Mansard Roof Extensions

A feeling of lack of space within the home is a common issue nowadays. No matter the size of family or household an often-limited floorplan means there never seems to be room to grow or do the things you want to.

Home working and modern life has brought this issue of lack of space to the fore, but with house moves expensive, disruptive, and stressful, more homeowners are left asking, “How can I stay in my community, create more liveable space and a better family lifestyle?”

With limitations around side and rear extensions particularly in densely populated urban areas such as London, the option which presents itself is to extend upwards. In this article we examine how to achieve this with mansard roof extensions.

These extensions effectively add an extra floor to a property by raising the ceiling height. Throughout the years they have proved to be an affordable and aesthetically pleasing solution for homeowners, particularly those living in Victorian and Georgian terraced properties built for smaller families over a century ago.

What is a mansard roof?
Before we look at mansard roof extensions, we need to examine what a mansard roof is. Sometimes confused with a gambrel roof, a mansard roof often known has a French roof or curb roof, has two slopes on each four sides. It is less steep at the top and very steep at the sides, the top is often flat or very nearly flat.

The lower slope characteristically contains box-shaped dormer windows (a window typically set vertically into a sloping roof).

Built to the front or the rear of a property usually with natural slate, when viewed from the ground, the upper slope is rarely visible.

There are design variations and which one a client chooses will depend upon the environment in which they live and the structure of their home. The variations are:

– Straight: slight slope on the upward panel portion
– Convex: curved outside on its lower slope portion
– Concave: curved inside on its lower slope portion, with a steep angle on the bottom

History of a mansard roof
We can trace the earliest known example of a mansard roof back to French Renaissance architect Pierre Lescot (1515-1578) who designed a double pitched mansard roof for the Lescot wing of the Louvre in 1550.

One of the most influential buildings of the Renaissance, this led to Francois Mansart (1598-1666), an accomplished architect of the French baroque popularising the design and giving the roof profile his name.

Seen as elegant, effective, and fashionable, the adoption of mansard roofs became widespread in Europe and North America following Haussmann’s renovation and rebuilding of Paris in the 1850s as instructed by Napoleon III.

Alongside the transformation of the city’s boulevards and public buildings, this saw modern apartment buildings created, characterised by mansard roofs and cream coloured limestone, such as those found to this day on Boulevard Haussmann.

His work underpinned a wider architectural movement, second empire style marked by an eclectic and decorative style which flourished between 1852 and 1871 and was widely used in large municipal and corporate buildings such as The Palais Garnier.

Mansard roofs in London
It was not until the late 18th Century that mansard roofs appeared in London and grew in popularity during the Georgian and Victorian eras due to a lack of space, replacing the traditional butterfly roof associated with terraced properties.

Named the London roof due to the prevalence within the city, the Victorian butterfly roof or a V roof is characterised by the inversion of a standard roof form, two surfaces which slope down and meet with a valley gutter in the middle. The look is of a butterfly’s wings hence the name.

The slope of this roof at 37.5 degrees allows them to accommodate a mansard roof extension building a mansard roof with a 70-degree slope, which effectively adds an extra floor to a property.

The very reasons why homeowners desired mansard roof extensions in the 18th and 19th Century exist up until this day, particularly in crowded and space-starved London as a front and rear extension allows up to 32.5sq m of generous indoor space compared to a loft conversion which may only add 23sq m.

This typically means an additional double bedroom with en-suite, or multi-functional open plan family living space. They can where practical also offer the opportunity to incorporate a roof terrace.

Compared to adding a rear or side extension this is a cost-effective way for people to maximise their living space and remain in their communities. It also in most cases adds significant value to the property.
Today mansard roof extensions are widespread across London with a desire by homeowners to ‘improve not move.’ They are particularly prevalent areas such as Fulham, Hammersmith, Kensington, Primrose Hill, Paddington, Bayswater, and Notting Hill.

At RISE Design Studio we have worked on a variety of London based projects which have seen the replacement of the traditional butterfly roof into a mansard roof extension such as our Queen’s Gate Sky home project in South Kensington.

Mansard roof extensions and roof terraces
In London as with any densely populated city private outdoor space is at a premium. As well as extra living space a mansard roof extension where feasible can give clients the option to add in an enviable external roof terrace.

They also add considerable value to a property with reports saying this can be between 10% and 30%. As with a mansard roof extension this return in investment is far greater than the outlaying costs.

Mansard roofs and planning
Due to the architectural importance of uniformed British terraces built in the 18th and 19th century, and their historical significance within London, successive Building Acts and Building Regulations governing height, windows, chimneys, and materials have meant that the design and form differs to that in France and other European countries.

In some areas such as Tower Hamlets in Bow, the design was even banned in the 1990s as local heritage officers felt it affected the look of the traditional London ‘butterfly’ roof. It was also in part because many of these historical terraces had survived the Blitz. This decision was reversed in 2017 with a view to have a more ‘permissable approach’.

Today in Bow and across London, it is almost always necessary to apply for full planning permission to construct a mansard roof. As discussed, this is due in part to historical significance as well as the structural alterations and the implication on the current roof profile.

All alterations conducted need to preserve the structure, character and appearance of a building looking at how that property fits into the wider context of the street and surrounds.

In conservation areas it can be difficult to get planning when the property sits alongside others which remain unchanged. However recent developments such as those in Primrose Hill, and across London, have seen neighbours come together so that several adjoining properties or a whole terrace applies at the same time.

This helps achieve uniformity, timeless design, and avoid the strange uneven ‘gap-tooth’ effect where some houses have extended upwards, and others have been refused.

A group application such as this can cost as little as £339 and as long as owners are informed you do not need their permission to proceed with the application. Furthermore, normally once planning is granted you have three years before it lapses so in practice as long as the person who made the application begins work in that time the other households don’t need to.

