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Design Guidelines for Extensions to Buildings

Physical Constraints

The obvious functional aspects such as structural stability and general housing standards are covered by the application for building warrant and are taken to be a separate consideration. Advice on these matters can he obtained from the Building Control Section. An extension may also affect road servicing by restricting vehicle manoeuvring space or the need to form a new access. An extension can also affect the neighhouring 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.

Form and location of side and rear extensions

The overall shape, size and position of side and rear extensions should not dominate the existing house. They should harmonise with the original appearance which should be taken as the starting point for any future changes. There are two alternative methods of achieving this aim: either the extension is integrated with the house. Or alternatively the extension is made to appear as an obvious addition which is subordinated to the main structure.

Great care must be taken in the selection of external materials which should normally match those already found on an 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 be unobtainable and as a result it may not be possible to achieve a perfect match. To overcome this problem it may be necessary to use substitute materials and when considering a side extension it will be advisable to set the extension back behind the face of the existing building.

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) Extension set back behind the front elevation.
(h) 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 which 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 which 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.


In order to achieve the airn of harmonising an extension to an existing house it will be required to use a pitched roof. The eaves of two storey side and rear extensions should also line up with those of the existing house and the pitch of the roof should also be similar. 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

Windows and doors are important features and an inappropriate choice can easily spoil an otherwise satisfactory design. In most cases it is worth trying to use the same kind of window throughout, with the proportional sizes of new window openings in general echoing those of the main house. The positioning of windows on the extensions should conform with existing symmetry and new lintel and sill heights should line up with those around existing openings. The internal divisions (mullions and transoms) within the window can completely alter the effect of the overall proportions. Ensure that new windows reflect the style and details of the existing windows.

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.

Gates, Fences and Walls

Boundary walls, where existing, should always be retained and not reduced in height. Where a stone wall requires repair or replacement the stone should match the original as closely as possible although in some cases a substitute material may be acceptable. Where possible cast iron works such as boundary and staircase railings should be retained and all gates and railings preferably painted black.


Trees growing within conservations areas are subject to special procedures and you are advised to contact your local council for advise if you are considering felling or pruning any trees.

Shop Fronts and Shop Signs

It is the aim of District Council policy that the character of the area is preserved and enhanced and it is important that shop fronts and shop signs should harmonise with and enhance the character both of the general scene and the individual buildings which contain them. The District Council have already prepared a leaflet on shop frontage design which can be obtained from the Planning and Development Department.


It is preferable that pipework or wiring for gas. T.V., etc. should be run up rear elevations wherever this is practical. T.V. aerials should be located in the roof space of buildings and satellite dishes should only be located on hidden elevations. Tn the case of satellite dishes planning permission will be required in specific circumstances and the advice of the District Council's planning officers should be sought before the installation of a satellite dish.

New Development

Applications for new development will be assessed against both the District Council's stated policies and the design criteria which has been set out in this leaflet. The District Council recognises it is possible to design a building which is architecturally good and uncompromisingly modern and which, by contrast, may add to the character of an area. It has however to be recognised that this is a difficult exercise which is only rarely successful. It is often best particularly when considering domestic buildings to follow a traditional approach. This involves studying the materials, proportions and detailing of surrounding buildings and using these elements in a traditional way, thus producing a building which blends easily with the existing character of an area.


Housing improvements, renovation works to listed buildings and other projects within the conservation area may attract grant assistance. A factsheet on the range of grants available can be obtained from your local Planning and Development Department. If you have any queries concerning any of the points or details raised in the leaflet, you are advised to contact the Environment Section of the Department of Planning and Development for advice and assistance.


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