A rear dormer adds a box to the back roof slope and often falls within permitted development. A mansard rebuilds the roof as a near-vertical structure with a shallow top, gains far more usable space, and in almost every case needs full planning permission.
Last checked against current guidance on 2026-08-09. Rules differ by borough — the local pages below say how.
A dormer projects from an existing slope and leaves the roof form intact; a mansard replaces the roof form itself.
That single structural difference drives everything else — the planning route, the party wall position, the head height you end up with, and how visible the result is from the street.
| Rear dormer | Mansard | |
|---|---|---|
| Roof form | Existing pitch retained; box added to the rear slope | Roof rebuilt: steep near-vertical face, shallow top |
| Usable floor area | Good — limited by the original ridge | Best — full-width, full-depth at near-full head height |
| Planning route | Often permitted development within the volume limits | Full planning permission in practice |
| Visibility from the street | Usually none, being to the rear | Visible; boroughs control the profile closely |
| Structural work | Moderate — trimming, new floor, steels | Substantial — new roof structure, often new party wall upstands |
| Party wall | Usually engaged | Almost always engaged on both sides in a terrace |
Often — if the total roof addition stays within 40 cubic metres on a terrace or 50 on a semi or detached house, and nothing extends beyond the roof plane of the principal elevation facing a highway.
The volume allowance is cumulative: an earlier conversion by a previous owner comes off your total. Materials must be similar in appearance, side windows must be obscure-glazed and non-opening below 1.7m, and other than a hip-to-gable the addition must be set back at least 0.2m from the original eaves.
Designated land removes the right — so in a conservation area a rear dormer is a planning application, not a certificate.
Because a mansard alters the front roof plane and the overall roof form, which takes it outside the permitted development conditions.
Several inner London boroughs publish detailed mansard guidance covering the pitch of the front face, the ridge position relative to neighbours, window proportions and materials. Where a terrace has an established mansard rhythm, matching it is usually the fastest route to approval; where a terrace has none, the case is harder and sometimes unwinnable.
This is where local knowledge outweighs national rules: the same mansard drawing can be routine in one borough and refused in the next.
Build cost moves with your builder, your specification and the month you tender. What you can pin down early is the design and compliance scope — how many drawing sets the council and building control will want, and whether structural input is in play. The estimator below works that out from the project type and floor area.
Move the controls for an indicative figure. It anchors your budget — it is not a quote. Every project is priced after a free survey.
Build costs are separate and quoted by your contractor. See our published fees.
A mansard, by a clear margin. Because the new roof rises almost vertically before turning, the floor area with full standing head height is much larger than under a pitched roof with a rear box.
Building regulations set the constraint that decides the layout: the stair needs 2m of headroom, and the loft needs a protected escape route. On a shallow roof the mansard is sometimes the only version that produces a compliant room at all.
Sometimes. The test is whether a compliant stair and 2m of headroom over it can be achieved. A mansard or a raised ridge may be the only way, and both need planning permission.
Yes. Designated land removes the Class B roof-addition right, so a rear dormer there is a full planning application.
It can be, within the same volume limits and outside designated land. It is also the one roof addition exempt from the 0.2m eaves set-back.
Structurally, yes — mansards in a terrace almost always involve work to the party walls, so notices under the Party Wall etc. Act 1996 are required.
A dormer within permitted development, via a lawful development certificate. A mansard runs the full householder application and may need pre-application discussion first.
