There are five loft conversion types: Velux (rooflight), dormer, L-shaped dormer, hip-to-gable and mansard. Velux conversions are cheapest and least likely to need planning permission; mansards create the most space but almost always require a full application. Every type requires structural calculations, because existing ceiling joists are rarely adequate as floor joists.
Loft conversions are among the most cost-effective ways to add usable space, because the structure is largely there already. But "loft conversion" covers five quite different approaches, with very different costs and planning implications. Here's how they compare.
Is my loft suitable for conversion?
Before the type matters, the roof has to be suitable. The usual constraints are head height, roof structure and access. As a rough guide you want a reasonable amount of height at the ridge — measured from the existing ceiling joists to the underside of the ridge. Traditional cut-and-pitch roofs are generally easier to convert than modern trussed roofs, where the webbing that makes trusses efficient also fills the space you want to use. Trussed roofs can still be converted, but the structural work is more involved.
| Type | Space gained | Usual planning route | Best suited to |
|---|---|---|---|
| Velux / rooflight | Lowest | Often permitted development | Roofs with existing generous head height |
| Dormer | High | Often PD, subject to volume limits | Standard terraced and semi-detached rear roofs |
| L-shaped dormer | Highest of the dormer types | Often needs full application | Period terraces with a rear outrigger |
| Hip-to-gable | High | Depends on scale and location | Detached, semi-detached and end-terrace hipped roofs |
| Mansard | Highest overall | Almost always full application | Maximum space; common in London conservation areas |
Don't forget the staircase. Building regulations require proper access, and the stairs have to land somewhere sensible — that's often the real constraint on the floor below, not the loft itself.
1. Velux (rooflight) conversion
The simplest option: the roof shape is untouched and windows are fitted into the existing slope. There's no extension of the roof volume, so it's the cheapest route and the least likely to require planning permission.
Best for: roofs that already have generous head height. Trade-off: no additional floor area with usable standing height — you're limited by the existing slope.
2. Dormer conversion
A box-shaped structure projecting from the rear roof slope, creating a section of flat ceiling and full head height. It's the most common conversion in UK terraced and semi-detached houses because it converts a cramped loft into a genuinely usable room.
Best for: maximising usable space on a standard rear roof. Trade-off: the external appearance changes noticeably, and rear dormers are subject to volume limits.
3. L-shaped dormer conversion
Two dormers joined at right angles, typically extending over the main rear roof and a rear outrigger — the back addition many Victorian and Edwardian terraces have. This creates the most space of any dormer arrangement, often enough for a double bedroom plus a bathroom.
Best for: period terraces with a rear outrigger. Trade-off: more complex structurally, and more likely to need a full application.
4. Hip-to-gable conversion
For houses with a hipped roof — one that slopes on three sides — this rebuilds the sloping side end as a vertical gable wall, squaring off the roof and reclaiming the volume the hip was wasting. Frequently combined with a rear dormer for maximum effect.
Best for: detached, semi-detached and end-terrace houses with hipped roofs. Trade-off: more building work than a simple dormer, and a bigger visual change.
5. Mansard conversion
The most substantial option: the roof structure is largely rebuilt with a steep, near-vertical rear slope and a shallow, near-flat top. It creates the most internal space and can look the most architecturally coherent on a period property, which is why it's common in London conservation areas — but it's essentially rebuilding the roof.
Best for: maximum space, and areas where councils prefer the mansard form. Trade-off: the most expensive route and almost always requires full planning permission.
What does every loft conversion require?
Whichever type you choose, the same fundamentals apply:
- Structural calculations — existing ceiling joists are almost never adequate as floor joists, so new floor structure is required
- Fire safety — a protected escape route, fire doors and alarms are building regulations requirements, not optional extras
- Insulation and ventilation — to meet current standards and avoid condensation problems
- Compliant stairs — with adequate headroom over the flight
Getting the drawings right
We produce planning drawings and full building regulations packages for all five conversion types, with engineer-signed structural calculations included in the Full Package at £1,650. Planning drawings alone start at £1,100, and every project is delivered within two weeks. You can see examples of completed loft conversions in our project portfolio.
Do loft conversion rules differ across London and Surrey?
The underlying volume allowances are national, but their application varies locally. London boroughs frequently operate conservation area controls and Article 4 directions that restrict rear dormers, while mansard forms are sometimes preferred on period streets for their appearance.
We prepare loft conversion drawings for authorities including Lambeth, Wandsworth, Merton, Croydon, Richmond upon Thames and Kingston upon Thames, and across Surrey including Elmbridge, Guildford, Woking and Waverley.
Please note: this guide is general information for UK homeowners, not a formal planning determination. Rules vary by local authority and by property — conservation areas, listed buildings and Article 4 directions all change what applies. We confirm exactly what your project needs as part of your free quote.