A loft conversion is one of the most effective ways to increase space and add value to your home without moving. Among the many types of loft conversions, mansard and dormer conversions are the most popular options in the UK. But choosing between the two isn't always straightforward. Each has its own design features, planning requirements, costs, and space benefits.
This guide compares mansard vs dormer loft conversions in detail, helping you understand the differences and determine which is best suited for your property and goals. Whether you live in a Victorian terrace in South London or a semi-detached home in the suburbs, this in-depth analysis, based on years of experience from dRAW Architecture, will give you all the insight you need to make an informed decision.
When deciding between a mansard and a dormer loft conversion, homeowners should consider planning permission requirements, budget, available space, and architectural style. Dormers are typically less expensive, quicker to build, and often fall under Permitted Development Rights, making them ideal for smaller budgets or homes with fewer planning restrictions.
Mansard conversions, however, involve more substantial roof restructuring and almost always require full planning permission. They tend to offer more internal space and a better external finish, especially for homes in period or conservation areas.
A Dormer loft conversion typically costs between £40,000 and £60,000, with planning permission often not required. This option provides moderate headroom and floor space, takes around 4–6 weeks to build, and suits most modern homes. Its visual appeal is more basic, featuring a box-like structure, but it still delivers a high uplift to property value. In contrast, a Mansard loft conversion generally costs £60,000–£80,000 or more, usually requires planning permission, and offers the maximum possible headroom and floor space. Build time is longer at 8–10 weeks or more, making it ideal for period or terraced homes. The Mansard design creates a seamless architectural fit and tends to add very high value to the property.
A mansard loft conversion involves altering the entire slope of one side of the roof, most often the rear, to create a much steeper angle, typically 72 degrees. The flat roof and almost vertical rear wall create a large, usable internal space, almost like adding a full extra storey to your home. It is named after French architect François Mansart and is commonly seen in Georgian and Victorian architecture across London.
This style is particularly well-suited to terraced houses, especially those in conservation areas, as it can maintain the historical architectural lines of the street. A rear mansard is the most common variant and may also include dormer-style windows for light and ventilation.
At dRAW Architecture, we regularly design and oversee mansard conversions in boroughs like Wandsworth, Lambeth, and Islington, where planning sensitivities require a more integrated and heritage-respectful roof addition. While the planning process is more involved, the long-term benefit in space, comfort, and value makes this a worthwhile investment for many homeowners.
A dormer loft conversion is one of the most common and cost-effective ways to expand a loft space. It involves extending a section of the existing roof vertically to create a box-shaped structure that protrudes from the sloping roof, adding headroom and usable floor area. Dormers are especially popular on semi-detached and detached properties, and they work well for bungalows too.
There are various types of dormers, including flat-roof, gable-fronted, shed-style, and L-shaped dormers. Each serves a slightly different architectural purpose and suits different roof types. In many cases, especially with smaller dormers on the rear roof slope, the work can fall under Permitted Development Rights, meaning you may not need to apply for full planning permission.
Dormers can accommodate bedrooms, home offices, or bathrooms, and their relatively low cost and quick build time make them ideal for families looking to expand on a modest budget. dRAW Architecture often recommends dormer conversions for clients looking to enhance their homes in the fastest and most efficient way, particularly where planning constraints are minimal.
Whether you're leaning toward a dormer or mansard loft conversion, getting the design and permissions right from the start is crucial. At dRAW Architecture, we offer comprehensive architectural services including feasibility studies, planning applications, and building regulations drawings tailored specifically for your loft project. With years of experience in residential design across London, we’re here to help turn your attic into a functional, beautiful living space. Contact us today for expert guidance and a free consultation.
In general, dormer loft conversions are more budget-friendly than mansard conversions. A standard rear dormer conversion can range from £40,000 to £60,000, depending on factors such as finishes, structural changes, and the inclusion of a bathroom or en-suite.
On the other hand, mansard conversions typically start at around £60,000 and can go well beyond £80,000, especially in London where structural complexity, access issues, and high-end finishes come into play. These costs also include the need for full planning applications, structural engineering, and longer build times.
At dRAW Architecture, we provide transparent cost guidance at the design stage and offer full building regulation drawings that allow for accurate contractor quotes. We help clients weigh the upfront investment against long-term space and value gains.
If your priority is maximising internal space, a mansard conversion offers a clear advantage. Because the roof slope is nearly vertical, mansards provide almost full ceiling height across the room. This creates a more natural, comfortable living space, particularly valuable for master suites or large home offices.
