If you want the absolute maximum space from your loft conversion and you’re willing to invest in something truly transformative, a mansard conversion is what you’re looking for. This is the premium option the type of conversion that turns your loft into a proper floor of the house rather than just a converted attic. It’s more expensive and almost always requires planning permission, but the results are spectacular.

We have been building mansard conversions across West London for over 25 years, particularly in areas like Hammersmith, Fulham, and parts of Ealing where the Victorian and Edwardian architecture lends itself beautifully to this style. A well executed mansard conversion can add 20 25% to your property value, often more in premium areas, and it creates space that’s genuinely comparable to adding an entire floor to your house.

Let us walk you through everything you need to know about mansard loft conversions what they are, what they cost, whether one would work for your property, and how to navigate the planning process in West London.

What Is a Mansard Loft Conversion?

A mansard conversion involves changing the angle of your roof to create near vertical walls, maximizing the usable internal space. Instead of having a steeply pitched roof that slopes at 40 50 degrees, a mansard roof has a steep lower slope (typically 70 75 degrees, almost vertical) and a shallow upper slope.

The name comes from François Mansart, a 17th century French architect who popularized this roof style. If you walk around Kensington, Chelsea, or Notting Hill, you’ll see loads of period buildings with mansard roofs they are a classic feature of Victorian and Edwardian architecture.

In a loft conversion context, we are essentially rebuilding your roof in the mansard style. We remove most of the existing roof structure and construct a new roof with those characteristic near vertical walls and shallow upper slope. This creates a loft space with full height walls around most of the perimeter, rather than sloping ceilings eating into your usable space.

The result is a room that feels like a proper floor of the house. You can walk right up to the walls without ducking, you can position furniture anywhere without worrying about head height, and you gain significantly more usable floor space than with any other conversion type.

Why Mansard Conversions Suit West London Properties

West London’s Victorian and Edwardian properties are perfect candidates for mansard conversions, and there is a historical reason for this. Many of these properties were originally built with mansard roofs or designed in an architectural style that accommodates mansard additions.

Walk down any street in Hammersmith, Fulham, or parts of Ealing, and you’ll see terraced houses with mansard roofs. The architectural style is established in these areas, which makes planning authorities more receptive to mansard conversions you are working within the established character of the area.

The property values in West London also justify the investment. A mansard conversion costs £55,000 70,000, which is significantly more than a basic dormer. However, when you are looking at properties worth £700,000 1,000,000+, and the conversion can add £140,000 250,000 in value, the investment makes perfect sense.

The space you gain is phenomenal. A mansard conversion typically provides 30 40% more usable floor space than a standard dormer conversion. This means you can create a genuine master suite with ensuite bathroom, walk in wardrobe, and still have room for a separate study or dressing area. Or you could create two good sized bedrooms where a dormer would only give you one.

Types of Mansard Conversions

There are several mansard styles, each with different characteristics and suitability for West London properties.

Full Mansard

A full mansard extends across the entire width and length of your roof, creating maximum space. This is the most dramatic transformation and provides the most usable floor area.

Full mansards are most common on detached properties or end of terrace houses where you have freedom to extend on all sides. They are less common on mid terrace properties because you are limited by party walls with neighbors.

The planning process for full mansards is more complex because you are making significant changes to your property’s external appearance. However, in areas where mansard roofs are common, planning authorities are generally supportive.

Rear Mansard

A rear mansard extends only on the rear slope of your roof, leaving the front slope unchanged. This is the most common type in West London terraced properties.

The advantage is that your property’s street facing appearance remains largely unchanged, which helps with planning permission. From the front, your house looks the same, but from the rear, you have that characteristic mansard profile.

Rear mansards still provide excellent space significantly more than a standard dormer while being more acceptable to planning authorities and neighbors.

Side Mansard

On corner properties or detached houses, you might extend the mansard around the side as well as the rear. This creates even more space and works well on properties where the side elevation is visible and important to the street scene.

Side mansards require careful design to ensure they complement the property’s architecture and don’t dominate the building’s proportions.

