Design
Dormer vs Mansard: Which Suits Your Home?
A comparison of two very different architectural approaches to added space.
Dormer and mansard are often spoken of as alternatives. They are, in truth, quite different pieces of architecture — with different structural implications, different planning routes, and quite different results in the finished room.
The dormer
A dormer sits within the existing roof, projecting from it. It preserves the original roof profile at the front and sides, and creates a flat-ceilinged room within the new box at the rear. It is quicker to build, less structurally involved, and usually falls under permitted development.
A well-proportioned dormer, set carefully within the roofscape and clad in materials that respect the house, is one of the most quietly successful pieces of domestic architecture available. A poorly proportioned one is one of the most common mistakes on a British street.
The mansard
A mansard rebuilds the roof entirely, creating a nearly vertical wall on all sides and a shallow, hidden flat roof above. It generates the largest possible usable floor area, and — done correctly — reads as a natural part of the elevation rather than an addition.
It is a more considered piece of work: full planning permission is almost always required, structural steelwork is invariably involved, and specification-grade slate or lead detailing is expected. It is the right answer for terraced London homes, period villas, and any house where the existing roof gives little back.
Choosing between them
The honest answer is that the house tells you. A generously proportioned 1930s semi in Surrey rarely needs a mansard; a Victorian terrace in Chelsea rarely benefits from a dormer alone. We begin every project by sitting with the house for an afternoon, sketching both, and letting the roof lead the conversation.
In summary
- A dormer works within the existing roof; a mansard rebuilds it.
- Dormers are quicker and often permitted development; mansards need full planning.
- Mansards yield more usable floor area and cleaner ceilings.
- Let the house — and the street — decide.