In the mansard vs dormer decision, a dormer is usually cheaper, quicker and often doesn’t need planning permission. A mansard gives you more usable space and a more finished look from the street, but it costs more and normally needs full planning permission. For most London terraces, the choice comes down to how much space you need, your budget and what your council will approve.
Mansard vs dormer at a glance
| Dormer loft conversion | Mansard loft conversion | |
|---|---|---|
| What it is | A box-shaped extension built out from the sloping roof | The rear roof slope is rebuilt almost vertical, with a nearly flat top |
| Usable space | Good: full headroom where the dormer sits | The most space: close to a full extra storey |
| Planning permission | Rear dormers are often Permitted Development | Usually needs full planning permission |
| Typical London cost (2026) | £45,000–£75,000 | £65,000–£110,000 |
| Typical build time | Around 8–10 weeks | Around 10–14 weeks |
| Looks | Practical; can look boxy if poorly designed | Looks like part of the original house |
| Best for | Most terraced and semi-detached houses on a budget | Victorian and Edwardian terraces, and conservation areas where councils favour them |
What is a dormer loft conversion?
A dormer loft conversion adds a box-shaped structure that projects out from the sloping roof, usually at the back of the house. It has vertical walls and a flat or pitched ceiling, which turns sloping, low-headroom space into full-height room you can stand and walk around in.
It is the most common type of loft conversion in the UK, because it adds a lot of usable space for a reasonable cost and often doesn’t need planning permission.
Types of dormer
- Rear flat-roof dormer: the most common choice. It runs across most of the back roof slope and gives the most space for the money.
- L-shaped dormer: two dormers joined at a corner, one on the main roof and one on the rear outrigger. Popular on Victorian and Edwardian terraces, it can fit two bedrooms and a bathroom.
- Hip-to-gable with rear dormer: on semi-detached and end-of-terrace houses, the sloping side roof is built up into a vertical wall first, then a dormer is added at the back.
- Pitched or hipped-roof dormer: smaller dormers with their own sloping roofs. They look more traditional, especially on the front of a house, but give less internal space and cost more to build.
See our guide to types of loft conversion for more.
What is a mansard loft conversion?
A mansard loft conversion replaces the rear roof slope with a new one that is almost vertical, usually at around 70 degrees, topped with a nearly flat roof. Windows sit in small dormers set into the steep slope. The result is close to a full extra storey, with head height across almost the whole floor.
The style is named after François Mansart, a 17th-century French architect. It is a familiar sight on London’s Georgian and Victorian terraces, which is one reason many London councils accept it, especially where neighbours already have one.
Types of mansard
- Rear mansard: the most common type, built on the back roof slope only.
- L-shaped mansard: a mansard on the main roof and the rear outrigger, giving the most space. Ideal for a large bedroom with an en-suite, or two bedrooms.
- Double mansard: mansards on both the front and back of the house. It gives the most space of all, but many councils restrict changes to the front roof, so it is harder to get approved.
Mansard vs dormer: the key differences explained
Space
A mansard usually gives more usable floor area than a dormer on the same house. Its steep slope keeps the walls close to vertical, so there is full headroom across almost the whole floor. A dormer gives full height where the box sits, but the front of the loft keeps the original roof slope. On a typical London terrace, an L-shaped mansard often fits a large bedroom with en-suite, or two bedrooms. An L-shaped dormer can do the same, but the rooms may feel slightly smaller.
Planning permission
This is often the deciding factor. Rear dormers within 40 cubic metres (terraced) or 50 cubic metres (semi-detached and detached) usually fall under Permitted Development, so no planning application is needed. A mansard almost always needs full planning permission, which adds around eight weeks and a £548 application fee in England. In conservation areas, the position can reverse: some councils will refuse a box dormer but approve a well-designed mansard. Read our planning guide.
Cost
A mansard costs more because more of the roof is rebuilt, the party walls are usually raised, and the finishes (often slate and lead-clad dormers) are more expensive. In London in 2026, a rear dormer typically costs £45,000–£75,000 and a mansard £65,000–£110,000. See our loft conversion cost guide.
Build time
A dormer is usually built in around 8–10 weeks. A mansard takes longer, typically 10–14 weeks, because of the extra structural work. Allow extra time before either for design, Building Regulations and party wall notices, plus planning for a mansard.
Looks
A well-designed mansard looks like it has always been part of the house, which is why planners often prefer it in period streets. A dormer is more practical than beautiful. It looks best when it matches the house’s materials and is set back from the eaves and party walls.
Value
Both add value by adding a bedroom, and often a bathroom. A mansard may add a little more because of the extra space and finish, but it also costs more to build. Ask a local estate agent what a four-bedroom house sells for in your street compared with a three-bedroom one.
Should I choose a mansard or a dormer?
The right choice depends mostly on your house and your street.
| Your house | Usually the better choice | Why |
|---|---|---|
| Victorian or Edwardian terrace with a rear outrigger | L-shaped dormer, or L-shaped mansard if neighbours have them | Both add a bedroom and bathroom; a mansard gives more space if the council supports it |
| 1930s semi-detached house | Hip-to-gable with rear dormer | Mansards are rare on semis; this combination gives the most space and is often Permitted Development |
| House in a conservation area | Mansard, or a small, carefully designed dormer | Many councils resist large box dormers in conservation areas |
| Flat or maisonette | Either, with full planning permission | Flats have no Permitted Development rights; you’ll also need the freeholder’s consent |
| Tight budget or timescale | Rear dormer | Cheaper, quicker and usually no planning application |
A good first step is to look at your street. If neighbours have mansards, the council has probably approved them before. If they have dormers, that tells you what’s normal too. See our guides to loft conversions in conservation areas and loft conversions in flats.
Not sure which would work on your house? We can visit, measure up and show you both options, with costs.
Mansard vs dormer: frequently asked questions
Which gives more space, a mansard or a dormer?
A mansard usually does. Its almost vertical rear wall gives full headroom across nearly the whole floor, while a dormer keeps the original roof slope at the front of the loft.
Is a mansard more expensive than a dormer?
Yes, usually by £20,000 or more in London. A mansard involves rebuilding more of the roof, often raising the party walls, and more expensive finishes. It also normally needs a planning application.
Does a dormer loft conversion need planning permission?
Often not. A rear dormer within the Permitted Development limits (40 cubic metres for a terraced house, 50 for semi-detached and detached houses) doesn’t need a planning application. Front dormers, flats and houses in conservation areas usually do.
Does a mansard loft conversion need planning permission?
Almost always. Because a mansard changes the shape of the roof significantly, it is usually outside Permitted Development limits. Many London councils approve them, especially where neighbouring houses already have one.
Can you convert a dormer into a mansard later?
It is possible, but it means rebuilding the roof again, so it is rarely good value. If you think you’ll want the extra space of a mansard, it is better to build one from the start.
Which adds more value to a house?
Both add value mainly by adding a bedroom and bathroom. A mansard can add slightly more because of the extra space and finish, but it costs more to build, so the return on what you spend is often similar.
Still deciding between a mansard and a dormer?
The best way to decide on mansard vs dormer is to see both options drawn for your own house, with real costs and a clear view of what your council is likely to approve. We can visit, measure your loft and talk you through both, with no obligation.
