This information provides design guidance on various aspects of new development. Its purpose is to provide helpful advice to anyone considering extending their property.
The Council is committed to promoting high standards of development throughout the Borough and encouraging proposals which make a positive contribution to the character of the area.

Introduction

Building an extension is often the greatest expenditure made by householders except for purchasing the house itself.

Good design is important not only because as an owner you will have to live with the results but it will also help avoid any adverse impact on your neighbours and therefore assist in obtaining planning permission from the Council.

Whilst a well designed extension may, in the first instance, appear to cost more, it is more likely to enhance the value of the property and may require less maintenance in the longer term.

This guidance sets out examples which illustrate the principles of good design.

Planning an Extension
A planning application is required for all but very small extensions and alterations, which if within certain criteria can be built under what is known as ‘permitted development rights’. However any structural alterations will need approval under the Building Regulations. You are advised to always check with the Council the need for any planning permission or Building Regulation approval.


Every planning application for an extension must be judged on its merits after careful assessment on site. Therefore compliance with the requirements set out on this page will assist, but cannot guarantee the grant of planning permission. In some areas, such as Green Belt or Conservation Areas, additional restrictions apply. The information here cannot cover every situation and further advice can be obtained from the Planning Division.

Every planning application for an extension must be judged on its merits after careful assessment on site. Therefore compliance with the requirements set out in this leaflet will assist, but cannot guarantee the grant of planning permission. In some areas, such as Green Belt or Conservation Areas, additional restrictions apply. This leaflet cannot cover every situation and further advice can be obtained from the Development Control Section of Planning Services.  This will include works to listed buildings.

When contemplating an extension or alteration it is very important to consider the siting, design and appearance from the outset. The scale, size, massing and use of materials are all elements of design which will determine how the extension will look, and how it will affect neighbouring properties and the appearance and character of the street scene.

Before starting detailed design work you should consider the effect any extension may have on your neighbour’s property and ideally discuss your proposals with them. This may avoid problems at a later stage. It is Council policy to notify neighbours immediately affected by planning applications and to take account of their views.

General Materials and Detailing

Whatever the form or size of an extension, the appearance will be improved if the materials compliment those of the original dwelling. On older houses, matching second-hand materials often provide the most satisfactory result. In the creation of a new roof it may be preferable to reuse the existing roof tiles at the front, with new tiles on a less visible elevation.

The use of alternative materials to produce a clear contrast may also be acceptable, provided the overall effect is sympathetic to the character of the existing building. Setting the extension back from the existing front elevation can be a useful device in lessening its impact, especially where differing materials are used.



The scale and proportions of the existing house should be reflected in the extension (Fig. 2). It is important to maintain the same relation ship of door and window openings, and to use the same design and style. Installation of inappropriate designs of double glazing or replacement windows can upset the proportions and appearance of a building. The cumulative effects of unsympathetic alterations can significantly alter the character of the street.

Outlook, Daylight and Privacy
Any extension, whether single or two storey, should not cause problems by overshadowing adjoining dwellings or blocking daylight and sunlight.

Remember to consider the levels of daylight and sunlight in any rooms which are to be extended.

Single storey extensions which project no more than 3 metres from the main original rear wall of a terrace dwelling and 4 metres for a semi-detached property will not, in most cases, cause a problem for neighbouring properties.  However orientation and proximity to adjoining windows will be significant factors.

Windows should be positioned so as not to cause problems of overlooking and undue loss of privacy for adjoining dwellings. Proposals which significantly reduce levels of daylight and sunlight in adjoining properties will similarly not be acceptable

Position of Extensions
Loss of light and overlooking of adjoining properties can be minimised if the extension is set back from the boundary. In the case of two storey extensions a reasonable outlook will normally be maintained if the first floor is set back behind a line drawn at 45 degrees to the rear of the property (Fig 3). It should also not encroach on the outlook of habitable room windows of adjoining properties. All proposals will require careful on-site assessment. Proposals which do not comply with the above requirements will not normally be acceptable.


Two storey side extensions can have a significant impact on the street scene. To avoid the loss of spaces between dwellings and the creation of a terracing effect, side extensions should be set in by at least 1 metre from the boundary (2 metres for three storey properties).

If an adjoining property already has a single storey extension this may reduce the impact the proposed new development will have on it.  Regard will be had to the position of first floor windows in an adjoining property and to the type of rooms they serve.  Obscure glazing (with an obscuration level of three or more) will often be required for windows in flank elevations.

For extensions of two storeys or more, it is important to maintain an adequate minimum separation distance between the rear of the extended property and the adjoining property to the rear. This should be 21 metres back to back for two storey dwellings and 30 metres where one property is 3 storeys.  Particular care is also required to avoid problems of overlooking from rear and side facing dormer extensions.

Roofs and Dormers
A traditional pitched roof which matches the style of the existing property will significantly improve the appearance of an extension and require little maintenance compared to flat roofs.  A pitched roof will be required on all extensions which are visible from the public highway.

A dummy or false pitched roof on the front elevation, which seeks to merely disguise a flat roof, is rarely satisfactory (Fig 4).

The construction of dormer windows and other roof extensions can provide useful additional accommodation but unless carefully designed can dominate the roof and be a very prominent feature in the street scene (Fig 5a).

Dormer extensions should not be a dominant feature of the roof and should allow the existing roof profile to be maintained. Such extensions should be set well back from the eaves and sides of the roof. They should normally only be constructed on the rear elevation so the impact of the dwelling on the street scene is minimised and also not cause problems of overlooking and undue loss of privacy.

Although a flat roof may be acceptable on a small dormer extension, it is preferable to reflect the style and character of the main roof by having hipped or gable ended dormers (Fig 5b). In some cases the use of roof lights may provide a more satisfactory solution.


Garaging and Parking Spaces

Extensions should not result in the loss of existing parking spaces and garages, which would lead to provision below the Council’s standards and cause a potential increase in on-street parking and highway danger.  Enough space for a garage should be retained, even if this does not form part of the proposal. New garages, as with other forms of extension, should reflect the style and materials of the existing house. In most cases, garages should be set back from the front wall of the house, although it may be appropriate to integrate with another feature of the house, such as a porch. A minimum distance of 6 metres should be maintained between the garage and the boundary of the property to allow a car to stand clear of the highway.

Where there is no space on a site to provide a garage, the front garden may be used to provide parking space, but it is important to maintain a balance between hard surfacing and planting.  Where space permits, not more than 50% of the front garden, or half the width of the garden, should be used as a hardstanding.

Summary - General Principles

Consider the appearance of the extension from the outset.

Consider the effect of your proposal on neighbouring properties and discuss your ideas with your neighbours at an early stage.

If possible, employ an architect or competent designer and discuss your proposal with the Council’s Planning staff.  The Planning Division operates a duty rota system when you can discuss your proposal with a Planning Officer at certain times of the day without an appointment.  

Do not allow the extension to dominate the original dwelling.

Match materials used on the existing dwelling and continue existing details.

Continue the vertical and horizontal emphasis of the property and match existing roof lines and styles.

Further Advice
The advice set out in this leaflet is based on policies set out in the Council’s Local Plan.  For further information on this or advice generally on a proposed extension or alteration please contact the Council.

Spelthorne Borough Council
Council Offices
Knowle Green
Staines
Middlesex
TW18 1XB
Tel: 01784 451499

E-mail: Planning.Development.Control@spelthorne.gov.uk