Extensions and improvements are a reasonable way of achieving more space and functionality within a dwelling, but where planning permission is required it may not always be granted. Whether in the open countryside or in a built-up area, proposed extensions should respect their local surroundings in terms of size, scale, aspect, design and materials. This Note is concerned with the external appearance of the dwelling, including its scale and mass. Although the planning system does not have the right to protect the views from householders’ properties, it should prevent detrimental impact on the amenity of neighbouring properties and on the character of the property and its surroundings. In this sense, the planning system operates to reconcile the needs of individuals with the wider community interest.

It is recommended that a suitably-qualified professional, Such as us, Eraldo Architects Limited, is used to prepare the proposals. Many mistakes have been made in the past where people have settled for solutions such as flat roofs because they appear cheap in the short term. Good design need not be more expensive; indeed, it can add value to the property and result in lower maintenance costs. Examples include designing in energy efficiency and designing to prevent (or reduce) crime. Whether or not planning permission is needed, this Guidance Note should help to bring about high quality development which will enhance the property and its neighbourhood.


At a general level, nearly all Local Authorities will have a similar planning guide publication  for "Householders" (it is not possible to give a full comprehensive guidance in all planning aspects, and for further detailed information your respective Local authority Planning Deprtment may prove invaluable). 

Certain minor changes – called "permitted development rights" – may be made without the need for planning permission, and these are explained in the general Planning booklets available.


A cautionary note: planning permission should only be applied for once the scheme has been finalised. Any changes desired after permission has been granted will require a fresh planning Application





All Local Authorities now take a firmer view on design matters.

At the local level, of greatest relevance to this topic the Policy (House Extensions and Alterations) of the will  state and assert:

"Extensions or alterations to existing dwellings will be permitted provided that the proposal

a. is subsidiary in scale and form to the existing dwelling, and does not represent an overdevelopment of the site;

b. respects the design and setting of the existing dwelling and surrounding area; and

c. will not have an unacceptable impact on people living nearby."


The supporting text notes that "As a general guide, house extensions should not be more than 50% of the original floorspace and extensions that are out of scale and character will generally not be permitted." For clarification, "original floorspace" means that of the original dwelling rather than the existing dwelling, thereby aiming to prevent several extensions over time which, cumulatively, would become overlarge.


Policy  (Annex Accommodation) is relevant to proposals for accommodation for dependent relatives (sometimes known colloquially as "granny flats"):

"Annex accommodation will only be permitted where:

a. it is created by an extension to an existing dwelling;

b. or, is a conversion of an existing building within the curtilage of a dwelling; and

c. its usage is ancillary to the residential use of the existing dwelling."


Account must also be taken of Policy (Building Design):

"Development will be permitted only where:

a. the proposed building and structures are of a high standard of design, form, scale and materials; and

b. it protects the character and amenity of the locality and adds to the quality and distinctiveness of the local area.

Some times, Applicants ( with the help of their Agent Architect, must include a short written explanation with their planning application illustrating the design principles adopted prior to the drawing up of their proposal. This will illustrate how the proposal will fit within the existing surroundings and improve the speed of the decision making process."





In the interests of good neighbour relations, it is advisable for any householder intending to extend their property to inform, and discuss their intentions with, their neighbours who will be affected by the development. On all planning applications the Council will undertake consultations with neighbours likely to be affected by the proposal.

It is the responsibility of the householder, applicant or agent making the proposals to ensure that any extension, solid wall or means of enclosure is constructed on land under the same ownership and to avoid encroachment of foundations or overhanging of guttering. Details of the Party Wall, etc. Act 1996 are available at the Planning Services office.




Additional special considerations


Whilst the same general principles apply across the Country, special considerations apply to dwellings which are:

  • Listed buildings

  • In a conservation area

  • In Area of Outstanding Natural Beauty (AONB)


  • In conservation areas and the AONB permitted development rights are more restricted, which means that it is necessary to apply for planning permission for certain types of work which do not need an application in other areas; for instance, the volume limits for extensions are lower, and permission is required to clad the outside of the house.

    Any alterations or extensions which affect the character or appearance of a listed building will require listed building consent as well as planning permission.

