Appendix G: Guidance for Residential Extensions and Alterations

Dúntadate_range16 D.F., 2019, 9:30am - 11 Nol, 2019, 5:00pm

The purpose of this appendix is to provide detailed guidance to provide a consistent basis against which to consider an extension and/or alteration to a dwelling house or flat, including those in multiple occupancy and any proposal for a domestic garage or an outbuilding. The guidance is intended to advise home owners on how to extend or alter their property in a neighbourly manner that is sympathetic with the original property, respects the character and appearance of the surrounding area and contributes towards a quality environment.

Context and Design

An extension or alteration to a residential property should be designed to become an integral part of the property both functionally and visually. Such works should not be designed in isolation solely to fit in a required amount of accommodation. Proposals that are badly sited or designed, or that are incompatible with their surroundings, can lead to an undesirable change in the character of the existing property and the area in which they are located. Success depends upon striking the right balance between adaptation and sensitivity to the original design. To ensure good design any extension or alteration will need to complement the host building and respect its location and wider setting.

An extension or alteration should not be so large or so prominent as to dominate the host property or its wider surroundings, rather development proposals should be in scale with existing and adjoining buildings. All such works should have proportion and balance, fitting in with the shape of the existing property. It is accepted that on occasion a larger extension may be required - for example to facilitate the renovation and upgrading of a small rural dwelling to meet modern amenity standards. It will not usually be appropriate to allow an extension to project above the ridge line of the existing dwelling and this will be especially important where uniform building height is part of the street scene.

Side Extensions

Proposals in an urban context should not overdevelop the site in terms of massing, plot size and proximity to boundaries thereby, for example, creating a visual ‘terrace’ effect. This is one of a number of problems associated with side extensions, where they can alter the character of the area by filling the visual gaps between residential properties. The need for adequate space alongside boundaries is also important to provide ease of access to the rear of the property and to allow for maintenance. This will also serve to eliminate the possibility of any part of the extension, including rainwater goods, overhanging neighbouring property.

A further concern may arise where a side extension to a semi-detached dwelling is proposed at the same height and follows the same building line as the block comprising an original pair of dwellings. This will often compromise the appearance and architectural integrity of the block, and if repeated throughout a neighbourhood is likely to have an adverse impact upon the character of the wider area. To address this particular problem, proposals of this nature should be ‘set back’ from the building line or front of the house and also ‘set down’ from the ridge line.

Front Extensions

Extensions or alterations to the front of a property require great care as the front elevation is often the most visible to public view. Poor design can upset the architectural integrity of the existing property and have an intrusive effect on the street scene. It is important, therefore, to ensure that extensions and alterations to the front of property do not detract from the street scene, especially where there is a clear and visually obvious ‘building line’ or architectural features. In such cases they should appear to be part of the existing property and not an obvious addition. This can be achieved by ensuring any such works are in proportion with the property, its fenestration and detailing, with matching materials, roof design and pitch.

Alterations or an extension to a dwelling should not infringe upon a neighbour’s property. For example, it is an infringement of a neighbour’s property rights should foundations or guttering encroach onto their land or if an extension overhangs or attaches to their property. Where an extension abuts or runs close to a property boundary, permission to enter neighbouring land will also be required to enable approved works to be carried out or for future maintenance purposes. Consequently, it is advisable to discuss proposals with any neighbours before submitting a planning application. It should be noted that infringement of property rights is primarily a legal matter between the relevant parties.

Garages and other associated outbuildings

Buildings within the residential curtilage, such as, garages, sheds and greenhouses can often require as much care in siting and design as works to the existing residential property. They should be subordinate in scale and similar in style to the existing property, taking account of materials, the local character and the level of visibility of the building from surrounding views. The use of false pitches should be avoided as these often detract from the appearance of these buildings, particularly when viewed from the side.

Garages or outbuildings wholly located in front gardens or those that extend in front of the established building line can over-dominate the front of the property and detract from the street scene and will therefore generally be resisted. In the countryside, ancillary buildings should be designed as part of the overall layout to result in an integrated rural group of buildings.

Roof Extensions

An extension or alteration which copies the roof type and angle of pitch of the original residential property will be more successful than those proposals that introduce a completely different type of roof. The roofing material of any pitched roof extension should seek to match that of the original. Flat or mansard roofed extensions to traditional buildings are seldom harmonious. However, they may be acceptable where they are not open to public views.

