Measure the head height at the ridge first. Under about 2.2m and a rooflight conversion will not work, so you need a dormer or a roof restructure. A Velux conversion is cheapest and adds no volume; a dormer adds usable floor area; hip-to-gable converts a sloping side into a vertical wall; a mansard rebuilds the roof and adds the most, at the highest cost.
- Take one measurement first: floor to underside of ridge. Everything follows from it.
- Velux and most rear dormers often fall within permitted development; mansards usually do not.
- Hip-to-gable suits semis and detached houses with a hipped roof, which is most interwar housing.
- Headroom over the new stair is the constraint that kills more schemes than anything else.
Measure before you choose
One measurement determines which of these is even available, and it takes two minutes.
Stand in the loft and measure from the top of the existing joists to the underside of the ridge. That is your starting head height, and the finished height will be less — a new floor structure and insulation take up height at the bottom, and insulation plus plasterboard takes more at the top.
As a working guide:
| Height to ridge | What is realistic |
|---|---|
| Over 2.8m | Any type; comfortable throughout |
| 2.4m to 2.8m | Rooflight conversion viable; dormer gives more usable area |
| 2.2m to 2.4m | Tight for rooflight only; dormer or restructure advisable |
| Under 2.2m | Rooflight will not work; dormer, mansard or lower the ceilings below |
| Trussed roof (W-shaped timbers) | Any type needs the roof restructured first |
The last row matters and is often missed. Houses built from roughly the 1960s onwards commonly have trussed rafters — prefabricated W-shaped frames whose diagonal members run straight through the middle of the space. They cannot simply be cut, so the roof has to be restructured with a new supporting arrangement before any conversion, which adds significant cost. Older cut roofs, with purlins and rafters, are far more adaptable. Note too that Building Regulations set a minimum headroom over the staircase rather than a minimum ceiling height for the room, and that staircase headroom is what most often defeats a marginal loft.
The four types side by side
| Velux / rooflight | Dormer | Hip-to-gable | Mansard | |
|---|---|---|---|---|
| Changes the roof shape | No | Yes, locally | Yes, one side | Yes, entirely |
| Adds floor area with full height | None | Significant | Significant | Most |
| Relative cost | Lowest | Middle | Middle to high | Highest |
| Usually permitted development | Commonly | Rear often | Often | Rarely |
| Suits which roof | Steep, tall roofs | Most | Hipped roofs | Most, incl. terraces |
| Build time | Shortest | Moderate | Moderate | Longest |
| External appearance | Barely changes | Visible at the rear | Changes the side | Changes the whole roof |
| Usual outcome | Room with sloping walls | Bedroom plus ensuite | Bedroom plus ensuite | One or two rooms |
Front-facing alterations are treated differently from rear ones throughout. A rear dormer frequently falls within permitted development where the same dormer on the front elevation would need an application.
Velux or rooflight conversion
The simplest option: the existing roof is left as it is and rooflights are fitted into the slope.
What you get. A room with sloping ceilings on both sides and a flat section in the middle. Usable floor area is limited to where the headroom is adequate, so a large loft can yield a surprisingly modest room.
Why it is cheaper. No structural alteration to the roof outline, no new roof covering, less scaffolding time, and usually a simpler planning position. The work is a new floor structure, insulation, the rooflights, the stair, and fit-out.
When it is right. A tall, steep roof with generous height at the ridge, and a brief that suits a study, a guest room or a child's bedroom rather than a principal bedroom.
Where it disappoints. Marginal head height. People measure 2.3m at the ridge, imagine a room, and end up with a space they stoop in at the edges. Mark out on the loft floor where the finished height exceeds 1.5m and look at what is left — that is your actual room.
Rooflights also perform well for daylight, giving more light than a vertical window of the same area, which is a genuine advantage of this type.
Dormer, hip-to-gable and mansard
Dormer
A box built out from the roof slope, with a flat or pitched roof and vertical windows. It converts sloping ceiling into full-height space, which is why it adds so much more usable area than rooflights. A full-width rear dormer is the standard answer on a terrace or semi and often falls within permitted development, subject to the limits on volume, set-back from the eaves and height against the ridge. The appearance at the rear changes noticeably, which matters less than people expect because it is rarely seen from the street.
Hip-to-gable
Where the roof slopes on the side as well as the front and rear — a hipped roof, which describes most interwar semis and detached houses — the sloping side is built up vertically to form a gable. That converts a large triangle of unusable space into full-height room, and it creates a flat wall that a rear dormer can then be built against. The two are frequently done together, which is the standard loft solution on a 1930s semi. It is more visible from the street than a rear dormer, and in streets where the hipped roofline is consistent that can attract comment.
