Local Planning Guidance Note (LPG) No 20 - House Extensions
This is one of a series of Local Planning Guidance notes amplifying Development Plan proposals in a concise format. The guidelines which follow have been prepared and adopted by the County Borough Council to inform prospective house owners, builders and agents about the issues relating to house extensions, and to encourage a better standard of design. If you have any queries, a planning officer will be pleased to discuss your proposals before you make an application. You may also wish to talk to your neighbour(s) to let them know what you are proposing to do.
The Development Plan stresses that generally house extensions will be permitted, providing that they reflect and enhance the appearance of the existing property, adjoining properties, and their setting in terms of scale, design and materials. However, there are a substantial number of detailed issues that need to be taken into account in designing domestic extensions. Listed buildings and older properties in conservation areas usually require special consideration, and specific advice can be given on request by the planning department.
Your house and garden
The following points should be borne in mind:
- it is always important to consider the specific character of the building to be extended and to take account of the context of the property. Extensions should reflect the existing dwelling and should retain the character, scale, design and materials of the original property, with existing details such as window shape and design and decorative ridge tiles repeated where possible, and similar materials used;
- the size and scale of the resulting extension should not over-dominate the existing dwelling;
- within settlement limits, an overall limit on the size of extensions would not be appropriate, provided that the extension complied with the Council's guidelines regarding garden size, separation distances, residential amenity, impact on townscape etc. Outside settlement limits, any extension should be clearly subsidiary to the existing dwelling, and should not normally exceed one third of the floor area of the original dwelling;
- in order to be in scale, the roof of the proposed extension should not be higher than the existing dwelling, and the pitch, angles and materials should match those of the existing dwelling; sometimes a lower ridge line will allow the extension to remain in scale with the existing;
- the extension should leave a minimum of 50 square metres of usable private garden space within the boundary of the site, (30 square metres in the case of small houses accommodating up to 2 people, 10 square metres in the case of flats and maisonettes). Exceptions may be made in the case of older terraced properties where basic facilities such as kitchens or bathrooms are required and cannot be provided within the existing property;
- extensions should not result in the loss of parking spaces if this would result in road safety or congestion problems. In order to reduce on street parking, parking may be acceptable in the front garden to maintain adequate provision, (usually two spaces) but only where sufficient landscaping and garden remains to protect the character of the area and reduce the visual effect of the development;
- if there are trees within or adjacent to the property they should be given careful consideration and every effort should be made to retain them. Guidance can be obtained from the Supplementary Planning Guidance note 'Trees in Development' available from the planning department.
Your house and your neighbour's house
- Your extension should not result in a significant loss of privacy, daylight or sunlight to neighbouring properties, or be visually overbearing when viewed from adjoining houses.
- Where two habitable rooms face one another such that direct overlooking is likely to occur, the windows shall be a minimum of 22 metres apart. Where a window faces a wall which exceeds the height of the top of that window, they must be a minimum of 13 metres apart. This standard applies on flat ground.
- On sloping ground, an increased distance will be required so that for for every metre (or part there of) difference in height, the distances in the standard shall be increased by two metres. Developers will be required to indicate on their plans the finished floor levels of their buildings and, where there are windows on adjacent existing properties, the level of these properties.
- In instances where these standards cannot be fully provided, overlooking can be reduced by:
- the erection of screen walls or fences at ground floor level;
- obscure glazing to windows and doors;
- the installation of high level windows or roof lights following the slope of the roof (minimum sill height 1.7m).
- The use of side windows in extensions adjacent to boundaries should be avoided, as a view across adjacent land/gardens, or adequate light, cannot be assumed.
- The majority of houses have their main windows facing front and back. An extension built close to the boundary with a neighbouring property may therefore reduce daylight and sunlight to the windows of that house, together with the outlook from these rooms. Extensions should not, therefore, be so large as to create a claustrophobic effect or cause a significant amount of visual intrusion and loss of light to neighbouring properties.
- The Council has adopted the Building Research Establishment guidance on site layout planning for daylight, and extensions should comply with the 45o test.