The use of loft space to provide bedrooms or other living space can often provide additional accommodation. However, alterations to the roof profile of any building can be particularly sensitive as roofs play an important part in contributing to a building’s appearance and the overall character of the area. An extension to the rear of a property should ensure that the roof of the extension does not project above the ridge of the existing dwelling as this can give an unsightly view along the streetscape. Rooflights, which lie parallel with the plane of the roof, are a particularly sympathetic way of providing light to a room within a roofspace.

The regular repeated rhythm and uniformity of roof forms and chimneys may be a particular feature of a group of similar buildings or the wider townscape and should therefore be retained. If elements, which are not part of the original property are proposed, for example, a dormer roof extension, these should be designed in a manner that complements the period and style of the original property, or to reflect the best examples of such features on properties of a similar period in the area.

Dormer Windows

Where a dormer is open to public view, it can interfere with both the original design of the existing building and cause a visual intrusion into the street scene or rural setting. Dormer windows to the front or side of a property will be resisted in areas where they are uncharacteristic, particularly large box dormers that are over-dominant often extending the full width of the roof. The size and number of dormers should therefore be kept to a minimum to avoid dominating the appearance of the roof and should be located below the ridge line of the existing roof. Positioning dormer windows vertically in line with the windows below and ensuring that they are smaller in size will usually avoid a top-heavy or unbalanced appearance.


Attention should be paid to design details such as the position, shape, proportion and style of windows, doors and other features to complement the existing property and respect the character and appearance of the area. To facilitate the integration of an extension or alteration with the existing property, new windows should be aligned to the existing fenestration and match the symmetry of the existing dwelling. The relationship between solids and voids is an essential component of any new proposal, but particularly when extending or altering an existing property where window size and height diminish on upper floors. Older residential properties in particular often have interesting arches, brick detailing and other special features or ornamentation which add character. Continuing or reflecting such ornamentation around doors, windows and at the eaves in the design approach followed can be an effective way of integrating any extension or alteration work with the existing property.

External Finishes

The external finish of a proposal should aim to complement the type of materials, colour and finish of both the existing building and those of neighbouring properties, particularly where certain materials strongly predominate. Using similar or complementary materials to those of the existing property is more likely to produce a successful extension or alteration. The re-use and recycling of building materials is encouraged and will be especially important when carrying out work to a listed building, or buildings within a conservation area or an area of townscape character.

Sustainable Design

The extension or alteration to a residential property can provide the opportunity to further sustainable development in terms of incorporating energy efficiency measures, renewable energy technologies and the re-use of existing materials. For example, additional insulation and rainwater recycling using water butts. Where existing walls are being demolished or roofs altered, existing materials can often be salvaged and re-used, which will benefit the visual appearance of the new work and its integration with the existing property. PV tiles are now available that look like traditional tile and slate roofs, allowing the installation of these systems to be sensitive to the character, colour and style of the existing roof.

Walls and Fences

Walls and fences, particularly in front gardens, can also have a significant effect on the appearance of the property and streetscape. When erected beside driveways or on corner sites they can have an impact on sightlines and traffic safety. Both the visual and road safety aspects of a wall or fence will be assessed when proposals are being considered. Materials should always complement the character of the property and the neighbourhood. Expanses of close-board fencing bordering public areas are visually unacceptable. It should be noted that some walls or fences may be permitted development.

The Countryside

The impact of an extension or alteration on the visual amenity of the countryside and, in particular, in our Area of Outstanding Natural Beauty needs to be considered. Proposals should be in keeping with the character of the existing property and its countryside setting. Through poor design the individual and cumulative effect of extensions and alterations which are disproportionate in size to the existing property, or which require the use of land outside the established curtilage of the property, will result in a detrimental change to rural character. Many rural dwellings occupy larger plots than their urban counterparts. Whilst there may be sufficient room on the plot to accommodate an extension in physical terms, great sensitivity is required to ensure the proposal integrates with the existing dwelling and surrounding landscape. In assessing the potential impact of development in the countryside, particular regard will be paid to the quality and nature of the landscape in the locality and at the particular site. The suburban boundary treatment of walls or fences and the introduction of ornate pillars are inappropriate in the rural landscape and will be resisted.

Residential Amenity

Single-storey extensions to the rear of a semi-detached or terraced dwelling will generally be acceptable where the depth does not exceed 3.5 metres from the back wall of the original building, at the boundary with an adjoining dwelling. Larger extensions will be assessed in light of the following guidance, although it is acknowledged that flexibility may be needed in respect of older properties with small plot areas or where the proposal seeks to meet the specific needs of a person with a disability.