Mansard
The roof is substantially rebuilt, with the rear or both slopes made nearly vertical — typically around 70 degrees — and a shallow roof across the top. It produces the most space of any type, with near-full height across almost the whole footprint, and it can create two rooms where other types create one. It is the most expensive and the longest, it almost always needs planning permission, and it is most common on terraces and in urban areas where it is an established local form. In a conservation area a mansard may be the only type acceptable, or the only one refused, depending entirely on local character.
L-shaped dormer
Worth knowing as a variant. On a house with an existing rear outrigger or back addition, a dormer over the main roof can be joined to one over the addition, forming an L. It yields more space than a single dormer — often two bedrooms and a bathroom — and it is common on Victorian and Edwardian terraces with a rear wing.
The things that decide it besides the roof
- Where the stair goes. It has to rise from a sensible place on the floor below, land with adequate headroom, and not destroy a bedroom in the process. More schemes fail here than on roof geometry.
- Fire escape and protection. A habitable loft storey requires a protected escape route, which usually means fire doors to the rooms off the stair enclosure and interlinked alarms. On a third storey the requirements step up further.
- Whether it counts as a bedroom. A loft room that does not meet the escape and headroom requirements cannot honestly be marketed as a bedroom, which affects value and is a question a buyer's solicitor asks.
- The water tank and the boiler. Both frequently live in the loft and both need relocating, which may mean changing to a system that does not need a tank.
- Insulation and ventilation. The roof build-up has to meet current standards and be ventilated, or condensation forms in the structure. Spray foam already in the roof is a problem to resolve first.
- The party wall. On a semi or terrace, building into or off the shared wall engages the Party Wall Act, as does most structural work at this level.
- Building Regulations approval, which is required regardless of whether planning permission is.
The planning position is covered separately, and the short version is that permitted development often covers rear dormers and hip-to-gable work but almost never a mansard or a front-facing alteration.
What it costs
These are the loft conversion figures published across this site:
| Job | Typical Coventry guide price | Notes |
|---|---|---|
| Velux / rooflight conversion | £25,000–£35,000 | Simplest type |
| Dormer conversion | £35,000–£50,000 | Adds head height & space |
| Hip-to-gable | £45,000–£65,000 | End-of-terrace / semi |
| Average loft conversion | ~£35,371 | Type & extras dependent |
A trussed roof needing restructuring adds to all of these, and it is the item most likely to move a quote substantially. Relocating a water tank or boiler, and upgrading the escape route and alarms, are also separate from the shell cost.
We measure the ridge height and look at the roof structure and the stair position, which between them usually answer it.
Frequently asked questions
What head height do I need for a loft conversion?
Measure from the top of the existing joists to the underside of the ridge. Over 2.8m works for any type; 2.4m to 2.8m suits a rooflight conversion; under 2.2m rules a rooflight out, leaving a dormer, a mansard or lowering the ceilings below. The finished height is always less than the measurement.
What is the cheapest type of loft conversion?
A Velux or rooflight conversion, because the roof outline is unchanged — no new covering, less scaffolding and usually a simpler planning position. The trade-off is that the usable area is limited to where headroom is adequate, so a large loft can yield a modest room with sloping walls.
What is a hip-to-gable conversion?
Where the roof slopes on the side as well as the front and rear, the sloping side is built up vertically to form a gable. That turns a large triangle of unusable space into full-height room and creates a flat wall a rear dormer can be built against. It is the standard solution on a 1930s semi.
What is a mansard loft conversion?
The roof is substantially rebuilt with the slopes made nearly vertical, around 70 degrees, and a shallow roof across the top. It produces the most space of any type and can create two rooms where others create one. It is the most expensive and almost always needs planning permission.
Does a loft conversion need planning permission?
Often not. Rear dormers and hip-to-gable work frequently fall within permitted development, subject to limits on volume, set-back from the eaves and height against the ridge. Mansards and front-facing alterations almost always need an application. Building Regulations approval is required either way.
Can a trussed roof be converted?
Yes, but the roof has to be restructured first. Trussed rafters are prefabricated W-shaped frames whose diagonal members run through the middle of the space, and they cannot simply be cut. A new supporting arrangement is needed, which adds significant cost. Older cut roofs with purlins are far more adaptable.
One measurement answers most of it
We measure the ridge height, check whether the roof is cut or trussed and look at where a stair can land. That tells you which types are actually available.