    In all these sensitive areas, the Council will be looking for the highest design standards. It is recommended that advice on such issues is sought at an early stage from the Development Control section in the Planning Authority



    Costs of the project

    the current standard application fee for a domestic extension is £202 payable to the respective local Authority.

    Eraldo Architects Limited charges a standard £300, for a the planning consultation, the carrying out of the planning drawings, liasing with the Local authority, and submitting Planning and application form on behalf of the client, (in some cases less).

    Nonetheless, outwith the planning administration fee, the cost of the project is obviously a matter for each applicant, and it is not a planning issue, therefore these comments are offered as advice. It is wise to be realistic about what can be afforded, and to beware of stretching the available funds too far. Better to put the major effort into the basic structure, because a skimped structure will be expensive and difficult to put right afterwards, whereas finishes and furnishings are cheaper and easier to improve later as more funds become available. From the planning viewpoint, clearly the external appearance must be acceptable in terms of the principles contained in this Note.




    Advice on extensions and alterations


    The advice on extensions which follows is looked at firstly from the viewpoint of general design principles and secondly with regard to particular forms of extensions, for example rear extensions.



    General principles


    To summarise, in altering a dwelling, several principles should be kept in mind:

  • Avoid over-enlargement or drastic changes in character. (See Figure 1)

  • Keep and develop the building’s good points whilst correcting its shortcomings by sympathetic modification.

  • Avoid the inappropriate, such as pseudo-historic details mimicking past styles not authentic to the building, and avoid also pseudo-foreign details intended for another climate and conditions. (See Fig.1)

  • Avoid unnecessary alteration of the building’s basic structure and avoid large structural works, unless these are repairs.

  • Respect the building’s site and surroundings.

  • These principles are now examined in greater detail in terms of scale and form.



    Scale and form

    The extended building should not become over-assertive or obtrusive, nor should it harm good public views into or out of the settlement, which may be of greater concern in small villages and hamlets. Any new extension should be subsidiary or subordinate in terms of overall floor area, size, height and proportion to that of the original dwelling. With regard to the 50% figure quoted earlier, it is difficult to give a precise percentage which would always be appropriate but any extension which came close to doubling the floorspace, for example, would be unlikely to appear subsidiary and therefore unlikely to receive approval. This should be borne in mind when thinking of buying a property. Choosing a building that needs only modest expansion will avoid many of the problems that are caused by attempting to add an overlarge extension. The aim should be to ensure that the extension harmonises with and complements the existing dwelling, rather than simply being added on to it. Fig. 2 illustrates these points. But it must also avoid over-dominating smaller adjoining properties, in other words it should fit with its surroundings.

    Good design principles when considering extensions include:

  • Continuation – where the extension continues the building line and roof of the original dwelling. This suits a relatively small extension. Windows, doors and materials should match the original.

  • Reflection - by using the same form as the original but at a smaller scale or set back in terms of building line, or with a lower roofline.

  • Separation – where the original house has special features or a distinctive character which it would be difficult to emulate by either of the two previous methods, the extension could be linked by a porch, hallway or gallery.


    The extension should not result in an overdevelopment of the residential curtilage.  (Space Around Dwellings) gives the Council’s minimum standards for garden areas, parking spaces, site boundaries, distances between properties with overlooking windows, and distances to plot boundaries from the building. Extensions should not result in the loss of existing parking spaces where to do so would cause road safety or congestion problems. Furthermore, extensions should not lead to the loss of large areas of garden which could then lead to future applications for garden extensions, particularly in rural settlements or in the open countryside.

     "Trees and Development",  aim to retain trees wherever possible and to avoid encroachment upon trees, which may still be growing.



    The external walls and the roof of the extension should normally be sympathetic to those on the original building in terms of the type, texture and colour of materials used, as should the pointing of brickwork and stonework. Should the building have been altered inappropriately in the past, it may on occasions be better to seek to relate the extension to nearby buildings which have better retained the vernacular, that is, the indigenous local style.

    Sustainability issues should be borne in mind, such as trying to use local materials (brick, stone or timber, as appropriate) and by using energy efficient materials and features.



    In order to produce a visually suitable design, account should be taken of the following considerations:

  • Punctuation – the giving of greater or lesser emphasis to different parts of a design to avoid monotony and incoherence.

  • Balance – the placing of the parts of a design to give a settled composure rather than a restless, unsettled appearance; try to ensure that the extended building does not look awkward or unbalanced.

