Mansard Loft Conversions Explained

A mansard loft conversion replaces most of one roof slope, usually the rear, with a near vertical wall and a shallow flat roof above it. It gives the most usable space of any loft conversion type, because the new floor runs close to the full footprint of the house below instead of being cut short where the original roof slopes down. It almost always needs planning permission, and we price it individually once the roof structure and party wall position have been assessed. On a Victorian or Edwardian terrace, it is often the only type that produces two proper rooms rather than one long one.

What a mansard loft conversion actually is

The existing rear pitch comes off, and a new wall goes up in its place at roughly 72 degrees. That is close enough to vertical that the floor area against it is usable, where a standard roof slope eats the room at exactly the spot you want a bed or a shower. Windows sit in small dormers set into the new wall, under a shallow flat roof.

Most are rear mansard loft conversions, built on the elevation that faces the garden rather than the street, because that is where the structural changes are least visible and the planning case is strongest. A mansard alters the entire roof profile rather than part of it, which is how it gives the maximum internal volume of any loft conversion without expanding the footprint of the house. The result is a new storey in all but name. Our guide to what type of loft conversion is best for your Bristol home compares every type, and our loft conversions in Bristol page sets out how we work.

Mansard or dormer: what you actually gain

A dormer loft conversion adds a box onto the existing roof. The slope stays, and the dormer creates standing height across the part of the loft it covers. It is the most common choice in Bristol: enough for most families, and often permitted development.

Removing the slope rather than building onto it is what separates a mansard. You gain significantly more headroom and floor space: full height walls out to the edges, across almost the whole width and depth, and room for several layouts, including two bedrooms with an en suite. A dormer gives you a room. A mansard gives you a floor.

That costs more structural work, a planning application and a longer programme. Whether a mansard roof extension is worth it depends on whether a dormer or a hip to gable loft conversion gets you close enough for less.

Which Bristol houses suit a mansard

Mansards belong on period terraces. The Victorian and Edwardian stock in Bristol, in Totterdown, Bedminster, Southville, Easton and St Andrews, is the shape a mansard roof was designed for: reasonable ridge height, a steep rear pitch, and a footprint that is long rather than wide. Built properly, the new wall and its small dormers read as a period roof form rather than an addition, which is why mansard extensions tend to suit period properties better than a boxy dormer does.

Where a house has a rear outrigger, an L shaped dormer over it can capture much of that volume for less construction, and on some terraces that is the better answer.

Mansard conversions are a terraced house solution first, common in dense urban areas and rare on open estates. Semi detached and end terrace houses usually have a cheaper route, since converting the sloping hip into a gable wall often stays inside permitted development. Bungalows in North Somerset are a separate case, because the staircase has to come out of the floor plan below.

Planning permission, conservation areas and Article 4

Plan on making a full planning application. The change to the shape of the building is too great for permitted development on the large majority of houses, and anyone who says otherwise before seeing your loft space is guessing. Significant structural changes to a roof are exactly what the planning system is there to look at.

Bristol adds a layer that national guides leave out. Large parts of the city sit within conservation areas, and in some an Article 4 direction has removed permitted development rights for roof alterations visible from the street. That cuts both ways: in an Article 4 area a rear dormer may need a full application too, which removes the main advantage a dormer usually holds.

The type and the planning route are decided together, not one after the other. We confirm the position for your address at the free feasibility visit and deal with the council for you. Our guide to loft conversion planning permission covers the national rules. We never guarantee an outcome.

Party walls and structure

On a mid terrace, a mansard meets a neighbour on both sides, and new structure bears on or against party walls, so party wall agreements under the Party Wall etc. Act are part of the job. An end terrace or a semi has one neighbour instead of two, which is one reason the same design costs different amounts in one street.

A structural engineer calculates the new floor, the staircase opening, the vertical walls and the flat roof, and those calculations form part of what Building Control signs off.

What you can fit in a mansard loft room

Because the new walls are close to vertical, the usable internal space is not confined to a strip down the middle. The mansard design can be tailored to a layout rather than the layout being dictated by the roof: two bedrooms, a bedroom with a dressing area, or a single large room with en suite bathrooms.

Natural light comes from the roof windows set into the dormers on the new wall, and on a rear elevation many mansard conversions take Juliet balconies over the garden. A roof terrace is occasionally possible, though it is a planning permission question long before it is a design one. That is a planning question as well as a design one in conservation areas, so we raise it early.

Storage is the part people forget. A mansard gives more extra living space than any other loft extension, and the temptation is to use every inch of it as room. Leaving some of the eaves as fitted storage usually makes the finished loft room work better.

Insulation, energy efficiency and Building Regulations

An unused attic space is usually the coldest part of a house. A mansard loft conversion rebuilds that envelope completely, so the new walls and the flat roof are insulated to current UK Building Regulations standards rather than patched around old rafters. The practical effect is that the finished rooms are warmer and cheaper to run than the loft space they replaced, and the energy efficiency of the house as a whole usually improves.

Because the walls are close to vertical and the roof is shallow, there is room for a better insulation build-up than a standard roof slope allows, and it is cheaper to specify at first fix than to revisit later. Building Control signs it off alongside the structure, the fire escape route, the staircase and the sound separation from the room below, then issues the completion certificate at the end of the loft conversion project.

What a mansard costs and how long it takes

We publish starting figures where one is honest. A Velux or roof light conversion starts from £40,000 + VAT, and a dormer or hip to gable from £55,000 + VAT. An en suite adds from £8,000 + VAT, and splitting the new space into more than one room from £10,000 + VAT.

How much does a mansard loft conversion cost? We do not publish a figure, and we are not going to. It is priced individually once the roof structure and the party wall position have been assessed, because the quantity of new structure differs too much for an average to mean anything. The relative order is settled: a rooflight conversion is least expensive, a dormer and a hip to gable sit in the middle, and a mansard is the most.

Most loft conversions take 8 to 14 weeks from the start of construction, excluding design, planning and Building Control, and a mansard loft conversion sits at the longer end. Planning runs in front of all of it, on the council’s timetable rather than ours.

This is an indicative range based on your answers. Your actual cost depends on your property’s structure, access, and final specification. A free feasibility visit gives you a detailed, fully specified quote so you know exactly what’s included and why.

Our loft conversion cost calculator gives an indicative range for the types that do have a figure, and our loft conversion cost in Bristol guide explains it. Loft conversions increase property value by up to 25 to 30 per cent, as our guide to how much value a loft conversion adds sets out.

Getting a price for your roof

We measure the loft, read the existing roof structure and the party wall position, and set out what each workable type means for your budget. See the full loft conversions service.

Free site visit. No obligation. We come to you. A member of our team will be in touch within 2 working days.

Frequently asked questions