Mansard roof extensions | London 

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A guide to Mansard roof extensions

Mansard roof extensions are the most cost effective way to maximise the floor area of your house or flat when compared to adding the same floor area with a rear or side extension.  Mansard roofs are a widespread and traditional method of adding an extra storey to the top of a Georgian, Victorian or Edwardian building. Named after a French architect, Francois Mansart, the mansard roof comprises a sloping roof clad in slate or tiles, set back behind a parapet, with dormer or attic windows, which either project from or are recessed into the slope. It emerged in the eighteenth century in Britain, when it was also referred to as a kirb roof. It allows extra accommodation at roof level, partially hidden behind a parapet wall, without having a great impact on the appearance of the façade below.


This type of roof extension is appropriate for properties with what is commonly known as “butterfly” or “V” roofs. Such roof extensions practically add a full new level to your property, and they can be the perfect solution to houses or flats in conservation areas. The planners will take a view on whether a mansard roof can be added and where. And here is the rule of thumb when it comes to planning: if the majority of the properties in your terrace have mansard roofs then it is likely the planners will approve an application for a mansard roof to your property.

There are two main types of mansard roofs:

Traditional | Traditionally most mansard roofs have two separate slopes or pitches, however they are more expensive to construct and less common these days.

Modern | Modern roof extensions often have a flat roof profile at the top, with a gentle slope to the rear.

There are several crucial elements to the correct appearance of a mansard roof, namely materials, slope of the roof, proportion of the dormer windows and extension of the party walls and chimneys. Such roof extensions always require planning permission from your local authority.

Design aspirations 

Designing the staircase is the single most important thing we will need to consider for any type of roof extension or loft conversion. There are building regulation restrictions in regards to the risers, goings, and head heights in regards to the staircase. There are fire regulations to consider, and plumbing/drainage issues when an en-suite is to be created. The design challenge is about how to exercise intelligent spatial planning around all the relevant restrictions so that both the new loft layout, and the layout of the floor below do not get compromised when the staircase is added to the existing floor plan.

Start considering how you would like to use your new space, and how it will interact with the rest of your home. It’s important that the new extension blends with the rest of the building and is easily accessible, so when considering the staircase think about its long-term use as much as its style.

During the design development stage we will get to explore lots of ideas but you can start the process now by considering answers to questions as such:

“How can we best use this new room? Will it be our new master-bedroom, a study space, a nanny’s room? What other bedrooms are available in the house, and what would work best for our changing needs and family?”

“Space permitting, how about adding an en-suite, a walk-in wardrobe, or even a coffee area in front of a Juliet balcony?”

“What do we think about rooflights in the bedroom? They bring more light into the room but how about the rain ticking on the glass; is it charming or just plain annoying?”

“Planning permitting, how about we sacrifice some of the internal floor area to create a small balcony with doors opening up to it directly from our new (master-) bedroom?”

Inspiration and ideas

Below we have selected some projects to lend architectural inspiration and design ideas for your Mansard roof extension. Enjoy!

purplebox Mansard roof extensions in London | Home ideasVisit our portfolio for a selection of case studies or alternatively visit our London Residential page for your local authority and information about planning.

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We see ourselves as creative problem solvers who will deal with any construction, planning, and design issues relevant to your project.

Contact Details

Hoxton Mix | 86-90 Paul Street | EC2A 4NE | Hackney
Elvaston Place | SW7 5NP | Kensington and Chelsea

t: 020 3872 2474


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Website | Blackledge & Sandall

Creative Director | Umi Ishimoto