Except in the most isolated rural location, few households can claim not to be overlooked to some degree. The protection of the privacy of the occupants of residential properties is an important element of the quality of a residential environment. It is a particularly important consideration where an extension or alteration is proposed adjacent to existing properties. Balconies, roof terraces, decking, dormer windows, windows in side elevations and conservatories all have the potential to cause overlooking problems, due to their position and orientation, particularly from upper windows. The use of obscure glass, velux windows and high-level windows in appropriate circumstances can often minimise this potential, for example, the use of obscure glass for bathroom and landing windows. However, this is not considered an acceptable solution for windows serving main rooms such as bedrooms, living rooms, dining rooms or kitchens.

Proposals should seek to provide reasonable space between buildings in order to minimise overlooking. This will also assist in providing acceptable levels of daylight to properties. In the case of dormer windows, restricting the size of the window and setting it back from the eaves is usually an adequate solution that can protect neighbouring privacy.

Overlooking of gardens may be unacceptable where it would result in an intrusive, direct and uninterrupted view from a main room, to the most private area of the garden, which is often the main sitting out area adjacent to the property, of your neighbours’ house. As a general rule of thumb this area is the first 3-4 metres of a rear garden, closest to the residential property.


Dominance is the extent to which a new development adversely impinges on the immediate aspect or outlook from an adjoining property. Neighbouring occupiers should not be adversely affected by a sense of being ‘hemmed in’ by an extension. This can often result from the construction of a large blank wall. Dominance can be increased when the neighbouring property is at a lower ground level to the development site. Loss of light is usually a consequence of dominance. Two storey rear extensions to semi-detached and terraced dwellings are usually very prominent when viewed from adjoining dwellings and can dominate outward views from adjoining ground floor windows, appearing excessively large and overbearing. It is appropriate, however, to take account of the prevailing local environment.

Overshadowing/Loss of Light

Sunlight and daylight are valued elements in a good quality living environment. Effective daylighting can reduce the need for electric lighting, while sunlight can contribute towards meeting some of the heating requirements of our homes through passive solar heating. In designing a new extension or alteration to a residential property care should be taken to safeguard access to sunlight and daylight currently enjoyed by adjoining residential properties.

Where an extension is poorly sited or badly designed it can cast a shadow that may reduce a neighbour’s daylight and adversely affect their amenity to an unacceptable level. It is important, therefore, that every effort should be made to avoid or minimise the potential for overshadowing to a neighbour when drawing up plans for an extension. Overshadowing to a garden area on its own will rarely constitute sufficient grounds to justify a refusal of permission.

In terms of daylighting, the effect on all rooms, apart from halls, landings, bathrooms and utility rooms will be considered. Where an extension would be likely to reduce the amount of light entering the window of a room, other than those indicated above, to an unreasonable degree, planning permission is likely to be refused.

Significant problems of sunlight or daylight loss are most likely to occur in terraced or semi-detached housing situations and it is here that most care needs to be taken. An extension should be kept as far as possible from neighbouring windows and boundaries to minimise impact.

To help assess the loss of light as a result of a proposed development to the front or rear of a residential property, the 60 degree and 45 degree lines, as shown in Figure G1 for single storey and two storey extensions respectively, will be employed. These lines will be taken from the centre of the closest neighbouring window. It should be noted that where the closest window is located at first floor level it may be more appropriate to consider this against the 60 degree line. The elevations and outline plans of adjoining properties should be shown on drawings, accurately scaled (in metric measurement) to allow proper consideration of this matter.

The guidance in Figure G1 is not however a rigid standard which must be met in every case. Rather it is an assessment tool which will be used in conjunction with other relevant factors in order to gauge the acceptability of proposals in terms of the overshadowing/loss of light impact upon neighbouring properties. Other relevant factors which will be considered in this assessment are set out below:

  1. The existing form and type of extension prevalent in the area. For example, where the majority of dwellings in a terrace have already been extended in a similar way to the application proposal this matter will be balanced against any adverse impact on neighbouring properties.
  2. The proposed design of the extension or alteration. For example, where a proposed extension incorporates significant glazing in the design, the impact on neighbouring properties may be acceptable in circumstances where alternatives might result in unacceptable overshadowing.
Figure G1 House Extensions

  1. The particular characteristics of the site and its context. For example, where daylighting to an adjacent dwelling is already impeded by an existing building or boundary wall and the proposal would not significantly exacerbate the existing situation.
  2. The orientation and position of a neighbour’s window in relation to the proposed extension, the room it serves and whether the window affected is the primary source of light for that room. For example, account will be taken as to whether a room affected by a proposed extension benefits from an alternative natural source of light.
  3. The potential size and form of an extension allowable under permitted development. For example, where a proposal would not have an impact significantly greater than that of an extension allowable under permitted development rights.

