The Most Important Decision in Your Loft Conversion
For the majority of London homeowners considering a loft conversion, the choice ultimately comes down to two options: a dormer conversion or a mansard conversion. Both create a full, habitable room in your roof space. Both add significant value to your property. But they differ substantially in cost, in the amount of space they create, in their planning requirements, and in which properties they are most suited to.
This guide explains both types clearly so you can make an informed decision — or at least arrive at your survey appointment with the right questions.
What Is a Dormer Loft Conversion?
A dormer is a box-shaped structure that projects vertically from the slope of an existing roof. It extends the height and floor area of the loft by breaking out of the existing roofline. The most common version in London is the rear dormer — a full-width box extending from the rear slope of a Victorian terraced or semi-detached roof. This is typically invisible from the street, which is why it usually qualifies for permitted development.
Dormers can also be constructed on the side or front of a roof, though front dormers almost always require planning permission and are subject to strict design criteria in conservation areas.
A mansard conversion typically adds 30–40% more usable floor area than a standard rear dormer on the same property — but costs approximately 25–35% more and almost always requires full planning permission.
What Is a Mansard Loft Conversion?
A mansard conversion takes a fundamentally different approach. Rather than adding a box structure to an existing roof slope, it rebuilds the rear roof slope almost entirely — replacing it with a near-vertical (72-degree) wall topped by a very shallow pitch roof. The result is a conversion with almost entirely flat walls and full ceiling height across most of the floor area, much closer to a conventional room than the sloped ceiling of a standard dormer.
Mansard conversions are named after the 17th-century French architect François Mansart and are common across Paris, which is why they dominate in London's period terraced streets. Because they alter the roofline more substantially than a dormer, they almost always require full planning permission.
Key Differences: Side by Side
| Factor | Dormer | Mansard |
|---|---|---|
| Typical Cost (London) | £45,000–£70,000 | £60,000–£95,000 |
| Usable Floor Area | Good — limited by slopes | Excellent — near-full floor plate |
| Ceiling Height | Full height in dormer zone, sloped elsewhere | Full height across most of the room |
| Planning Required? | Usually Permitted Development | Almost always Full Planning Permission |
| Build Time | 6–8 weeks build | 10–14 weeks build |
| Best For | Single bedroom + bathroom | Large master suite or two rooms |
| Conservation Areas | Possible with restrictions | Possible — design-led approval required |
| Property Types | Most terraced, semi-detached | Terraced with sufficient roof height |
Our surveyors will assess your roof, property type and planning position and recommend the conversion that gives you the most space for your budget.
Space: How Much Difference Does It Make?
This is where the mansard's superiority is most apparent. A standard rear dormer on a typical two-storey Victorian terrace might deliver 18–22 square metres of usable floor area — enough for a double bedroom and a decent en suite bathroom. A mansard conversion on the same property might deliver 26–32 square metres — enough for a large master bedroom, a full bathroom, and potentially a dressing area or study space.
The reason is geometry. In a dormer conversion, the original rafters remain on either side of the dormer box, creating sloped sections of ceiling that reduce effective floor area. A mansard removes the original rear slope entirely, replacing it with a near-vertical wall — giving you effectively rectangular walls on all sides.
Planning: The Critical Difference
For most London homeowners, permitted development is the single most attractive feature of a rear dormer conversion. It means you can proceed without a planning application, saving 8 to 10 weeks and the associated cost and uncertainty. A standard rear dormer that does not exceed the permitted development volume limits, is not visible from a public highway, and is set back from the eaves, will typically qualify.
Mansard conversions almost never qualify for permitted development because they alter the roofline too substantially. A full planning application is nearly always required. If your property is in a conservation area — which applies to large parts of inner London — the planning authority will also scrutinise the design carefully. Our complex loft conversions page covers conservation area requirements in detail.
In London, over 60% of Prime Loft's conversions are dormer-based. Mansard conversions account for around 20% — concentrated primarily in terraced properties in conservation-area boroughs such as Islington, Hackney and Wandsworth.
Cost: Why the Mansard Costs More
The additional cost of a mansard conversion reflects several factors: more structural steel, more roofing material, a longer build programme, and in almost all cases a planning application fee and additional architectural time. The planning process also means a longer overall timeline before work can begin. That said, when the cost per square metre of usable space is calculated, the mansard often comes out favourably — delivering more room for every pound spent on the conversion structure.
Which Should You Choose?
The right choice depends on your property, your budget, your timeline and your planning position. As a rough guide:
Choose a dormer if: you want the fastest, most cost-effective route to one new bedroom and bathroom; your property is not in a conservation area; and your budget is between £45,000 and £70,000.
Choose a mansard if: you want to maximise space and create a proper master suite; you are willing to go through planning; and you have a budget of £60,000 or above. Mansards also tend to add more value per pound to the property — particularly in higher-value areas of London where buyers expect larger, better-proportioned rooms.
Not sure which applies to your home? Our free survey will assess your roof, give you accurate dimensions for both options, and recommend the conversion that delivers the most space for your budget. See also our full guide to all types of loft conversion.
Frequently Asked Questions
Dormer conversions are generally cheaper, with typical London costs of £45,000 to £70,000 compared to £60,000 to £95,000 for a mansard. The mansard's higher cost reflects more structural steelwork, more roofing material, a longer build, and in almost all cases a planning application. However, the mansard delivers significantly more usable floor area, so the cost per square metre of new space is often comparable.
In almost all cases, yes. Because a mansard conversion rebuilds the rear slope of the roof to a near-vertical position, it alters the roofline more substantially than permitted development allows. A full planning application is therefore nearly always required. Permitted development rules for loft conversions do not cover this scale of alteration. Our planning permission guide explains the rules in detail.
Yes — but it requires careful design and planning authority approval. Conservation area status does not prevent a mansard, but it means the design must be sympathetic to the existing streetscape and character of the area. Our in-house architects have experience producing mansard designs that satisfy conservation officers, and our planning team handles the application end-to-end. See our complex conversions page for more.
Both types typically add significant value. In London, a well-executed loft conversion adding a bedroom and bathroom generally increases property value by 15–25%. A mansard conversion, which adds more usable space and a more premium finish, can add at the higher end of this range — particularly in sought-after inner London areas where buyers are comparing room sizes closely.
An L-shaped dormer is a variant specifically suited to Victorian and Edwardian houses with a back addition (the lower rear extension that runs along one side). It combines a full-width rear dormer with a side dormer over the back addition, creating an L-shape when viewed from above. It delivers more space than a standard rear dormer and usually qualifies for permitted development, making it an excellent middle ground between a standard dormer and a mansard.
For a standard two-storey Victorian or Edwardian terrace in London, the rear dormer is the most common choice — fast, cost-effective, and usually available under permitted development. For properties where maximising space is the priority, or where the existing roof height gives enough room, a mansard delivers a materially better result. The L-shaped dormer is the best option for properties with a back addition.
Prime Loft has completed over 500 loft conversions across London. Our free survey includes a recommendation on the best conversion type for your specific property.