Draft Residential Extensions & Householder Development Supplementary Planning Document

Ended on the 29 January 2018
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3.1 Thinking of extending your home? The following guidance will help you plan your extension, by identifying some of the key points to consider for achieving a high quality design whilst respecting residential amenity.

General Advice for Extensions

3.2 Alterations and extensions must be of a high quality of design which responds positively to the opportunities for improving the original building.  To achieve this goal it is essential to assess the factors which make up the character, appearance and setting of your home. 

3.3 The general form of an extension, its roof pitch and eaves details, its profile/outline, the materials used, positions of windows and doors and their details should all relate to the original building and the existing street scene and building patterns. Extensions are generally most successful when they are subservient to the host dwelling.

This diagram shows an example of a well thought-out extension and garage, which are subservient to the original house

By contrast, this diagram shows an example of poorly-designed extensions that dominate and detract from the original house

3.4 Consideration should also be given to the size of an extension and whether it would adversely affect your neighbours.  For example, would it result in overshadowing or overbearing of their gardens/amenity space, or cause a loss of daylight or sunlight into the existing rooms of a neighbouring property? Would the position of new windows result in overlooking of neighbouring properties? Impacts are unlikely to be acceptable where new windows for ground or first floor habitable rooms allow unrestricted views into the rooms of neighbouring buildings. No part of the extension (including rainwater goods and foundations) should physically extend beyond the site boundaries.

3.5 An extension that is in-keeping with and subservient to the original house can be achieved by various means, such as by:

  • positioning the ridge of the extension at a lower height than the ridge (and sometimes also the eaves) of the existing dwelling;
  • setting the extension back from the front elevation;
  • respecting the existing fenestration and "solid-to-void" ratio on public elevations;
  • using building materials and finishes that are similar in colour, appearance and texture to those of the existing dwelling.

3.6 Generally extensions should look as if they could be part of the original design, although very skilful and imaginatively designed extensions can also be successful.

3.7 In terms of roof design, a pitch that matches that on the existing house is advisable. Flat-roofed extensions are unlikely to be acceptable unless the roof of the original building is flat.

3.8 Key Points for General Advice:

  • Extensions should be subservient to the host dwelling.
  • Extensions should reflect the appearance of the host dwelling.
  • Extensions must not cause unacceptable overshadowing of, or overbearing impacts on neighbouring property.
  • Windows to habitable rooms should not allow unrestricted views into neighbouring windows or private amenity areas.

Specific Advice

3.9 Each of the following sections provide more specific advice about certain types of residential extension and should be read in the context of relevant policies in the Development Plan, including any relevant Neighbourhood Plan.

Rear Extensions

3.10 Proposals for new rear extensions will be considered in terms of their impact on the privacy, amenity and possible dominance over neighbouring properties. For purposes of applying JCS policies CP27 and CP29, this consideration will typically include an assessment of the proximity to the neighbours, degree of projection from the established building line, and proposed height and building mass (i.e. how bulky the extended building would appear). New extensions should not detract from the residential amenity or character of the local area, by adversely affecting these relationships between adjoining properties.


Two versions of a rear extension, showing how the degree of projection can affect the character of the host dwelling. The image on the right hand side illustrates a more sympathetic rear extension.

3.11 The potential impact on amenity will typically be considered by taking measurements from the proposed extension to the window wall elevations of adjacent properties.

3.12 The maximum limit to which extensions will be favourably considered is generally determined by drawing a straight line at an angle of 45o towards an adjacent property from the nearest projecting corner of the extension, when viewed from above. If the centre of a habitable room window on a neighbouring property lies on this line (or lies within the triangle created by the vertical surfaces of the properties and this line; see image below) then the extension is likely to have an unacceptable impact on the daylight received by this window.

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3.13 This is a useful rule of thumb, but other material considerations will be taken into account to decide on the likely impact of a proposal on local residential amenity, such as window size, the orientation of properties relative to the trajectory of the sun and the presence of intervening physical features (such as the height of boundaries and the presence of trees). These factors can also affect the amount of daylight/sunlight received by adjoining properties, so the above rule of thumb will not be the only consideration. 

3.14 In addition to the above, two storey rear extensions should respect a 21 metre back-to-back separation distance between properties in order to avoid any undue overlooking of the neighbouring dwellings and their private amenity space. Separation distances can sometimes be less than this when dwellings are orientated at an angle to one another.

3.15 Key Points for Rear Extensions:

  • Pay careful attention to the impact of a rear extension on the scale and character of the original building and on the character and appearance of the area.
  • Adhere to the 45o rule to safeguard residential amenity.
  • Windows should not result in the overlooking of windows and private amenity areas to the neighbouring property/properties.

Side Extensions

3.16 In most urban, suburban and village situations, the design of two-storey and first floor side extensions should allow for a visual break from a communal boundary to ensure separation from adjacent dwellings and to prevent the creation of a 'terracing effect' where this would be inappropriate, given the character of the local area.

The perception of terracing can be an inadvertent consequence of badly-designed side extensions. The first image illustrates two extensions that "close the gap" between adjacent dwellings to such an extent that the character of the area is likely to be adversely affected. The second image illustrates an example that is more acceptable, where a reasonable gap has been maintained between the dwellings.

3.17 Generally speaking, two storey extensions should not be flush with the front main wall of the house but should be set back slightly. This can help to ensure their subservience to the original building and can help to avoid the perception of terracing, when little or no gap to the communal boundary can be achieved. It is often preferable to have a lower ridge height for a two storey side extension, to lessen its impact on the street scene and ensure that it does not dominate the original building.

