Extensions: The design rules

When designing an extension it's important to consider the end goal - how do you want to use the extra space? Will the extension add value to your home? Experienced renovator Michael Holmes explains the rules for designing an extension, including advice on style, access and light.

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ABOVE: This end-of-terrace house in north London has been extended over two storeys at the rear and remodelled for an open-plan ground floor kitchen/breakfast room and a first floor study. Find out more about this project…

Experienced renovator Michael Holmes explains the rules for designing an extension, including advice on style, access and light.


Make sure the extension’s scale and form, and the materials used in its construction, are complementary to the existing house.A seamless extension that looks like it was constructed as part of the original house is a clever option and what many people aspire to, but it is difficult to get right. You need to pay attention to both the original materials and detailing.Choosing a different period style can work very well, designed as if the house has evolved over time, and it is often easier to achieve convincingly than trying to match existing materials and create a seamless extension.

Contemporary-style extensions can work very well with period houses, providing a sympathetic style and materials are chosen. Setting an extension apart from the existing house with a fully glazed link can be a good way to overcome any objections from the planners, especially when extending a listed building.

An extension can be part of an overall remodelling scheme to give your home a totally new look. This is a particularly good option for a mid-to-late 20th-century property with little architectural style, or for a property with several mismatching extensions, which have been added on a piecemeal basis.


Careful thought needs to be given to where the access to a new extension will be, in order to minimise the amount of existing space sacrificed for circulation.

If the extension adds a new principal reception room, such as a kitchen, breakfast room, living room, family room or dining room, then access should always be from the main entrance hallway. Where this is not immediately possible, consider integrating the extension with existing rooms and reconfiguring the room plan.

If the extension adds secondary rooms, such as a study, playroom, music room or utility room, than access via another room is not an issue.

Make sure that new rooms are sufficiently large to serve their proposed function. Small extensions may be best used to enlarge existing rooms.

At first floor level, new bedrooms should be accessed from the main centrally located landing at the top of the stairs. Where this is not easily achieved, it can make sense to sacrifice all or part of an existing bathroom or small bedroom to enlarge the landing to form a corridor to provide direct access.

Linking new bedrooms through an existing bedroom is not a good solution – if this is the only option and you can’t afford to sacrifice a room, then consider creating a separate staircase within the extension to reach the new first floor rooms. Make sure each bedroom has easy access to bathroom facilities.


Introduce as much light as possible into your extension, from as many different directions as is practical. As well as conventional windows and glazed doors, consider adding roofl ights, glazed ceilings and fl oors, high-level windows, and using obscured glass to overcome privacy issues while still allowing in light.

Never leave an existing room without natural light by extending around it and leaving it with no window openings. The only exceptions are if you plan to build a utility room, cloakroom or storeroom, all of which can function without windows providing they have ventilation. The best option is to remodel and go open-plan, combining existing space that is losing its windows with the new extended area.

There are many ways to comply with the Building Regulations for energy-efficiency (Building Standards in Scotland), and they do not necessarily restrict the amount of glazing you can include in your design.

If your designer says you can’t have all the glazing you want, suggest they look at the Overall Carbon Index Method of complying. This allows energy loss through a large area of glazing to be offset by improvements in energy-efficiency elsewhere in the building.