This kind of group application may be stemmed in the future due to the introduction of Community Infrastructure Levies for those applications adding more than 100 sq m (1076 sq ft) of floor space to a street. Consideration for smaller batches of group applications may need to be the way forward.

It is essential when applying for listed building consent whether individually or as a group, to hold pre-application discussions with the conservation officer within the local authority at an early stage, this helps avoid later problems and establishes a clear line of communication.

Our professional team will consult with the appropriate departments throughout the process from design through to build. We will also assist with Party Wall Agreements which are required if the work impacts on adjoining properties.

In addition, many local authorities have published residential design guides if they are in an area with a high number of mansard roof extensions. These guidance notes such as this one from City of Westminster Council will answer questions including:

– Maximum floor to ceiling height allowed – 2.3m
– Maximum pitch of the front and back walls, usually 72-degree angle – set back from existing elevation
– Which materials to use in line with the appearance of the building

The local authority planning department will inspect the renovations to ensure that all of the regulations are followed.

With considerable experience in historic building renovation, planning applications, and mansard roof extensions we guide clients through the process to put forward the strongest case for their design. Our applications are always supported with photographs and detailed architectural drawings in line with PPG15 ‘Planning and the Historic Environment’.

If you live in an area where there are no listed buildings,’ then it is easier to gain planning permission under permitted developing rights. These allow you to perform certain works without the need for planning permission and the Local Authority will have that information. Again, we can help to guide you through this process.

How much does a mansard roof extension cost?
There is a considerable amount of structural work involved in adding a mansard roof extension to a property. It is for this reason that they are not cheap.

In London for example costs would range dependent on the building and quality of installation, from £40,000 on average to £70,000 and over.

However, what must be considered is the financial value that they bring to a home in adding what is in essence a whole extra floor. A property which costs approx. £1000 per sq ft, can increase roughly by £100,000 when a front and rear mansard roof extension is added.

As well as that, putting in an additional floor and going from three to four floors for example, will still work out to be considerably cheaper than a house moves. Relocating from say a £1.2million home to something further up the scale which may not even be in the same area could set you back £80,000 in Stamp Duty alone.

Advantages and disadvantages of Mansard roofs
As with any significant alteration to a home, it is important to consider upfront the advantages and disadvantages that a mansard roof extension presents.

Here at RISE Design Studio, we always advise our clients thoroughly and present them with the most appropriate and sensible solution for their property, environment, budget, and needs.

As well as being aesthetically pleasing, the primary advantage of constructing a mansard roof extension is that it increases the volume of the interior of a property thereby creating additional space and functionality.

The almost vertical bottom slope achieves this. The use of dormer windows along this slope also ensures that the space benefits from ventilation and plentiful natural light further adding to a sense of space.

Extending upwards means that homeowners retain their garden space. In addition, the space created can often be more significant and cost-effective than a rear or basement extension and offer a more discreet design.

Whilst popular amongst terraced properties, they are also suitable for most homes where the finish can match an existing building style.

The disadvantages are that you are creating an entire new roof, which is a significant and often costly undertaking and, in most cases, full planning permission is required, which can be stringent particularly in conservation areas. However, our team will help advise on local authority regulations.

In today’s congested urban environment with tightly packed housing well-designed mansard roof extensions present numerous benefits.

Not only do they solve immediate problems modern households have by creating contemporary liveable space, adding value, and ensuring families remain in their communities, but in a wider sense as evidenced by Dr Samuel Hughes proposal ‘Living Tradition’, adding to and adapting these historic properties can improve housing supply and regenerate neighbourhoods and local areas.

Furthermore, in environmental terms, they do not encroach on green spaces, have a lower carbon footprint as a new building is not being constructed and if done correctly have little impact on the skyline.

Why employ an architect to design your mansard roof extension?
There are many specialist builders who work on loft conversions and mansard roof extensions, an architect however brings with them an independent design approach to help you achieve your specific and bespoke brief, as opposed to a firm’s in-house designer who may have a more generalistic ‘cookie cutter’ approach based around reducing costings.

• They see the bigger picture
Rather than a generic one-size fits all design based around saving money and time, a reputable and experienced architect will work collaboratively with you to understand your vision. They will look at your property holistically to create a well-thought-out design that meets your needs, budget, and your overall aesthetic.

Working alongside you they will give your home much-needed bespoke space, maximising the potential of your property and adding value to it whilst enhancing your lifestyle.

They can also present different spatial options that you may have not considered, suggest new technologies in terms of energy efficient heating and lighting, allowing you to reach your desired outcome and get the most from the space in ways you may not have considered.

• They are up to date on planning and legislation
Architects are well-versed in current planning and legislation, and usually have established relationships with local authorities, conservation departments, and planning.

Planning applications, and permissions can seem daunting, and an experienced architect can help to navigate these and any challenges, with professional guidance and advice, ensuring a smoother process which is timely and a lot less stressful.

• They use innovative technology to bring your vision to life
Architects such as ourselves at RISE Design Studio use the latest in technology such as Virtual Reality (VR) to help you to experience and visualise your design in superb detail before construction even begins.

Not only does this ensure that you get the design you want but it helps to avoid any issues with contractors and specialists as the build progresses because every little detail has been taken into account in advance.

• They can act as a contract administrator
If you employ a specialist company to carry out your mansard roof extension there is no-one overseeing the work apart from the company themselves, they are carrying out the work and project managing it.
An architect can act independently with your interests at heart to oversee the work and ensure the quality is as expected in both build and finish, and nothing is rushed through.

At RISE Design Studio we have considerable experience in mansard roof extensions and roof terraces. For more information and to discuss your project contact us at mail@risedesignstudio.co.uk or give us a call on 020 3973 9073.