Mansard loft conversion in London and Essex
A mansard is the largest loft conversion a house will take. Rather than pushing a box out of the existing roof, it rebuilds the roof structure so the rear wall rises almost vertically, at around 72 degrees, with a near-flat roof on top.
What you get is a full extra storey rather than a converted attic. Ceiling height is consistent across the whole floor, not just down the middle, and the space feels like part of the house instead of a room in the roof.
It is the most expensive and most involved loft conversion, and it almost always needs planning permission. For the right property, usually a period terrace with a steep roof and awkward head height, nothing else comes close for space gained.
We build mansard loft conversions across North London, Brentwood, Chingford and the wider Essex area.
What is a mansard loft conversion?
The mansard takes its name from Francois Mansart, the seventeenth century French architect the style is credited to. The defining feature is the roof profile: a steep, near-vertical lower slope and a shallow upper section, which maximises the usable volume inside.
In practice the existing roof is largely removed. The party walls are built up, the rear wall is raised at around 72 degrees, and a new near-flat roof goes over the top. Dormer windows sit in the steep face.
Because so much is rebuilt, a mansard is the conversion that gives you the most control over the finished space. Ceiling height, room layout, window positions and the staircase are all designed rather than worked around.
Mansard, dormer, or hip-to-gable?
A dormer works with the roof you already have, pushing a box out of one slope. Faster, cheaper, usually no planning application. Right when your existing height is nearly sufficient and you need one good room.
A hip-to-gable squares off a sloping side roof to recover wasted volume. It suits semi-detached and detached houses with a hipped roof, and it is often combined with a rear dormer.
A mansard rebuilds the roof entirely. It gives the most space and the best ceiling height, and it suits terraced period properties where the roof is steep and the head height is poor. It is also the most expensive and the slowest.
The honest test: if a dormer gives you the room you actually need, take the dormer. Choose a mansard when you are adding a floor, not a room, or when the existing head height rules everything else out.
Mansard loft conversion cost in 2026
| Mansard type | Best suited to | Typical London and Essex cost | Build time |
|---|---|---|---|
| Standard mansard | Mid-terrace period homes adding a full storey | £70,000 to £95,000 | 10 to 14 weeks |
| Mansard with en-suite | Two bedrooms and a bathroom on the new floor | £82,000 to £105,000 | 12 to 16 weeks |
| Double mansard | End-terrace and wider properties, front and rear | £95,000 to £120,000 and above | 14 to 18 weeks |
| Mansard on a semi-detached | Semis where the roof and party wall allow it | £75,000 to £100,000 | 11 to 15 weeks |
Figures cover the structural build, staircase, insulation, glazing, plastering, electrics and standard finishes. They exclude VAT, professional fees, planning costs, and any en-suite.
Why a mansard costs more
The roof comes off. A dormer leaves most of the roof in place. A mansard removes and rebuilds it, so there is more demolition, more structure, more scaffolding and more time on site.
Party walls have to be raised. On a terrace, both party walls are built up to the new roof height in brick or block. That is structural masonry work at height, and it is a significant part of the cost.
Weather protection. With the roof off for longer, a temporary roof or full scaffold sheeting is normally needed. It is a real cost and it belongs in the quote rather than appearing later.
More structural engineering. New steels, new floor joists, new roof structure, and calculations for all of it.
Planning permission. Most mansards need a full application. Budget £2,000 to £5,000 and eight to twelve weeks before work can start.
Party wall surveyors. On a terrace with two attached neighbours, that is two processes and two sets of fees.
Planning permission for a mansard
Assume you will need it. A mansard alters the shape and height of the roof enough that Permitted Development rarely covers it, and on flats it never does.
Some councils have adopted mansard-friendly policies in specific areas, particularly where a terrace already has mansards on neighbouring properties and the council wants the roofline consistent. In those streets a mansard can be straightforward. In a conservation area with an intact original roofline, it can be refused.
The practical first step is to look at your own street. If several houses in the terrace already have mansards, precedent is on your side and the application is far more likely to succeed. If none do, expect a harder conversation with the planners and factor that into your timeline.
Building Regulations approval is required in every case, covering structural work, fire escape, the staircase, insulation, ventilation and thermal performance.
How long a mansard takes
Ten to sixteen weeks on site for a standard mansard, longer for a double mansard or where the structure below needs strengthening.
Before that: four to six weeks for design and structural calculations, then eight to twelve weeks for planning. Realistically, from first conversation to finished room is six to nine months.
The roof-off phase is the disruptive part and typically lasts two to three weeks. After that the house is weathertight again and the work moves inside.
Is a mansard worth it?
On space gained per pound spent, a mansard is strong. You are adding a full storey, often two bedrooms and a bathroom, to a house that could not otherwise take them without losing garden.
The check that decides it is your street's ceiling price. Find what the largest, best-finished houses on your road have sold for in the last year. If your current value plus the full mansard cost lands comfortably below that number, the project stands up financially as well as practically. If it pushes past it, the return narrows.
In much of North London and the Essex borders, where terraced family houses are in short supply and gardens are small, a mansard that adds two bedrooms tends to hold its value well.
Mansard loft conversion: frequently asked questions
How much does a mansard loft conversion cost?
Between £70,000 and £120,000 in London and Essex in 2026. A standard single mansard runs £70,000 to £95,000, and a double mansard or high specification build runs £95,000 to £120,000 or more.
Do I need planning permission for a mansard loft conversion?
Almost always yes. Mansards rarely fall under Permitted Development because they alter the roof shape and height. Flats never have Permitted Development rights. Building Regulations approval is required in every case.
What is the difference between a mansard and a dormer?
A dormer pushes a box out of the existing roof slope and leaves the rest of the roof intact. A mansard rebuilds the roof so the rear wall is near vertical, creating a full extra storey with consistent ceiling height throughout.
How long does a mansard loft conversion take?
Ten to sixteen weeks on site, plus four to six weeks of design and eight to twelve weeks for planning permission beforehand.
Can any house have a mansard?
No. Mansards suit terraced and semi-detached period properties with sufficient structural capacity and a supportive local planning position. Flats and many modern houses with shallow-pitched trussed roofs are not suitable.
Thinking about a mansard?
The first question worth answering is whether your street and your structure will support one, and that is a conversation rather than a form. Tell us your address and what you are trying to gain from the space, and we will give you a straight answer on whether a mansard is realistic before anyone talks about money.