Planning Permission for Mansard Conversions

Let us be straight with you: mansard conversions almost always require planning permission. The changes to your roof’s external appearance are too significant to fall within permitted development rights.

Why Planning Permission Is Required

Mansard conversions exceed the volume limits for permitted development (40 cubic metres for terraced, 50 cubic metres for detached/semi detached). They also significantly alter the roof’s external appearance, which triggers the need for planning permission.

Even if you are only doing a rear mansard, the change is substantial enough that planning permission is required.

Planning Considerations in West London

West London planning authorities have varying attitudes toward mansard conversions, depending on the area and the property.

In areas where mansard roofs are already common parts of Hammersmith, Fulham, and Ealing planning authorities are generally supportive of well designed mansard conversions. You are working within the established architectural character rather than introducing something new.

In conservation areas, planning scrutiny is more intense. Areas like Chiswick, Kew, and parts of Richmond have strict conservation controls, and planning authorities will carefully assess whether your mansard conversion preserves or enhances the character of the conservation area.

The key planning considerations are design quality and proportions the mansard must be well proportioned and complement your property’s existing architecture. Materials must match or complement the existing property. The impact on neighbors, particularly regarding overlooking and loss of light, must be minimized. And in conservation areas, you must demonstrate how the conversion preserves or enhances the area’s character.

Improving Your Planning Chances

Use high quality materials that match your existing property. Natural slate or traditional tiles are far more acceptable than modern alternatives.

Design the mansard to be proportionate to your property. A mansard that dominates the building or looks top heavy is likely to be refused.

Minimize the impact on neighbors. Set windows back from party walls, use obscure glazing where appropriate, and design the mansard to avoid direct overlooking.

Engage with neighbors early. Neighbor objections don’t automatically mean refusal, but they make the process more difficult. Explaining your plans and addressing concerns early can prevent objections.

Use an experienced architect who knows West London planning authorities and their expectations. At Loft Conversion West London, we work with architects who regularly obtain planning permission for mansard conversions across West London.

Planning Timeline

The planning process takes 8 12 weeks from submission to decision, sometimes longer in conservation areas. Factor this into your project timeline mansard conversions typically take 7 9 months from initial consultation to completion, with planning permission accounting for a significant portion of that time.

The Construction Process

Mansard conversions involve more extensive work than other conversion types because you are essentially rebuilding your entire roof.

Timeline

Construction typically takes 14 18 weeks for a mansard conversion, compared to 10 14 weeks for a standard dormer. The extra time is because we are removing and rebuilding the entire roof structure rather than just adding a dormer.

Including design, planning permission, and building regulations approval, the total timeline is 7 9 months from initial consultation to completion.

What’s Involved

We start by erecting scaffolding around your entire property. This provides safe access for the extensive roof work and stays up for the duration of the project.

The existing roof structure is carefully removed. This is done in sections to ensure your property remains weatherproof throughout. We don’t leave your house open to the elements we work methodically and ensure everything is watertight at the end of each day.

Steel beams are installed to support the new roof structure. Mansard conversions typically require substantial steelwork because you are creating a large open space without internal supporting walls.

The new mansard structure is built, including the near vertical walls and shallow upper roof slope. This is precision work that requires experienced craftsmen the angles and connections must be exact to ensure structural integrity and weatherproofing.

The mansard is clad with your chosen roofing material typically natural slate or traditional tiles to match the existing property. Windows are installed, usually dormer windows in the near vertical sections and roof lights in the shallow upper slope.

Floor joists are installed or strengthened to create a solid floor that meets building regulations load requirements.

The staircase is installed, connecting your new loft space to the floor below.

Internal work includes insulation throughout (roof, floor, walls), electrical wiring for lights, sockets, and heating, plumbing if you are having an ensuite bathroom, plasterboarding and plastering, and all finishing work (decoration, flooring, doors, fixtures).

Building control inspects at key stages and issues your completion certificate once everything is complete and compliant.

Cost of Mansard Loft Conversions in West London

Mansard conversions are the most expensive type of loft conversion, typically costing £55,000 70,000 in West London, sometimes more for larger properties or premium finishes.

