The Mansard Roof: Frenchmans Tax Trick That Became Londons Favourite Loft
Here’s something most people don’t know. The mansard roof exists partly because of tax. Back in 17th century France, homeowners were taxed on the number of floors below the roofline. So a clever roof shape that added a full story inside the roof, without counting as a floor, saved people money. The Frenchman Francois Mansart made it famous, and if you’re weighing this option, its worth going through your loft conversion options explained before committing to the biggest one of all.
Today the tax reason is gone. But the space saving genius remains. That’s why mansards still dominate London terraces. Lets look at what this roof gives you, and what it asks in return.
First, What Makes a Mansard Different From a Normal Loft?
Most loft conversions work within your existing roof. A mansard doesn’t. It rebuilds the roof completely. The sides go almost vertical, around 72 degrees, with a near flat top.
That one change is why a mansard gives so much more room. Youre not squeezing space out of an existing slope. You’re building a proper new story. A typical mansard adds 35 to 55 square metres. Thats often a full extra floor.
Compare that to a dormer, which just pushes a box out from the roof. The difference in usable space is big. A mansard is the biggest loft type you can build, but its not always the right fit.
The Three Mansard Shapes Most People Never Hear About
Heres something even many homeowners miss. A mansard isnt just one shape. There are three main variations, and the one you pick changes the whole look.
A straight mansard has a slight slope on the upper panel. Its the most common and the simplest. A convex mansard curves outward on the lower slope, giving a softer, rounded feel. A concave mansard curves inward, with a steep angle at the bottom, which suits grander period homes.
Most London terraces use the straight version. But on a period property, the right curve can make the roof sit far more naturally against its neighbors. This is the kind of detail a good architect raises early, before the design is locked in.
The Space Winner, Compared Simply
Heres how a mansard stacks up against the other common loft types:
| Loft type | Space gained | Planning needed? | Rough London cost |
| Velux | Smallest | Usually no | £18k to £50k |
| Dormer | Medium | Often no | £45k to £95k |
| Hip to gable | Medium to large | Usually yes | £47k to £95k |
| Mansard | Largest | Almost always yes | £80k to £120k+ |
The pattern is easy to read. More space means more cost, and more planning. A mansard sits at the top of all three.
Why a Mansard Almost Always Needs Planning Permission
This is the part that catches people out. Many loft conversions are permitted development, so no full application is needed. The rule allows up to 40 cubic meters for terraced houses and 50 cubic meters for semi-detached and detached homes.
A mansard usually blows past that. And even when it doesn’t, it changes the roof shape so much that it falls outside permitted development anyway. So in nearly every case, you need full planning permission.
Eugene Kim, founder of Extension Architecture, hears the same thing often: “People assume a mansard is permitted development because their neighbor did a dormer without planning. But a mansard rebuilds the whole roof. Its a different job. We tell clients on day one, because it changes both the timeline and the budget from the start.”
Where Mansards Get a Warm Welcome, and Where They Don’t
Not every council treats a mansard the same way. In some heritage areas, planning officers actually prefer a mansard to a flat-roofed dormer. It sits better against old rooflines. Camden and Islington, with their big conservation areas, often lean this way.
Other places are tougher. Westminster and Kensington and Chelsea look very closely at design. Conservation area status means almost every loft there needs full permission, whatever the type. Outer boroughs are usually easier, though a mansard still needs planning even where a dormer might not.
One rule holds everywhere. If your home is listed or in a conservation area, you need permission no matter what. And the design has to match the street, using the right materials and proportions.
Is the Extra Cost Worth It?
A mansard is the priciest loft type, and the reasons are structural. Bigger steels. Party wall work. A full roof rebuild. Brickwork to match the house. All of that makes it roughly 60 to 90% more expensive than a dormer.
But the value can be strong. In inner London, a mansard that adds a bedroom and en suite can lift a home’s value well beyond the build cost. To weigh it up properly, its worth reading the full mansard roof pros and cons before you decide.
The Rules You Can’t Skip
Whatever you choose, building regulations still apply. Fire safety, insulation, and staircase design are all checked under Approved Documents B, K, and L. Fire safety matters more here because a mansard adds a full story, which changes the escape route rules.
The Party Wall Act 1996 usually applies too. A mansard on a terrace or semi means raising a shared wall, so you need to serve notice on your neighbors early. Leaving this late is one of the most common causes of delay.
Most mansard conversions, once design and building regulations work is underway, take 5 to 7 months from start to completion. The planning stage before that can add time on conservation or listed properties, so its worth starting early.
