Mansard Loft Conversion

A mansard loft conversion changes the whole shape of the rear roof, replacing it with a near vertical wall set at a steep angle, usually around seventy two degrees, topped with a flat or gently sloped section. This design creates a large amount of usable space and head height, far more than most other loft conversion types can offer, which makes it popular for homeowners who want a proper full sized extra floor rather than a single added room.

Because a mansard changes the fundamental shape of the roof, it almost always needs full planning permission rather than falling under permitted development. This adds time to the project, usually around eight to twelve weeks for a decision once the application is submitted, but it also allows for a much larger and more versatile finished space.

Mansard conversions are especially well suited to houses with a low, flat roof between two party walls, sometimes called a London roof, which is common across many Victorian and Edwardian terraces. On these properties a mansard can make the difference between a cramped loft and a genuinely spacious top floor.

Who this for

This service is most commonly chosen for Victorian and Edwardian terraced houses with a low, flat rear roof, though it can also suit semi detached and detached properties where a large amount of extra space is the priority. It suits homeowners who are prepared for a full planning application and a longer project timeline in exchange for the largest possible increase in usable space.

Project Process Steps

We survey the existing roof shape and assess whether a mansard is the right approach compared with other loft conversion options for the property.

We prepare and submit a full planning application, including the drawings and structural information the council will need to make a decision.

Once planning permission is granted, our structural engineer designs the new roof structure, since the original roof is largely rebuilt rather than simply extended.

The new mansard structure is built and made weathertight, followed by the usual interior stages of insulation, plumbing, electrics, staircase and finishing.

Frequently asked questions (FAQ)

Does a mansard loft conversion need planning permission?

Almost always, yes. A mansard changes the whole shape and pitch of the roof, which goes beyond what permitted development allows, so you should expect to submit a full planning application. In London this typically takes around eight to twelve weeks to be decided.

Why are mansard conversions so common on London terraces?

Many London terraced houses have what is sometimes called a London roof or a butterfly roof, which is low and flat between two party walls with very little natural head height. A mansard conversion is designed to work with exactly this roof shape, and it can create a genuinely large amount of usable space where other conversion types would struggle.

Will my mansard application definitely be approved?

Not necessarily. Approval depends on your council's local planning policy and how the design fits with neighbouring properties and the character of the street. Working with a designer who understands your specific borough's usual expectations for mansard designs improves your chances, but no council will guarantee approval before an application is properly submitted.

Do I still need a party wall agreement for a mansard conversion?

Yes. Since mansards are common on terraced houses, and terraces share a wall with a neighbour on both sides, you will usually need to arrange party wall agreements with both neighbouring households, in addition to your planning application.