Basic Mansard Conversion: £55,000 60,000

This includes removing and rebuilding the entire roof structure, substantial steelwork to support the new structure, new floor joists and flooring, mansard construction with windows, full insulation to building regulations standards, new staircase, electrical work (lighting, sockets, heating), plastering and decorating, and standard flooring throughout.

This creates a large, usable loft space that feels like a proper floor of the house.

Mansard with Ensuite: £60,000 70,000

Adding an ensuite bathroom increases the cost by £8,000 15,000 depending on fittings quality. This includes all plumbing work, bathroom suite, tiling, and fixtures.

Most clients who invest in a mansard conversion include an ensuite because the space allows for a proper master suite.

Premium Mansard Conversion: £65,000 80,000+

High end finishes and features push costs higher. This might include premium roofing materials (natural slate, handmade tiles), underfloor heating throughout, luxury bathroom fixtures and fittings, hardwood flooring, extensive custom joinery and built in wardrobes, designer lighting and electrical fixtures, and high specification windows.

What Affects the Cost

Property size is a major factor. Larger properties have bigger roofs, which means more material and labor to rebuild.
The extent of steelwork required varies. Mansard conversions typically need substantial steel beams, and the size and number affect cost significantly.
Roofing materials make a big difference. Natural slate costs significantly more than concrete tiles, but it looks far better and is often required in conservation areas.
Planning permission costs include application fees (£206) plus architectural drawings (£3,000 5,000 for mansard conversions, which require more detailed drawings than simple dormers).
At Loft Conversion West London, we provide fixed price quotes that include all these elements, so you know exactly what you are paying.

Design Considerations for Mansard Conversions

Good design is crucial for mansard conversions because you are making significant changes to your property’s external appearance.

Proportions and Scale

The mansard must be proportionate to your property. A mansard that’s too large or too tall looks top heavy and dominates the building. Planning authorities will refuse applications where proportions are wrong.
The height of the near vertical walls, the pitch of the upper slope, and the overall scale must all be carefully balanced to create a harmonious addition that complements rather than overwhelms your property.
Experienced architects understand these proportions instinctively. At Loft Conversion West London, we work with architects who specialize in mansard conversions and know how to get the proportions right.

Materials and Finishes

Materials are crucial for planning permission and for the final appearance. In conservation areas, planning authorities often require natural slate or traditional tiles that match the existing properties in the area.
Even outside conservation areas, quality materials make a huge difference to how the conversion looks. Natural slate ages beautifully and maintains its appearance for decades. Cheap concrete tiles look exactly that cheap.
Windows should be proportionate and well positioned. Dormer windows in the near vertical sections should be evenly spaced and sized to create a balanced appearance. Roof lights in the upper slope should be discreet and not dominate the roof.

Internal Layout

The beauty of a mansard conversion is the flexibility it provides. With near vertical walls around most of the perimeter, you can position furniture anywhere without worrying about head height.
The most popular layout is a master bedroom suite with ensuite bathroom, walk in wardrobe or dressing area, and perhaps a small study or seating area. The generous space allows for all these elements without feeling cramped.
Alternatively, you could create two good sized bedrooms, each with their own ensuite. Or a large bedroom plus a separate home office. The possibilities are far greater than with other conversion types.
Storage is less of an issue in mansard conversions because you have so much usable wall space. You can use standard freestanding furniture rather than needing extensive built in storage in eaves spaces.

Natural Light

Mansard conversions can accommodate generous windows, bringing in loads of natural light. Dormer windows in the near vertical sections provide light and views, while roof lights in the upper slope bring light from above.
The combination creates a bright, airy space that feels nothing like a cramped loft conversion. Many clients tell us their mansard loft is the brightest, most pleasant room in their house.

