space but involving the most substantial structural work.
Comparison table
The table below sets out indicative costs, head-height suitability and planning implications for each type, based on a typical UK semi-detached or terraced house.
| Type | Typical cost range | Head-height needs | Planning implications |
|---|---|---|---|
| Rooflight/Velux | £20,000–£30,000 | Needs at least 2.2m existing ridge height | Usually permitted development |
| Dormer | £35,000–£50,000 | Suits most standard roofs, adds full headroom to rear | Often permitted development, subject to volume limits |
| Hip-to-gable | £40,000–£55,000 | Adds headroom at the hip end, best with hipped roofs | Often permitted development, but check corner-plot rules |
| Mansard | £50,000–£70,000+ | Delivers the most headroom across the space | Usually needs full planning permission |
Indicative UK ranges for 2026, varying by region, roof size and finish specification. A site visit and roof survey are needed for a real figure.
Head-height and ridge height
Building regulations do not set a strict minimum ceiling height, but in practice a usable habitable room needs around 2.2 to 2.3 metres of clear height over a reasonable portion of the floor area, factoring in insulation and floor build-up added during conversion. Existing ridge height, measured from the loft floor to the underside of the ridge board, is the single biggest factor in whether a straightforward rooflight conversion will work, or whether a dormer or more substantial structural change is needed to gain enough usable space.
Permitted development limits
Most loft conversions on houses (not flats or maisonettes) fall under permitted development rights under the GPDO, subject to volume limits of 40 cubic metres for terraced houses and 50 cubic metres for detached and semi-detached houses. Extensions must not go beyond the existing roof plane at the front, and materials must be similar in appearance to the existing house in most cases.
- Volume added must sit within GPDO limits for the house type
- No extension beyond the principal elevation roof plane
- Side-facing windows should be obscure-glazed and non-opening below 1.7m
- Article 4 directions or removed permitted development rights override the above
When full planning is needed
Mansard conversions almost always need full planning permission because they alter the roof shape substantially, changing the building's overall appearance. Conservation areas, listed buildings, and properties where permitted development rights have already been used up by a previous extension also typically require an application regardless of loft conversion type.
Structural and staircase considerations
Every loft conversion needs a structural assessment of the existing roof and floor joists, as loft floors are rarely designed to carry habitable loads without strengthening. A new staircase also has to be accommodated within the existing floor plan below, which can be one of the more disruptive parts of the project even though it rarely dominates the budget.
Building regulations
Regardless of planning route, all loft conversions require building regulations approval, covering structural adequacy (Part A), fire escape provision including protected stairwells and fire doors (Part B), thermal insulation (Part L), and sound insulation between the new room and the floor below.
Typical timeline
A rooflight conversion typically takes six to eight weeks on site, while a dormer or hip-to-gable conversion usually runs ten to fourteen weeks given the additional roof structure, weatherproofing and staircase work involved. A mansard conversion, being the most substantial structural change, can take fourteen to twenty weeks. Add several weeks either side for drawings, structural calculations, and where full planning permission is needed, the eight-week statutory determination period, which can extend further if the application is called in for committee or needs revisions.
Comparing loft conversion quotes
Loft conversion quotes vary widely depending on whether they include the staircase, en suite plumbing, roof windows, and making good to the rooms below where new structural supports pass through. Ask each contractor to confirm whether structural engineer fees, building control fees and party wall costs — relevant on semi-detached and terraced houses where the roof structure is shared or close to a boundary — sit inside or outside the headline figure, since these can otherwise appear as unexpected additions later.
- Confirm whether the staircase and any structural support work below is included
- Check if roof windows, dormer cladding and insulation meet current Part L standards
- Ask whether party wall costs are included, relevant on semi-detached and terraced roofs
- Clarify who arranges building control inspections and sign-off