Noise and General Disturbance

Residential areas can be sensitive to noise and general disturbance, particularly in the late evening when there is an expectation that surrounding background noise will remain low. An extension or alteration such as a balcony, roof-terrace or high level decking can all increase the level of noise and general disturbance experienced by residents of adjacent properties and will be subject to particular scrutiny.


Landscaping is a vital consideration for all development and should form an integral part of any proposal. Landscaping can create a high quality setting, help integrate new development into its surroundings and assist the promotion of biodiversity of native species or other species characteristic of a particular area. Proposals for landscaping should therefore always be considered as part of any application for an extension. The retention of existing trees, hedges and other significant landscape features will often be an important element in this and will usually help to reduce the impact of an extension on the character of the surrounding area more readily than walls or fences. Where important trees and landscape features exist within a site, care should be taken that extensions are not sited too close to them.

Best practice in relation to this matter can be found in the publication ‘Trees and Development’ co-sponsored by the Department, the Forest of Belfast and the Construction Employers’ Federation. To ensure that full account is taken of existing trees and landscape features within the residential curtilage, such features should, as part of a planning application, be accurately detailed on a site survey map in accordance with British Standards BS 5837 (2012) ‘Trees in Relation to Design, Demolition and Construction’. Where it is proposed that existing trees or significant landscape features are to be removed, the layout plan should indicate proposals for compensatory planting.

Private Amenity Space

Amenity space is an essential part of the character and quality of the environment of residential properties. It is important therefore to ensure, when bringing forward a proposal to extend, that adequate amenity space-particularly private space, is left. Garden space around a residential property is an integral part of its character and appearance and should not be reduced to a point where it is out of scale or fails to meet the present and future occupiers need for adequate useable private amenity space.

All residential properties require some in-curtilage private open space, usually to the rear, compatible with the overall size of the plot, for normal domestic activities. This space should enjoy a high degree of privacy from the public street and from any other public places. Residents may now have up to three bins per household to facilitate recycling. It is inappropriate for these to be stored in front gardens, which are rarely private, as they provide a public aspect and can adversely affect the character and appearance of the area.

Extensions, particularly to the side of a residential property, whereby refuse and garden equipment will need to be carried through the house or stored in the front garden, will not normally be permitted. An exception may be made where a route can be maintained through the extension via a garage or utility room on the ground floor.

Access and Car Parking

An extension or alteration to a residential property that involves the conversion of an attached or integral garage to create additional living space can result in the loss of in-curtilage car parking provision. Proposed works that would result in the significant loss of car parking spaces or a turning area, with no reasonable alternative being available, will not be acceptable. Similarly the use of an entire garden area to provide car parking or a turning area will be resisted. Garages should be positioned where they can be accessed safely. To ensure the highway is not blocked while the door is being opened, a new garage which gives access to the public highway should retain a minimum of 6.0m driveway within the residential curtilage. Further detailed guidance in relation to in-curtilage driveways, hardstandings and vehicle turning facilities is set out in the ‘Creating Places’ design guide.

Extensions and Alterations to provide for Ancillary Uses

An extension or alteration to a residential property to provide an ancillary use, such as additional living space for elderly or dependent relatives should be designed to be subordinate to the main dwelling and should provide limited accommodation and demonstrate dependency on the existing residential property for example shared facilities. Such additional accommodation should normally be attached to the existing property and be internally accessible from it. Ancillary uses that could practically and viably operate on their own will not be acceptable.

Where an extension to the existing house is not practicable and it is proposed to convert and extend an existing outbuilding, planning permission will normally depend on the development providing a modest scale of accommodation. The purpose of this is to ensure the use of the building as part of the main dwelling. The construction of a separate building, as self-contained accommodation, within the curtilage of an existing dwelling house will not be acceptable, unless a separate dwelling would be granted permission in its own right.

In all cases Council will need to be satisfied that the proposed accommodation will remain ancillary to the main residential property and careful consideration will be given to the impact of proposals on neighbouring dwellings. Where permission is granted it will be subject to a condition that the extension will only be used for ancillary residential purposes in connection with the main dwelling, and not as a separate unit of accommodation.

People with Disabilities

Council will give sympathetic consideration to proposals where an extension or alteration is required for a person with a disability or whose mobility is otherwise impaired. The specific needs of a person with a disability are however an important material consideration and exceptionally the policy criteria may be relaxed to meet these needs.

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