- What Is a Dormer Loft Conversion?
- What Is a Mansard Loft Conversion?
- Dormer vs Mansard: The Key Differences
- Which One Costs More in London?
- Planning Permission: What You Need to Know
- Which Conversion Adds More Value to Your Property?
- Which Is Right for Your Home?
- Why the Design-Build Relationship Matters Here
- FAQs
You've decided the loft is the answer. Maybe you need a proper bedroom, a home office, or a bathroom that isn't squeezed into a corner. The question now isn't whether to convert — it's which type will actually work for your home, your budget, and your plans.
Two options come up more than any others for London properties: the dormer and the mansard. They look different, cost different amounts, and suit different situations. Here's what you need to know before you commit to either.
What Is a Dormer Loft Conversion?
A dormer is a box-shaped structure that projects vertically from the slope of your existing roof, adding both headroom and usable floor area in one go.
The most popular version in London is the rear dormer, which runs across the back of the roof. It's relatively straightforward to build, and because it faces the garden rather than the street, it often falls within permitted development rights — meaning no full planning application in many cases.
What a dormer gives you
- A significant increase in usable floor space
- Full-height ceilings across most of the converted area
- Natural light through vertical windows or a Juliet balcony
- A more cost-effective route to a liveable room than most alternatives
Dormers suit semi-detached and terraced houses with a pitched roof and a reasonable ridge height. They work particularly well on Victorian and Edwardian properties, which make up a large share of London's housing stock.
What Is a Mansard Loft Conversion?
A mansard is a more substantial structural change. Rather than adding a box to the existing roof, it replaces the rear slope almost entirely — creating a near-vertical rear wall (typically at 72 degrees) with a shallow flat roof on top.
The result is a much larger, more rectangular space that feels less like a converted loft and more like an additional storey. In many cases, the room is indistinguishable from any other floor of the house.
What a mansard gives you
- Maximum usable floor area — more than any other loft conversion type
- Full ceiling height across the entire space
- A premium finish that suits high-value London properties
- The option to add a terrace or roof garden in some configurations
Because a mansard involves significant changes to the roofline, it almost always requires planning permission. That adds time and cost. However, in conservation areas and on terraced streets where a uniform roofline is expected, mansards are often approved precisely because they maintain the visual consistency of the street when viewed from the front.
Dormer vs Mansard: The Key Differences
| Factor | Dormer | Mansard |
|---|---|---|
| Space gained | Good | Excellent |
| Headroom | High at centre, lower at edges | Full height across the room |
| Planning permission | Often permitted development | Almost always required |
| Build time | 8–12 weeks typically | 12–20 weeks typically |
| Cost | Lower | Higher |
| Best suited to | Terraced, semi-detached, period homes | Terraced rows, conservation areas, high-value properties |
| Visual impact | Moderate (rear only) | Significant (rear roofline replaced) |
Which One Costs More in London?
Both conversions vary considerably depending on your property's size, the specification you choose, and the complexity of the structural work involved.
As a general guide, a rear dormer in London tends to start from around £40,000 to £60,000 for a single room with a bathroom. A mansard typically starts from £65,000 to £100,000 or more, reflecting the additional structural work, planning process, and larger footprint.
These are ballpark figures. Your actual cost depends on factors like the existing roof structure, party wall agreements, the floor area being created, and the finish level you want.
The fastest way to get a realistic number for your specific project is to use the instant cost estimator at MVV. It gives you a ballpark figure before any conversation or commitment.
Planning Permission: What You Need to Know
For many London homeowners, this is the deciding factor.
A rear dormer on a standard terraced or semi-detached house frequently qualifies as permitted development, provided the property hasn't already used its allowance and isn't in a conservation area. No planning application means a faster start on site and lower upfront costs.
A mansard almost always needs planning permission. That's not necessarily a barrier — on many London streets, mansards are the norm, and local planning authorities are well used to them. But you need to factor in the application fee, the time to decision (typically eight weeks or more), and the possibility of conditions being attached.
If your property is in a conservation area — which covers large parts of Chelsea, Notting Hill, and other central London neighbourhoods — both types will likely need permission. In those cases, a mansard can actually be easier to get approved, because it maintains the established roofscape of the street.
Always check with your local planning authority or a qualified architect before assuming permitted development applies.
Which Conversion Adds More Value to Your Property?