Dormer conversions, while effective, typically add headroom only to part of the loft. The front and side slopes remain untouched, limiting the overall usable floor area. Still, for homes with a high original ridge height, dormers can provide plenty of room for bedrooms and bathrooms.
dRAW Architecture often recommends mansards for clients who want maximum transformation or are adding multiple rooms, while dormers work well for clients with tighter budgets or less invasive goals.
Aesthetically, mansard loft conversions often blend more seamlessly with the existing architecture, especially in period properties. The change in roofline can be designed to complement original brickwork and window proportions, making it less visually intrusive.
Dormer conversions, while functional, are more visibly "added on." Flat-roof dormers can look boxy and may clash with the existing structure if not well-designed. However, with thoughtful detailing and materials selection, dormers can still be an attractive option, especially on more modern properties.
Our team at dRAW Architecture places a strong emphasis on visual integration, ensuring every loft design, whether dormer or mansard, respects the character of the existing home while enhancing kerb appeal.
Planning rules are one of the key differences between these two loft conversion types. Mansard conversions almost always require full planning permission, as they alter the roof structure significantly. They’re subject to height restrictions, conservation area rules, and local design codes.
Dormer conversions, especially on the rear of the property, are often covered under Permitted Development Rights, provided they meet certain criteria (e.g., not extending above the existing roof ridge, staying within volume limits). However, these rights can be removed in Article 4 Directions or conservation zones.
At dRAW Architecture, we handle all aspects of planning, including design statements, drawings, and liaison with your Local Planning Authority, to ensure smooth and successful applications, regardless of project complexity.
Dormer conversions generally take less time to build, usually between 4 and 6 weeks. This makes them appealing for homeowners who need space quickly or wish to minimise disruption.
Mansard loft conversions, due to their complexity, require more time, often 8 to 10 weeks or longer, depending on structural alterations, scaffolding, and external finishes. The longer timeline also reflects more extensive work to the roof and the need for staged inspections.
Working with professionals like dRAW Architecture ensures your timeline is well-planned, realistic, and coordinated with trusted contractors and engineers.
Dormers offer a range of variations: flat roof, gable front, L-shaped, and even eyebrow dormers. This flexibility makes it easier to tailor the design to your roof type and desired layout.
Mansards, while more uniform in appearance, still allow for customisation in window placement, materials, and integration with rear extensions. They're especially useful in planning a larger open-plan space or multiple rooms.
We regularly advise our clients at dRAW Architecture on the best variation for their needs, incorporating both spatial optimisation and planning constraints into the design process.
Both dormer and mansard loft conversions must comply with Part L of the UK Building Regulations, which governs thermal efficiency. However, mansard conversions typically offer better insulation potential due to their full roof reconstruction. This allows for thicker insulation and airtight construction.
Dormers, by contrast, can sometimes lead to thermal bridging if not properly detailed, particularly around junctions between the dormer and the original roof. Energy-efficient glazing and insulation are vital for both.
At dRAW Architecture, our building regulations drawings always factor in high-performance insulation, thermal breaks, and low-U value glazing to maximise comfort and energy efficiency year-round.
In terms of return on investment (ROI), both dormer and mansard conversions can significantly boost your property’s value. On average, a well-designed loft conversion adds between 15–25% to a home’s resale price, depending on location and finish.
Mansard conversions often command a higher uplift due to the extra usable space and architectural integration. They are especially desirable in London boroughs, where square footage is at a premium.
Dormers still add great value but may offer slightly less ROI unless paired with a high-quality finish and thoughtful layout. dRAW Architecture always advises clients on the balance between budget, space, and market value.
Choosing between a dormer and a mansard loft conversion depends on your home’s existing structure, your budget, and your long-term plans. If you want the maximum internal space and live in a terraced or period property, a mansard may be the best investment. If you want a faster, simpler, and more budget-friendly upgrade, especially in a suburban semi, a dormer may suit you better.
At dRAW Architecture, we tailor each loft conversion to our client’s unique requirements. During our feasibility phase, we’ll assess your property’s structure, planning constraints, and spatial goals to determine the ideal solution, whether that’s a dormer, mansard, or hybrid design.
With over a decade of experience in loft conversions across London, dRAW Architecture has delivered dozens of successful dormer and mansard projects in areas like Clapham, Wandsworth, Putney, and Dulwich. Our expertise spans initial design, planning permission, building regulations, and construction monitoring.
We’ve helped homeowners unlock unused attic space and transform their homes with stylish, energy-efficient, and regulation-compliant loft conversions. Our work is regularly featured in design publications, and we consistently deliver high satisfaction through collaborative design and technical rigour.
