Our Mansard loft conversion guide breaks down everything from typical costs to planning rules, so you can budget smartly and build with confidence.
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Considering a Mansard loft conversion but unsure about the costs, planning rules, or what’s involved? This expert guide covers everything, from average prices and regulations to design features, pros and cons, and how to maximise your new space.
Use the quick links below to jump to the sections that matter most, making it easy to explore every part of your Mansard loft conversion confidently.
A Mansard loft conversion typically costs between £35,000 and upwards of £55,000. The final price is influenced by the materials, property location, and specific features you choose.
Here’s an expert breakdown of the main cost components of a Mansard loft conversion in the UK.
Note: location can be a big factor in pricing. Please consider this and use our loft conversion cost calculator, or if based in London, please check out our London loft conversion costs page.
A Mansard loft conversion can vary in cost based on several factors. Here’s what typically drives the price up or down:
A Mansard loft conversion transforms your roof with steep, near-vertical sides and a flat top, creating an extra floor beneath. Originating in 17th-century France and popularised in 19th-century Paris, this design maximises space in tight urban areas by building upwards.
A Mansard loft conversion changes the shape of your roof to create maximum usable space. Here are the standout design features:
Because of how the roof is altered, a Mansard loft conversion creates more internal space than most other types, making it ideal for homeowners who want to maximise both head height and floorspace. You should look into our expert guide on raising a roof for your loft conversion to give you a bit more detail on considerations surrounding roof features on a Mansard.
If you're comparing loft conversion types, you'll likely come across Mansard and Dormer conversions, two of the most popular options in the UK. While both add valuable living space, the design, cost, and planning requirements differ.
If you want a quick overview, here's our concise overview table of Dormers vs Mansards.
A Dormer loft conversion extends from the existing sloped roof by adding a box-like structure, which increases headroom and usable floor space without requiring significant alterations to the original roof shape.
In contrast, a Mansard loft conversion involves entirely rebuilding one or more sides of the roof. The new structure features a steep rear slope and a flat top, resulting in a much more significant structural change that effectively creates a full additional storey.
Visually, Dormers are designed to work with the existing roofline and often blend well with traditional pitched roofs. They can be added to a property's front, side, or rear with minimal impact on the home’s overall appearance.
Mansard conversions, however, completely reshape the roof, typically at the rear, and give the home a more modern, boxy look with a flat roof and steep, almost vertical walls. This change is often more noticeable and dramatic from the outside.
Rear Dormer loft conversions are often classed as permitted development, meaning they don’t require formal planning permission if they meet certain size and placement limits. In contrast, Mansard conversions almost always require full planning permission due to the extensive structural changes and alterations to the roofline.
Dormer loft conversion costs start from £25,000 to £40,000 depending on the design and finish. Mansard conversions cost more, typically ranging from £40,000 to £60,000 or higher, but they offer far greater space and can significantly boost a property’s long-term value, particularly in high-demand areas.
Here's a quick overview to make it easy to digest the key differences between the two.
You are now probably wondering, which is best for you? Our experts say to choose a Dormer loft conversion if you want to add usable space without major roof changes or planning hassle. Opt for a Mansard if you're after maximum space and are happy to go through the full planning process.
In most cases, planning permission is required for a Mansard loft conversion. Unlike simpler loft types (like rear dormers), Mansard conversions involve significant roof alterations that change the shape and height of your property. Because of this, they don’t usually fall under permitted development rights.
Planning departments consider the visual impact, neighbouring properties, and any changes to the roofline. Planning permission is always required if you live in a conservation area, a flat, or a listed building.
Note: consider speaking to your local planning authority early. Each council can interpret the rules differently, especially in well populated or historic areas.
Even if planning permission isn’t needed (which is rare for Mansards), your loft conversion must still comply with building regulations.
These rules ensure your loft is safe, energy-efficient, and structurally sound. Key areas covered include:
A building control officer will inspect the work at key stages and issue a final completion certificate when it meets the standards.
Here are five specific guides on the above regulations to consider:
1. Fire safety regulations for loft conversions.
2. Loft conversion insulation.
3. Ventilation in loft conversions.
4. Loft conversion stairs.
5. Minimum height for your loft conversion.
If your property is terraced or semi-detached, a Mansard conversion will likely affect a shared wall with your neighbour. In this case, you’ll need a Party Wall Agreement under the Party Wall etc. Act 1996.
This involves:
Although you don’t need your neighbour’s permission to build, you do need to follow the process correctly, otherwise your project could be delayed.
It's also worth discussing the plans informally with neighbours beforehand. A friendly heads-up can make the formal process much smoother.
A Mansard loft conversion offers one of the most effective ways to add an extra floor to your home. It combines maximum internal space with a clean, modern finish, increasing your property’s value and unlocking flexible layout options. From bright bedrooms to home offices or en-suites, a well-designed Mansard can completely reshape how you live.
Space is often the primary motivation for loft conversions, and the Mansard style excels in maximising usable area with headroom and flexibility for multiple room types.
Adding both space and a modern look can significantly boost property value, making this benefit particularly compelling to homeowners focused on investment returns.
Homeowners are increasingly interested in personalisation, and the Mansard design supports features like dormers, balconies, and adaptable room layouts.
Large windows not only create a welcoming, bright atmosphere but also enhance the loft’s appeal and usability, which many buyers or residents find attractive.
With growing interest in energy efficiency, modern insulation in Mansard lofts offers a valuable selling point by lowering energy bills and improving comfort.
The Mansard design’s architectural charm adds curb appeal and elegance, transforming both the interior and exterior look of the home.
With its unique design and transformative impact, a Mansard loft conversion is more than just an upgrade, it’s a way to elevate your home’s style, space, and value. Let’s explore the standout benefits that make this loft style a popular choice for homeowners.
A Mansard loft conversion typically takes 3 to 6 months, depending on the project’s scale and any unique requirements. Here’s a breakdown of each stage to help you plan:
Planning a Mansard loft conversion requires special attention to structural requirements to ensure the conversion is stable, safe, and built to last.
A Mansard conversion involves transforming the roof into a steep, nearly vertical slope, adjustments to the roof structure, weight support, and insulation are essential - Here is a breakdown of the key structural considerations:
Pro tip: Engaging professionals and addressing these requirements early in the planning process can help your mansard loft conversion go smoothly and ensure a safe, long-lasting addition to your home.
Mansard loft conversions are among the most effective ways to add an extra storey to your home. But with that level of transformation comes a lot of questions. Here, we’ve answered the most common ones to help you decide if a Mansard is right for you.
Mansard conversions are costlier due to the scale of structural work involved. Rebuilding the roof, installing steel supports, and creating a full-width rear extension require more labour, materials, and time than simpler loft types. The result is a full-height extra floor that feels like a natural part of your home.
Mansards offer more internal space and head height, making them ideal for homeowners looking to create an extra floor. Dormers are simpler and cheaper but don’t offer the same level of transformation. The right choice depends on your goals, space, and budget.
Build time is typically 10 to 14 weeks, but planning, design, and party wall agreements can add a few more. Weather, property type, and specification also affect the timeline. Start to finish, allow several months for the full process.
Yes, particularly in high-demand areas. While build costs are higher, the value added can reach 20%. It’s often cheaper than moving, avoids stamp duty, and creates the necessary space without leaving your current location.
