Why has this note been prepared?
This advice note has been produced to inform applicants submitting planning applications for housing extensions. Policy on this type of development is set out in Local Plan Policy CO17 (Householder Development). In particular, it includes further guidance on how floorspace will be calculated when considering the policy’s requirement for a 30% limit on extensions, including when using and extending existing roof space.
What is the policy?
The Local Plan contains Policy CO17 - Householder Development, which contains the following criterion:
a) Any extension should be clearly subservient to the main part of the building and should not increase the total habitable floorspace by more than 30% unless there are compelling planning considerations in favour of a larger extension;
The supporting text to the policy explains its rationale – that very large extensions can be overbearing and proposals which incrementally extend small dwellings beyond their original size can have a detrimental impact on the character of an area and a mix of dwelling types needed to sustain balanced communities. Limiting the size of new extensions can help avoid the loss of smaller more affordable dwellings in the National Park.
All proposals for extensions to existing dwellings should be considered for their impact on local character and appearance. Proposals which fail to respect local character, or complement the scale, height, massing, appearance and character of the existing dwelling, or have an overall adverse impact on the landscape of the National Park will be resisted irrespective of compliance or otherwise with the size-based threshold of Policy CO17.
Permitted development rights
You can make certain types of minor changes to your home without needing to apply for planning permission – known as "permitted development rights". The Planning Portal is the UK Government’s online planning and building regulations resource and has lots of useful information about the types of minor works you can do to your home without the need for planning permission.
Permitted development rights for house extensions are set out in the 2015 General Permitted Development Order 2015 (GPDO). The extent of such rights may differ in the National Park from elsewhere - within the Order National Parks are classed as ‘Article 2(3)’ land, where rights may not be as extensive. For example, under Part 1 Class A.2 of the GPDO it not possible to extend beyond the side elevation of the existing building in a National Park, which is possible elsewhere without planning permission.
Class B of the GPDO also allows the enlargement of a dwelling house consisting of an addition or alteration to its roof. This right does not, however apply in the National Park (GPDO B1(f)). Any proposals to alter the shape of a roof – for example through the installation of dormer windows will require planning permission.
As stated in Policy CO17, where extensions are granted permission the Authority may remove permitted development rights to prevent further extension occurring without the need for a planning permission.
Calculating the floorspace of the existing dwelling
The supporting text of the policy states that the 30% figure is calculated based on existing ‘habitable floorspace’ of the main domestic dwelling. This is the extent of the main domestic dwelling as it was on 1 July 1948 (or if built after that date, as it was originally designed) excluding extensions erected after 1July 1948.
Habitable floorspace is calculated from the gross internal floorspace defined as the total floor space measured between the internal faces of perimeter walls that enclose the dwelling. This includes columns, chimney breasts, entrance halls, corridors, service accommodation, internal garages, flights of stairs and voids above stairs. External garages (including attached garages), conservatories (defined as a room attached to the main dwelling with glass roof and walls) and outbuildings will not be counted in the calculation of floorspace when calculating the existing habitable floorspace (see 7.88 of the Local Plan).
For the avoidance of doubt, garages which are attached to the main building or within an outbuilding will not be considered part of existing habitable accommodation. However, integral garages (defined as those built within the walls of the main property and an element of the building's structure) and any additional floorspace located above detached garages used for ancillary habitable accommodation, such as guest accommodation or a home office or studio will be included within the percentage floorspace increase.
Outbuildings should be excluded from the habitable floorspace calculation as these are classed as separate from the main structure – even if they provide habitable accommodation (e.g. as a bedroom or study). Outbuildings are normally defined as subsidiary buildings which have not been designed or built for habitable use as part of the main dwelling. Examples include log stores, sheds, fuel store, balconies, stables, carports, detached garages and greenhouses.
Where extensions have occurred as permitted development, applicants will be expected to provide details of these such as the extension size and the date of completion. Planning officers may also be able to check details through the site’s planning history and building control records. The onus will be on the applicant to evidence this and where there is any doubt, the case officer will presume that previous extensions do not form part of the ‘existing dwelling’ for the purposes of calculating the percentage floorspace increase.
A modest basement will not normally be regarded as habitable floorspace. However, some judgement will need to be applied in terms of whether it is genuinely a secondary space in association with the main dwelling.
Some applications for substantial extensions cite the extensions that could be built under Permitted Development Rights in support of the proposal. The calculation will be applied to existing habitable floorspace and will exclude any floorspace that could be built in the future under permitted development rights, or any floorspace that could be built in accordance with an existing permission that has yet to be implemented (or both).
Extending into existing roof space
If additional habitable floorspace is being created in an existing roof space (for example through a loft conversion) and there are no changes to the external shape of the roof this area should be included in the total habitable floorspace of the original dwelling. It would not therefore form part of the percentage increase to the existing dwelling and would not count towards the 30% allowance applicable to further extensions.
Likewise, if small alterations are needed to allow the existing roof space to become habitable which don’t fundamentally alter the shape or size of the accommodation within the roof space, such as the addition of small dormers, this would also be considered to form part of the existing habitable floorspace of the original dwelling, and would therefore not count towards the 30% allowance.
If, however more substantial alterations are needed to create habitable floor space in an existing roof void which go beyond alteration and are more in the realms of an extension, then the 30% threshold will be applied. Such instances could include:
- Where the dormer(s) is/are excessive to the point that it would be considered a first floor extension;
- Where the proposal would result in an increase in roof height above the current highest point;
- Where a proposal would result in an increase in eaves height;
- Where a new storey is put on a house.
Evidencing this information in a planning application
Applicants will be expected to provide calculations with their applications. Such information should include:
- Calculation of the total habitable floorspace of the existing dwelling (as defined above) clearly stating which building element(s) have been used to create this figure,
- Calculations of the total habitable floorspace of extensions already built (whether through permitted development or previous approvals)
- Total additional floorspace being provided by the proposal
When totalling B and C together, the additional floorspace should not increase A by more than 30%.
Further information
Further information on how to submit a planning application, along with details on how it will be assessed can be found on the Authority’s website
The Authority also offers a structured pre-application advice service through which officers can advise on the 30% limit.
Note: Paragraph 4.2 of this note was modified on 6 September 2021 to further make clear that whilst internal garages (within the footprint of the main building) will be included in the calculation of the original habitable floorspace, external garages (including those attached to the house will not).