The short answer
A dormer is cheaper, faster and often achievable without planning permission; a mansard delivers more internal volume at higher cost and almost always needs planning. The choice typically comes down to budget, planning environment and how much space you need. See loft conversion costs for a side-by-side cost comparison across all types.
The dormer and the mansard are the two conversion types that maximise usable floor area in a loft. They are not interchangeable — one involves adding a box to an existing roof while the other involves rebuilding the roof itself. The right choice depends on your planning situation, your budget, how much space you need and what the street scene in your area expects. In London in particular, planners have strong opinions about which type fits the local character, and understanding that context before designing your project saves a planning application that was always going to fail.
Dormer vs mansard at a glance
- Dormer cost £30,000–£55,000 typical
- Mansard cost £45,000–£75,000+ typical
- Dormer planning Rear usually PD; front needs permission
- Mansard planning Almost always required
- Space: dormer Large rear zone at full height
- Space: mansard Maximum — near-vertical walls both sides
What a dormer involves
A dormer adds a box-shaped structure projecting vertically from the roof slope. The key advantage is that it can often be built on the rear of the property as permitted development, avoiding a planning application entirely. It creates a substantial zone of full-height floor area within the new room — typically the rear two-thirds of the loft on a full-width dormer — while the remaining front section retains the original sloping ceiling. The dormer is more limited than a mansard: the space in front of the dormer box is still constrained by the original roof pitch, so a full dormer room has both full-height and lower-ceiling zones. Most homeowners find this entirely acceptable for a bedroom; it can be more limiting for a larger, open-plan room.
What a mansard involves
A mansard conversion rebuilds one or both sides of the roof as near-vertical walls (typically 70–72 degrees from horizontal) topped by a shallow flat or low-pitched roof. The result is that virtually the entire footprint of the house is at full ceiling height in the new loft room — you lose very little space to sloping eaves. A mansard gives the maximum possible volume within the existing house envelope and, in many London streets, matches the established local precedent for conversion. It is significantly more expensive because the construction is more complex: the roof is effectively rebuilt, with new structural walls, a new roof structure and a new external envelope. It also requires planning permission in almost every case.
| Factor | Dormer | Mansard |
|---|---|---|
| Typical cost | £30,000–£55,000 | £45,000–£75,000+ |
| Build time | 7–12 weeks | 10–14 weeks |
| Planning (rear) | Often PD | Almost always required |
| Full-height area | Partial (dormer zone only) | Near full footprint |
| Disruption | Medium | High — roof is rebuilt |
| London prevalence | Common | Very common in inner London |
When a mansard makes sense
A mansard tends to be the right choice when: you are in London or another city where planning policy or street precedent expects a mansard; you need a large, full-height room or two rooms with maximum floor area; your planning position means a planning application is unavoidable anyway; or the additional space gained is important enough to justify the cost premium. In many inner-London terraces, a mansard is what neighbours have done and what planners expect — a dormer application may be refused while a mansard would be approved. In those situations, the “cheaper” dormer is not actually cheaper if it requires a refused planning application, an appeal and then a mansard application.
When a dormer is the smarter choice
Outside London, in most suburban and provincial streets, a rear dormer is the dominant conversion type and planning officers are familiar with approving it. If your property is outside a conservation area and the rear dormer fits within permitted development limits, avoiding the planning process entirely saves both time (typically 8–10 weeks) and money. The space produced by a full-width rear dormer is adequate for almost all bedroom and en-suite combinations. The £15,000–£25,000 premium of a mansard over a dormer is difficult to justify on added value alone in most non-London markets. See does a loft conversion add value? for the cost-versus-return analysis. This page is general information and not professional planning or design advice. Your architect and local planning authority are the definitive sources for what will be approved in your area.
Get the right recommendation for your property
A specialist can assess your planning position, local precedent and budget to recommend whether a dormer or mansard is the right call for your specific property and street.
Frequently asked questions
Is a mansard always better than a dormer?
No. A mansard gives more full-height space but costs significantly more and almost always needs planning permission. A dormer is often achievable as permitted development, is faster and is the more cost-effective choice for most properties outside London.
Does a mansard need planning permission?
Almost always. A mansard conversion fundamentally changes the roof profile and appearance of the building, which takes it outside permitted development rules. A planning application — including architectural drawings — is needed in virtually all cases.
Which is better in London — dormer or mansard?
In many London boroughs, particularly inner London conservation areas and streets where precedent is established, a mansard is preferred by planners. Check local planning decisions on neighbouring properties before fixing your design — a mansard that matches the street may be easier to approve than a dormer.
How much more does a mansard cost than a dormer?
Typically £15,000–£25,000 more for an equivalent project, with a mansard ranging from £45,000 to £75,000+ versus a dormer at £30,000–£55,000. The premium reflects the more extensive structural work and larger external envelope.
Sources & further reading
- Planning Portal — planning permission requirements for mansard conversions and dormer volume limits
- GOV.UK — Town and Country Planning (General Permitted Development) (England) Order 2015
- RICS — guidance on loft conversion value and planning considerations
- LABC (Local Authority Building Control) — building regulations for mansard and dormer conversions
This is general information about loft conversions in the UK, not professional planning, structural, building or legal advice. Costs are typical illustrations, not quotes; timescales and outcomes vary with your property, location and chosen specialist. Always consult a qualified specialist and your local planning authority before starting work.