Both types add value. The question is how much, and whether the return justifies the investment.
A well-executed dormer adds a usable bedroom and often a bathroom, which typically lifts the value of a London home by somewhere between 15% and 20%, based on general market evidence from estate agents and property analysts.
A mansard adds more space and commands a higher premium, particularly in prime London postcodes. For a property in SW3, W10, or SW6, the additional floor area can translate directly into a stronger sale price or rental yield — especially when the space is finished to a high standard.
Both conversions offer solid returns in London's property market. The mansard delivers more space and potentially more value uplift, but at a higher upfront cost and longer timeline. The dormer offers a faster, more affordable route to a genuinely useful room.
Which Is Right for Your Home?
There's no single correct answer. The right choice depends on your property, your goals, and your budget.
Choose a dormer if:
- You want a cost-effective way to add one or two rooms
- Your property has a standard pitched roof with reasonable ridge height
- You'd rather avoid the planning process where possible
- You need the work completed within a defined timeframe
- You're adding space for personal use rather than maximising resale value
Choose a mansard if:
- You want to maximise usable floor area in the loft
- Your property is in a conservation area or on a terraced street where mansards are common
- You're investing in a high-value property and want the conversion to match
- You're planning to sell or let and want the strongest possible return
- You have the time and budget to go through the planning process
For properties in areas like Chelsea, Notting Hill, or Wimbledon, a mansard often makes sense as a long-term investment. For a semi-detached house in a less central postcode, a dormer frequently delivers excellent value at a lower cost and a faster pace.
Why the Design-Build Relationship Matters Here
Choosing between a dormer and a mansard isn't purely a structural decision. It shapes your design options, your planning strategy, your budget, and the final feel of the space.
That's why who you work with matters. When your designer and your builder are the same team, the decisions made at design stage are grounded in what's actually buildable at your budget. There are no surprises when drawings go to tender, and no gap between what was designed and what gets built.
MVV takes loft conversions from concept to completion across London, managing design, planning, structural work, and fit-out under one team. Every project includes monthly progress reports with budget tracking and on-site photography, so you always know exactly where your project — and your money — stands.
FAQs
Is a dormer or mansard loft conversion better for a terraced house in London?
Both work well on terraced houses. A dormer is typically faster and less expensive, making it a practical choice for adding one or two rooms. A mansard creates more floor space and suits terraced streets where the roofline is already uniform, which can make planning approval more straightforward.
Do I need planning permission for a dormer loft conversion in London?
Not always. Many rear dormer conversions fall within permitted development rights, so no formal planning application is needed. However, if your property is in a conservation area, is listed, or has already used its permitted development allowance, you will need to apply. Always confirm with your local planning authority before starting.
How much does a mansard loft conversion cost in London in 2026?
Costs vary significantly by property size, specification, and location, but a mansard in London typically starts from around £65,000 and can exceed £100,000 for larger or more complex projects. Use an online cost estimator to get a ballpark figure for your specific property before speaking to a contractor.
Which loft conversion type adds the most value to a London property?
A mansard generally adds more value in absolute terms because it creates more floor space. That said, a well-executed dormer also delivers strong returns — typically adding 15–20% to property value. The best choice depends on your budget, your property's location, and whether you're optimising for personal use or resale.
How long does a dormer loft conversion take compared to a mansard?
A dormer typically takes between 8 and 12 weeks on site. A mansard takes longer — usually 12 to 20 weeks — due to the more extensive structural work involved. Planning permission for a mansard adds further time before work can begin.
Can I add a bathroom to either type of loft conversion?
Yes. Both dormer and mansard conversions can accommodate an en-suite or full bathroom, provided the structural design allows for the necessary drainage and ventilation. This is best resolved at the design stage rather than retrofitted later.
What is the difference between a dormer and a mansard in terms of appearance?
A dormer adds a box-shaped projection to the existing roof slope, visible from the rear of the property. A mansard replaces the rear roof slope almost entirely with a near-vertical wall and a shallow flat roof, creating a more substantial visual change. From the front of the property, a mansard is typically less visible than a rear dormer.
The right loft conversion is the one that fits your home, your timeline, and your budget — not just the one that looks impressive on paper. If you're weighing up your options, get a ballpark cost for your project at themvv.co.uk before your first conversation with a contractor. It takes a few minutes and gives you a realistic starting point.