Value Added by Mansard Conversions

Mansard conversions add significant value to West London properties typically 20 25% of your property’s value, often more in premium areas.
On a £800,000 property (fairly typical for a family home in Ealing or Hammersmith), that’s £160,000 200,000 added value for an investment of £60,000 70,000. The return on investment is excellent.
The value added is particularly strong when you create a high quality master suite with ensuite bathroom. Properties with luxury master suites command a premium in the West London market, where buyers expect quality and are willing to pay for it.
Estate agents consistently tell us that properties with mansard conversions sell faster and achieve higher prices than comparable properties without. The conversion needs to be done well poor quality work doesn’t add value but a professional mansard conversion from a reputable company is one of the best investments you can make in your property.

Frequently Asked Questions

How much does a mansard loft conversion cost in West London?

A mansard conversion typically costs £55,000 70,000 in West London, making it the most expensive type of loft conversion. However, it also provides the most space and adds the most value typically 20 25% of your property’s value. On a £800,000 property, that’s £160,000 200,000 added value for a £60,000 70,000 investment. At Loft Conversion West London, we provide fixed price quotes with no hidden costs.

Do I need planning permission for a mansard conversion?

Yes, almost certainly. Mansard conversions exceed permitted development limits and significantly alter your roof’s external appearance, which means planning permission is required. The planning process takes 8 12 weeks and costs around £206 for the application plus £3,000 5,000 for architectural drawings. We handle all planning applications as part of our service and have extensive experience obtaining permission for mansard conversions across West London.

How long does a mansard loft conversion take?

Construction takes 14 18 weeks, longer than other conversion types because we are rebuilding your entire roof. Including design, planning permission, and building regulations approval, the total timeline is 7 9 months from initial consultation to completion. The planning permission process (8 12 weeks) accounts for a significant portion of this timeline.

Will a mansard conversion work on my terraced house?

Probably yes, particularly if you are doing a rear mansard that leaves the front roof slope unchanged. Rear mansards are common on West London terraced properties and generally acceptable to planning authorities. Full mansards extending across the entire roof are less common on mid terrace properties because of party wall constraints, but they are possible on end of terrace or detached houses.

How much space does a mansard conversion create?

Significantly more than other conversion types typically 30 40% more usable floor space than a standard dormer conversion. The near vertical walls mean you have full head height around most of the perimeter, rather than sloping ceilings eating into your usable space. A typical mansard conversion in a West London terraced house creates 25 35 square metres of usable floor space enough for a master bedroom suite with ensuite, walk in wardrobe, and seating area, or two good sized bedrooms each with their own ensuite.

Is a mansard conversion better than a dormer?

If you want maximum space and can afford the higher cost, yes. Mansard conversions cost £55,000 70,000 vs £45,000 60,000 for dormers, but they provide 30 40% more usable space. They also add more value typically 20 25% of your property’s value vs 15 20% for dormers. However, mansards always require planning permission (8 12 weeks) and take longer to build (14 18 weeks vs 10 14 weeks). Choose a mansard if you want the absolute maximum space and the best return on investment. Choose a dormer if you want good space at lower cost and faster completion.

Can I create two bedrooms in a mansard conversion?

Absolutely. The generous space in a mansard conversion easily accommodates two good sized bedrooms, each with their own ensuite if desired. This is one of the major advantages over dormer conversions, which typically create one bedroom. Two bedroom mansard conversions are popular with growing families who need multiple children’s bedrooms or want a guest bedroom alongside their master suite.

What materials should I use for a mansard roof?

Natural slate or high quality traditional tiles are best, particularly in conservation areas where planning authorities often require these materials. Natural slate costs more than concrete tiles but looks far better, ages beautifully, and is more likely to gain planning permission. In conservation areas, matching materials to the existing properties in your street is often a planning requirement. We advise on appropriate materials during the design stage and include them in our fixed price quote.

Will my neighbors object to a mansard conversion?

It’s possible, particularly if they are concerned about overlooking or loss of light. However, neighbor objections don’t automatically mean planning refusal. Planning authorities assess applications on planning grounds, not personal preferences. To minimize objections, we design mansards to minimize overlooking (setting windows back from party walls, using obscure glazing where appropriate), engage with neighbors early to explain plans and address concerns, and ensure the design is high quality and proportionate. Most well designed mansard conversions gain planning permission even if some neighbors object.