  • Resolution – the giving of major emphasis to one important element of the composition, to give a focus and avoid the confusion of different parts competing for attention in an unbalanced jumble of unrelated parts.

  • The focal point should be clear, and from there the other elements of the design should be placed, proportioned and detailed so as to allow the eye to "read" the whole design in an orderly way. These principles are explained better by illustrations than words. Fig. 3 shows these considerations in both good and bad terms.


    be higher than that of the existing dwelling, and its pitch, angles and materials should match those of the existing struDetails of ridges, eaves, gutters and downpipes should be shown on the drawings. In conservation areas and on listed buildings, materials used (e.g. cast iron) will be a significant consideration. Walls, railings and gates of merit should be taken into account, with good examples being retained.




    Particular forms of extensions


    Front extensions

    Normally these should not project forward of the existing building but in certain situation to do so would reflect an existing feature of the locality, they may be acceptable. Also, where the existing building is set well back and the front elevation reflects the design of the existing should not cause detriment to the neighbouring dwellings . The 45° guide, describer rear extensions, may be applicable. Care should be taken not to reduce car parking below the required level, normally two car parks per dwelling are acceptable.


    Side extensions

    Flat roofed extensions, which do not fit with the present building, are not generally acceptable. A side extension should not fill the gap between residential properties so as to create the impression of a terrace in line of detached or semi-detached houses, but setting back the extension by a metre or two could maintain a visual break. Corner plots, prominent from two streets, may warrant additional criteria in which the width of the extension should not exceed half the width of the existing frontage of the property, nor should it exceeds half the width of the  garden/plot between the property and adjacent road.



    Rear extensions

    As always, care should be taken to avoid adversely affecting the amenities of neighbours, as previously explained.

    Note 2. Figure 6 shows examples of how not to do it.

    The Council uses as a guide the 45° code, explained in Figure 7. In summary, a 45° line is drawn from the midpoint of a sill of a window in a habitable room in an adjacent house. If the proposed extension would go beyond that line it would probably result in an unacceptable loss of light. (But this requirement would normally exclude windows on side/gable elevations, otherwise spaces between dwellings fronting a highway would have to be widened to an unacceptable level.) The impact of the height of an extension can be assessed by drawing a upwards at an angle of 25 °. For a bungalow the extension should not be higher

    Two storey extensions should not normally be within 2 metres of a boundary that forms a party wall between terraced and semi-detached properties, and 1 metre of other properties. (Occasionally the only way properties can be extended is by building at the side up to the boundary, but this must be visually relieved by setting back the façade and/or lowering the ridge height.)



    Attic and roof extensions


    If the roofspace has sufficient height to allow for standing then light may be obtained by means of rooflights, but otherwise the only physically practical answer may be to use sizeable dormer windows, which may well be unacceptable to the Council because they almost inevitably affect the character of the building and area for the worse. Where small gables are a local feature they may provide an acceptable design solution. Roof alterations and dormer windows should normally be kept as small as possible, and should not project above the ridge line of the property. Sloping rooflights are cheaper to install than dormer windows and less intrusive, and are also likely to avoid or reduce overlooking. If the attic or loft conversion is to be used as a habitable room (e.g. a bedroom), the stair access should not be from an existing habitable room.




    Annexes and dependent relatives’ accommodation ("granny flats")


    A residential annex is accommodation ancillary to the main dwelling within the residential curtilage and must be used for this purpose. It is acknowledged that an extension of the house or conversion of an outbuilding may provide an opportunity to accommodate elderly or sick relatives, or older teenagers, in the curtilage of the main dwelling whilst giving them some degree of independence. However, the annex (or "granny flat") should form part of the same "planning unit" (by sharing the same access, parking area and garden) because the Council will wish to avoid the annex becoming a self contained dwelling and the creation of two separate dwellings, and it will attach conditions to prevent this occurring. (If the applicants’ wish is to separate off part of their curtilage to form a new planning unit then there may be an acceptable way of doing so in built-up areas should be discussed with Planning Services staff, but not in the open countryside where new dwellings are not normally allowed.) The layout, design and physical relationship between the house and the proposed annex will be important when considering applications.. As a guide, the scale should be such that the annex could be used as part of the main dwelling once the dependency need has ceased.