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House Extensions Design

If you are planning to add an extension to your house, one of the first things to consider is the design. There are four basic variations: a single-storey or a double-storey extension with either a flat roof or a pitched roof. 

Architectural plans of a single storey rear house extension with a flat roof
Rear house extension with a flat roof design
Architectural plans of a two storey side extension with a pitched roof
3D Design of a double storey side extension with a pitched roof design
Architectural drawings of a two storey rear house extension in London
Two storey rear house extension with a flat roof

Planning your Home Extension

Deciding on the right design is not as easy as it sounds. What might look like a fabulous home improvement to you, might be your neighbour's worst nightmare. Your extension will need to comply with the Local Development Framework, which you can view at the planning department of your local Council or online. To get an idea of whether or not your extension is likely to be permitted, take a walk around your local area to see what size and type of extensions other people have built. As a general rule, house extensions should complement the existing building instead of overwhelming it. Flashy designs are often rejected. This does not necessarily rule out contemporary extension designs that contrast with the original house, as using original natural material might add to the character of an older property. A highly glazed extension on an older period property, for example, can appear very modern and striking, yet at the same time, it somehow enhances the look of the house. If you stick to the traditional design, using matching bricks, or rendering your extension with the same type of plaster as the original building, will help it blend in.

Two storey side extensions will usually need to have a roof of a pitched design similar to the original house roof, with the roofline preferably being slightly lower. The roof tiles or slates will need to match too. Single storey extensions and sometimes even two-storey rear house extensions can be designed with flat roofs, which are generally cheaper. Pitched roofs tend to look better and last longer, but there are sometimes technical obstacles preventing them from being built, such as the house's first-floor windows being too low.

Some extensions might qualify under the Permitted Development, and will not require Planning Permission. If your extension does not qualify, you will need to submit a Planning Application to your Local Authority, along with a full set of scale drawings in metric measurements, showing all the work you are planning to carry out including the locations of doors and windows.

What should a set of house extension drawings include?

  • Separate drawings for each of the required elevations, floor plans and cross-sections.

  • Measurements marking the dimensions of external walls and the positions of window and door openings.

  • Specification document explaining how the design will comply with the building regulations in key areas.

  • Structural drawings showing the positions of supporting steel posts and beams. 
  • Architectural design and floor plans of a single storey rear extension with a pitched roof
    Rear house extension structural drawings and specifications

    Home Extension Materials and Construction Methods

    When it comes to choosing the building materials for your extension project, you can choose between the traditional masonry walls, a prefabricated timber structure, or a contemporary glass room. 

    Masonry wall house extension construction

    Traditional Masonry

    Masonry walls consist of three layers: An outer brick wall tied to an inner leaf which is often built from concrete block, and a gap or an insulated cavity in between the two. It is mostly the inner leaf that supports the roof or additional floors, so the correct type of block must be used. An architect or structural engineer will specify the type of block with enough strength to support the loadings. A masonry structure makes your house extension feel more solid while storing heat at night and helping reduce noise from the outside. The external bricks or blocks can be finished with render, or, if you are extending a period house with brick walls, it is best to match your new extension by using similar bricks and leaving them exposed.

    Having the best insulation
    Being the longest lasting

    Taking longer to complete

    Construction of a prefabricated timber house extesnion

    Prefabricated Timber Frame

    Modular timber extensions are generally cheaper and faster to build than traditional brick ones. Still, you will have to order the made to measure components weeks, if not even months, in advance. Contrary to what you might expect, a prefabricated timber extension will look similar to a regular one once built. That's because the outer wall is usually constructed with the same brick or rendered blockwork. The timber frame forms the inner leaf of the construction and supports the building's structural loadings. 

    Being the least expensive
    Taking less time to construct

    Not lasting as long as traditional masonry

    Highly glazed house extension in North West London


    Highly glazed extensions (where the total area of glazing exceeds a quarter of its floor area) have been gaining in popularity amongst home-owner in London because of their stylish contemporary look. If you don't care that the neighbours will know what you are up to half of the times, and there isn't an army of pidgeons living on your roof, you might want to opt-out for a frameless 'glass box'. Or you could compromise by building a masonry or timber extension with a whole wall composed of sliding glass panels. You might be allowed to add a glass extension to a period house, or even a listed property, as their transparency stops them from clashing with the existing building. The biggest problem to overcome when you are building a highly glazed extension is thermal performance. Even when using low emission glass, you might still be forced to improve the thermal performance of your existing house to comply with Building Regulations.

    Being able to blend in with the surrounding
    Allowing a lot of natural light into the house

    Being high maintenance

    Office Address

    Kingsbury House
    468 Church Lane
    NW9 8UA


    Phone: 0208 200 2327

    About Us

    We are a Design and Build construction company specialised in building new homes, house extensions and loft conversions in North and North West London and Hertfordshire. We cover Notting Hill, Kensington, Maida Vale, Camden Town, Golders Green, Hampstead, Finchley, Hendon, Wembley, Colindale, Mill Hill, Harrow, Barnet, Hadley Wood, Borehamwood, Watford, Welwyn Garden City and St. Albans.