It depends on your roof and how much space you need. A Velux (roof light) conversion keeps the roof shape and costs least. A rear dormer adds the most usable space for the money. Hip-to-gable suits semis and detached houses with a hipped roof. A mansard gives the most room but usually needs planning permission.
Is my loft suitable for a conversion?
Start with head height. As a rule of thumb, designers look for at least 2.2 m or so from the top of the ceiling joists to the underside of the ridge, because the new floor will sit higher than the old joists. You can check this yourself with a tape measure at the highest point of the loft. The pitch matters too: a steeper roof gives more floor area with comfortable standing height, and that is the space you will actually use.
Then look at how the roof is built. Older houses with traditional rafters and purlins are usually simpler to convert. Roofs made from trussed rafters, the W-shaped timbers common from the 1960s onwards, can still be converted, but trusses must not be cut without a structural engineer's design, and the steelwork adds cost. Water tanks, chimney stacks and room for a staircase all need planning in from the start.
What is a Velux loft conversion?
A roof light conversion adds windows set into the existing roof slope and leaves the roof shape alone. 'Velux' is a brand name that has become shorthand for this type. It is the least disruptive and usually the cheapest option, but you only gain the floor area that already has headroom. It suits lofts that are already tall, often in older houses with steeper roofs.
Roof lights are normally permitted development if they don't project more than 150 mm beyond the roof slope, don't sit higher than the highest part of the roof, and any on a side-facing slope are obscure-glazed and non-opening below 1.7 m. Typical West Midlands range: £22,000–£32,000.
What is a dormer loft conversion?
A dormer is a box-shaped extension built out from a roof slope, usually at the back, with vertical walls and a flat roof. A full-width rear dormer turns a sloping roof into near full-height space. It is a common choice on semis and terraces because it can fit a double bedroom and a small shower room.
Rear dormers are often permitted development within the volume limits: 40 m³ of extra roof space for a terraced house and 50 m³ for other houses, including any earlier roof extensions. The dormer must be set back about 20 cm from the eaves as far as practicable, must not extend beyond the roof slope at the front facing a road, must use materials similar in appearance to the house, and needs obscure-glazed side windows. Typical range: £38,000–£55,000.
What is a hip-to-gable loft conversion?
A hipped roof slopes down at the sides as well as the front and back. A hip-to-gable conversion builds the side wall up into a vertical gable and extends the ridge, so the roof space becomes much larger. It is often combined with a rear dormer and suits semi-detached and detached houses with a hipped roof, such as many 1930s semis.
Hip-to-gable work can fall under permitted development within the 50 m³ allowance, provided the other conditions are met. On a semi, new steel beams may bear on the party wall, which brings in the Party Wall Act. Typical range including a rear dormer: £48,000–£68,000.
What is a mansard loft conversion?
A mansard replaces one roof slope, usually the rear, with a near-vertical wall at around 70 degrees and a flat top, with windows set into small dormers. It gives the most space and a regular room shape, and is often seen on terraced houses.
Because it changes the shape of the roof so much, and often means raising party walls, a mansard usually needs full planning permission. It also takes longer to build. Party wall notices are almost always needed, and the roof is open for longer, so scaffolding and temporary weather protection make up a bigger share of the price. Mansards are priced individually, usually from around £60,000.
What Building Regulations apply to every loft conversion?
Whichever type you choose, converting a loft into a habitable room needs Building Regulations approval. A full plans application with structural calculations is the usual route, and the inspector visits at key stages, typically when the steels are in, before the insulation is covered and at completion. Building control will check:
- Structure: new floor joists sized for bedroom use, steel beams where needed, and any strengthening of the roof.
- Fire safety: mains-powered, interlinked smoke alarms in the hall or landing on every floor. Where the house becomes three storeys, a protected stair with fire doors to the habitable rooms, giving a safe route to an exit.
- Escape windows where they are needed: a clear opening of at least 0.33 m², at least 450 mm wide and high.
- Stairs: clear headroom of at least 2 m, although building control may accept 1.9 m at the centre and 1.8 m at the side in a loft.
- Insulation and ventilation to current standards, and sound insulation in the new floor.
- Party walls: in a semi or terrace, steels that bear on the party wall usually need a party structure notice under the Party Wall etc. Act 1996, served at least two months before that work starts.
How do the four loft conversion types compare?
The prices in this guide are typical West Midlands ranges for 2026, including VAT, for a standard specification with a shower room. Your survey and written quote set the real figure. If your house is in a conservation area or covered by an Article 4 direction, permitted development for roof extensions doesn't apply, so speak to the council first. Side by side:
- Velux: least space gained, least disruption, usually permitted development, typically 4–6 weeks on site.
- Rear dormer: most space for the money, often permitted development, typically 6–10 weeks on site.
- Hip-to-gable with rear dormer: a large gain on hipped roofs, often permitted development, typically 8–12 weeks.
- Mansard: the most space, usually needs planning permission, typically 10–14 weeks.
Sources
- Planning Portal – Loft conversion: planning permission
- legislation.gov.uk – GPDO 2015, Schedule 2, Part 1, Class C (roof alterations)
- Warwick District Council – Building regulations: loft conversions
- GOV.UK – The Party Wall etc. Act 1996: explanatory booklet
This guide is general information for homes in England, not legal or professional advice for your property. Rules change — check with your council or the Planning Portal before you rely on it.