Original dwelling With proposed side extension

Two versions of a side extension, showing an unsympathetic extension with a flat roof and an acceptable alternative, with a roof that is in-keeping with the original dwelling, with a slightly lower ridge height.

3.18 To avoid an overbearing impact on a neighbouring property, a minimum separation distance between the flank wall of a two storey side extension and the rear elevation of a neighbouring property of 12 metres will be encouraged.

3.19 Key Points for Side Extensions:

  • Side extensions should not lead to an unacceptable loss of space between an original building and its neighbours. In order to avoid terracing, two storey extensions should leave a gap to the communal boundary and incorporate a set-back from the front elevation.
  • Side extensions should be subservient to the original building, with a lower ridge height.

Front Extensions

3.20 Virtually all forms of front extensions require planning permission where they extend beyond a wall which fronts onto a highway and forms either the principal or side elevation of the dwelling. This may exclude small porches and recessed areas.

3.21 Front extensions will normally only be accepted on dwellings located well back from the road; or where the street scene already includes a staggered building line as part of the area's distinctive character. Where applicable, the 45o rule should be respected (see advice on rear extensions for details).

3.22 Front extensions may be more appropriate in countryside locations, where the character of the built environment is low density with houses set back and/or well screened from the public highway. In all cases front extensions should be subservient to the original dwelling: the extension should not project significantly beyond the front main wall of the original building, or be visually prominent from the street.  Two-storey front extensions may only be accepted where the house is set well back from the road, is well screened and in areas where there is not a uniform building line.

The two proposals in red show front extensions that project significantly from the front wall of the original dwelling, disrupting the building line as a visually-prominent feature. The third proposal in blue is clearly subservient and does not project far beyond the front wall.

3.23 Front extensions should be well designed to avoid disfiguring the face of the house. New windows should be in proportion with and reflect those of the host dwelling: this could mean respecting the solid-to-void ratio and/or pattern of fenestration of the front elevation. The materials and design details (such as the eaves, weatherboards) should also be in-keeping.

3.24 Key Points for Front Extensions:

  • Front extensions might not be acceptable in suburban areas, where they could upset regular building lines and local character. Houses will often need to be set back from the road for a new front extension to be acceptable.
  • In cases where porches may be acceptable, they should be subservient to the original building and complement rather than compete with existing features.
  • Sympathetic fenestration and design details will be important for ensuring that a front extension enhances the character of the house.

Extensions on Corner Plots

3.25 Whilst extensions on corner plots should adhere to the guidance outlined elsewhere in this document, there are a number of further matters to consider, particularly in urban and suburban locations.

3.26 Corner plots can be especially difficult as any extension must achieve a degree of compatibility with two, potentially very different, streetscapes.  Consideration will need to be given to an active frontage along both street frontages. Where a house faces the public realm from more than one aspect, particular attention should be given to the fenestration and design details of the new extension. There should be windows facing both street frontages, to allow for natural surveillance and visual interest from the street. Any extension should be set back from the front elevation of the host dwelling and should respect all building lines, to avoid becoming an incongruous feature within one or more streetscape and to help the building "turn the corner". A gap must be maintained between the extension and existing boundaries.

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Plan-view of a proposed corner plot extension that is unacceptable because of a failure to respect one of the building lines and because it impinges on the existing boundary.

3.27 Key Points on Corner Plots:

  • Corner extensions are visible from a number of locations. It will therefore be especially important to assess their impact and produce a high standard of design and detailing.
  • The extension should assist in presenting an active frontage for both streetscapes.
  • Extensions should respect all building lines and should be stepped back from the front elevation, whilst maintaining a gap with existing boundaries. 

Roof extensions (including dormers and rooflights)

3.28 The design and profile of a domestic roof makes an important contribution to the dwelling's overall appearance. Roof extensions can therefore have a positive or a negative impact on the character of the property and its surroundings. Significant changes to the profile or pitch of the roof are generally unacceptable because they are likely to be unsympathetic to the overall design of the house.

3.29 The insertion of dormers can be appropriate where they would complement the proportions, size, arrangement and details of existing windows. New dormer windows need to be well designed, which normally relies on ensuring that they are not over-scaled and do not dominate the roof. This can be achieved by insetting from the existing eaves, verges and ridge. They should not protrude significantly from the roof profile; a small dormer will be preferable to a large 'box' dormer. Such box dormers are nearly always over-scaled and unsympathetic to the character of the host building and are unlikely to be supported.

3.30 The detailing also needs to be good quality and the materials need to be well related to the existing roof in terms of their colour and profile. Particular attention should be given to the roof design of a dormer. A hipped dormer will usually be suitable only on a building with a hipped roof, as it could otherwise appear incongruous. A pitched roof will normally be preferred to a flat roof, unless the character of the dwelling suggests otherwise. The size of the dormer should take into account the necessary insulation, so that the structure avoids 'gaining weight' later.

Two of these proposals are unacceptable due to their size, and their intrusive effect on the roof of the existing dwelling. The smaller dormer is in-keeping with the roof of the dwelling and of a more appropriate size.

3.31 Dormers and rooflights should be well related to the existing fenestration on the building, for example by lining up with the existing windows, or being centrally located between existing windows, and they should be smaller than the windows beneath them. New rooflights should not protrude significantly from the roof, should be evenly spaced and should not dominate the roof.

3.32 Key Points for Roof Extensions:

  • Roof extensions should not significantly alter the pitch or profile of the existing roof
  • New dormers should not dominate the roof and should complement the proportions, size, arrangement and details of existing windows. They should relate well to, but be smaller than the windows beneath them.
  • The design of the dormer should be in-keeping with the existing roof, particularly in terms of its materials and roof design.
  • New rooflights should also be well-related to the existing fenestration and should not dominate the roof.
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