The most space of any conversion type, the most structural work, and the one most likely to need planning permission.
Arrange a site visitA mansard rebuilds the roof rather than extending it. The rear slope is replaced with a wall set at around 70 degrees — close to vertical — with a shallow, almost flat roof across the top. The result is very close to a full additional storey, with near-vertical walls on the main elevation and usable floor area right out to the edges.
It is the most expensive and most involved of the conversion types, because a large part of the existing roof structure is taken away and rebuilt. In return it gives more usable space than any other option, and it is often the only type that yields two rooms plus a bathroom out of a single roof.
Because a mansard alters the shape of the roof substantially, it usually exceeds the permitted development volume allowance and generally needs a full planning application. That is not a reason to avoid it — mansards are consented regularly, particularly where the form already exists on the street — but it does mean the timeline includes a determination period, and the design has to respond to what the council expects on that elevation.
Cardiff's conservation areas are worth checking early. In some of them a mansard is the preferred form precisely because it reads as part of the original roof rather than as a box bolted onto it. In others it will not be entertained. That answer is worth having before drawings are commissioned. See planning permission and building regulations for the detail.
A mansard removes a substantial part of the load path and replaces it, which means steelwork, a new floor structure, and party wall work on both sides if you are mid-terrace. Two Party Wall notices rather than one is common, and both carry notice periods. Access for materials and for removing the old roof is a genuine planning consideration on a terraced street without rear access.
Published UK guides put mansard conversions at roughly £45,000 to £60,000 for around 30 square metres in 2026, with inner-London figures running far above that. These are national averages, not our quotes. On site, a mansard is the longest of the conversion types — ten to sixteen weeks is a more realistic expectation than the six to eight a simple dormer takes — and the roof is open for part of it, so the programme has to allow for weather.
A short site visit tells us the roof type, the head height and where the stairs can land — which is most of what decides the design and the price. We will tell you if a conversion is not the right answer for your house.
Arrange a site visitAll loft conversions