The 45o Test
- To use, take the elevation of the wall in which the neighbour's window is placed and draw diagonally down at an angle of 45o away from the near top corner of the extension;
- Then take the plan and draw diagonally back at an angle of 45o towards the window wall from the end of the extension;
- If the centre of the main window of the next door property lies on the extension side of both these 45o lines, then the extension may well cause a significant reduction in the light received by the window.
This is guidance only and passing the test does not mean automatic approval, nor the reverse.
Side extensions - specific guidelines
Where the property stands in a line of detached/semi detached dwellings and the extension would fill in the gap, there is a risk that the extension will create a terraced appearance. This is not always in the interests of maintaining the character of the street, and, in the interests of visual amenity, should be avoided. One way of maintaining a visual break would be to set back the extension behind the front of the dwelling by a metre to create a clear break.
The following criteria should be met:
- a pitched roof will normally be essential. The roof of the proposed extension should match the original in terms of pitch and shape. The ridge line should either follow, or be lower than the original dwelling;
- to allow for maintenance and access, it is recommended that a distance of 1m be left between the extension and the side boundary.
Great care is needed in the design of extensions on corner plots, which often provide an open appearance and greenery, and are prominent from both streets. The following criteria for corner plots should be met:
- the width of the extension should not be more than half the width of the original frontage of the property;
- the width of the extension should not be more than half the width of the garden/plot between the property and adjacent highway;
On side plots there can be problems in incorporating the minimum parking standards. Where proposals involve access, garages or car ports, the following criteria should be met:
- garages and carports should have a driveway at least 5.5m clear of the highway. If the garden is not deep enough to provide a 5.5m driveway the garage must be set back from the main front wall to provide this minimum driveway length;
- where existing parking spaces are lost as a result of an extension and this would cause road safety or congestion problems, replacement spaces must be provided.
Rear extensions - special guidelines
Rear extensions, including conservatories, should not dominate, nor materially alter, the existing levels of sunlight, privacy and daylight to adjoining properties.
Two storey rear extensions should not come nearer than 2 metres of a boundary that forms a party wall between terraced and semi-detached properties and 1 metre of other boundaries. The maximum projection from the dwelling should be one third of the garden width. Proposals should satisfy the 45o test.
Any extensions within one metre of the boundary will normally be limited to a maximum of 3.5 metres in length. For every additional metre from the boundary this can be increased by one metre.
Front extensions - specific guidelines
Front extensions will clearly impact on the street scene and will rarely be acceptable. Front extensions must be sympathetic to the form, scale, proportion and design of the dwelling and neighbouring properties, and should not:
- detrimentally affect the overall character and appearance of the street scene;
- project a significant amount from the front wall of the existing dwelling
- result in the loss of existing parking facilities where this would cause road safety or congestion problems
- Front extensions should satisfy the 45o test
It is sometimes possible to construct some porches without the need for planning permission. Whether they require planning permission or not, they should match the character and design of the existing property and should:
- be simple in design and respectful of the original lines of the house;
- incorporate matching materials and design features;
- provide a pitched roof;
- not exceed the sill height of the first floor windows.
Roof extensions - specific guidelines
Roof alterations and dormer windows should be kept as small as possible so as to minimise the visual impact on the appearance of the property and the surrounding area, and to minimise the loss of privacy to neighbours. The following criteria should be met:
- dormer extensions should not normally be incorporated on front elevations;
- dormer extensions should be as small as possible and in proportion to other features of the house. Where dormers would cover and dominate a large area of the roof they will normally be refused;
- extensions must not project above the ridge line of the property;
- dormer windows should follow the vertical lines of existing doors and windows;
- dormer extensions should re-use recycled or matching materials where possible;
- where the use of roof space is desired, consideration should be given to the incorporation of sloping roof lights or traditionally styled dormer windows to create the sloping ceilings associated with attic rooms. Sloping roof lights are cheaper to install and are less intrusive than dormer windows, and can reduce the problems of overlooking.
Updated March 2010