What is a mansard roof?

A mansard is a type of roof that was fashionable in Europe in the 18th and 19th centuries. Mansard roof extensions provide a larger floor area in comparison to a typical loft conversion, as the ceiling height is maximised around the whole perimeter of the space. They have a steeper angle to the outside edge (usually 70-80 degrees), and a lower slope on top, which is usually invisible from the street level. This increases the amount of standing area inside.

Mansards with dormer windows

Although dormer windows can be added to many different types of roofs, they are commonly paired with mansard roofs.

Dormer windows are set vertically on a sloping roof. Such windows will have their own roof structure, which can be a variety of different styles, such as hipped, gabled, arched, or flat.

Like the mansard roof, dormer windows raise the pitch and can be used to increase the habitable areas inside.

Mansard loft conversions in London today

Mansard loft extensions may be added to your home through permitted development rights, or they may require planning permission, depending on your particular property and its context.

Permitted development rights

If your property is not Listed and is not within a Conservation Area, it is possible you will not need planning permission to add a mansard and can take advantage of Permitted Development Rights (PD) instead.

In August 2020, a new permitted development (PD) right was introduced to the General Permitted Development Order. This PD allows people to extend the height of their home by 1-2 storeys and allows for loft conversions without the need for planning permission.

The previously existing permitted development rights may allow for a rear mansard roof loft conversion, provided that the following provisions are met:

  • the cubic content of the resulting roof space would not exceed the cubic content of the original roof space by more than—

  • 40 cubic metres in the case of a terrace house, or

  • 50 cubic metres in any other case;

  • no part of the dwelling house would, as a result of the works, exceed the height of the highest part of the existing roof;

  • no part of the dwelling house would, as a result of the works, extend beyond the plane of any existing roof slope which forms the principal elevation of the dwelling house and fronts a highway;

  • it would not consist of or include—

  • the construction or provision of a veranda, balcony or raised platform, or

  • the installation, alteration or replacement of a chimney, flue or soil and vent pipe; or

  • The property is not a listed building, in a conservation area or area of outstanding natural beauty

Working with an architect is the best way to ensure that your plans for a loft extension will be permitted. While there are PDs that will allow for mansard loft conversions, the wider scope of work may require further substantial alterations to a property. In these instances, PDs may not be appropriate and planning permission will instead be required.

Advantages and disadvantages of mansard roofs

As with all building works, there are advantages and disadvantages to be aware of before deciding if a mansard loft conversion is the best choice for you and your home. Working with an architect is the best way to ensure that the space is best utilised and beautifully designed.

The primary advantage of a mansard roof over other loft extensions is that they create greater usable space than other types of roof conversions. They can also be more cost-effective than a ground floor extension and allow for an increase in square footage without reducing the size of the garden. In addition, as the roof structure will require insulating there is a scope for improving the thermal performance of your property.

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