Design Guide to Extensions

Extension to side of dwelling

External Design and Appearance

The careful consideration of the external appearance of an extension must not be ignored if a sympathetic addition to your dwelling is to be achieved. It is usually easier and more successful to copy the appearance of the existing house, however, a contrasting design using different materials can achieve satisfactory results when the effect is to change the overall appearance of the dwelling.

Physical Constraints

The most obvious constraint to any extension is the amount of physical space available for the extension, the location to the existing house and its relationship to the nearest boundary. The obvious functional aspects such as structural stability and general construction standards are covered by the application for Building Control approval. An extension may affect car-parking arrangements by restricting vehicle manoeuvring space or the need to form a new access. An extension can also affect the neighbouring house and gardens in terms of potential loss of sunlight, daylight and privacy. A window in the side of an extension for example or even a new window on an existing wall can cause problems of overlooking, that may be rejected by the local Planning Office.

Form and location of side and rear extensions

The extension must not dominate the appearance of the existing house, its shape, size and position must be carefully considered, if a harmonising between old and new is to be achieved. Great care must be taken in the selection of external materials, which should normally match those already found on the existing house. The aim will be to integrate the extension with the original house keeping the number of materials used to a minimum. In the case of many older houses the existing materials will have changed colour and texture due to age and weathering and may require cleaning to restore their appearance. The reuse of existing materials such as roofing tiles can greatly improve the integration of the extension into the existing house. In general the following factors could be said to contribute to a well-designed extension.

  • (a) A new extension harmonises with the original appearance of the house.
  • (b) It has a pitched roof compatible with the existing roof.
  • (c) The colour and shape of roof materials and ridge tiles match existing.
  • (d) Eaves line up with those of the main house.
  • (e) The type and proportions of new windows echo those in the original house.
  • (f) New windows positioned to conform to the existing symmetry.
  • (g) Materials and detailing match those already found on the house.

The following factors could be said to contribute to an unsightly extension.

  • (a) The extension ignores the style, character and appearance of the original house.
  • (b) It has a flat roof that clashes with the existing pitch roof and gable.
  • (c) There is a clumsy junction between the old and the new roofs.
  • (d) It has inappropriate windows that differ in shape, in alignment, and materials from the existing.
  • (e) The new materials for walls and roofs of the extension are unattractive and incompatible with the existing house.


To achieve the aim of harmonising an extension to an existing house, the new roof must match the style, pitch, colour, material and shape of the existing. The line of the eaves of two storey side and rear extensions must also line up with those of the existing house. The colour and shape of new tiles and slates should match those of the existing roof, this is particularly important when the new roof connects directly into the existing.

Windows and Doors

The style, shape and detailing of windows and doors to the extension must correspond with the appearance of their counterparts in the existing house, if a satisfactory integration of design is to be achieved. The proportional size, sill and lintel heights of new window openings should conform in general terms, echoing those of the main house. The symmetry of window and door positions should be maintained to preserve the character of the existing house.

Dormer Windows

It is rarely desirable to add dormers to the front of the house, but where necessary they must have a pitched roof and materials to both the roof and the sides of dormer windows should match those existing on the roof. Try to ensure that the dormers relate to the shape, position, design and size of the existing doors and windows and ensure that the construction of dormer windows and other roof extensions do not dominate the original house. This will require any new dormer to have a clear area of roof around its perimeter and to be kept as small as possible.


The location of services to the existing house must be investigated and established so that the extent of any alteration to them can be realised. This is particularly true for rainwater down pipes, soil vent pipes and underground drainage runs. It is preferable that pipe-work or wiring for gas. T.V., etc. should be run up rear elevations wherever this is practical.