A side extension can provide valuable additional space without changing the overall character of a home too significantly. In England, certain side extensions can be built under permitted development rights without submitting a full planning application. However, strict rules apply to the extension’s size, height, position and design.
Understanding these requirements before starting work can help you establish whether your proposed side extension qualifies as permitted development or whether planning permission is required.
Can a Side Extension Be Permitted Development?
Yes. A side extension can qualify for permitted development under Class A of the householder permitted development rights, provided it satisfies all the relevant conditions and limitations. The rules apply to houses, but not generally to flats, maisonettes or other types of buildings. The most important requirements for a side extension are that it must be single storey, no more than 4 metres high, and no wider than half the width of the original house. Other restrictions can also apply, including rules concerning the position of the extension, the amount of land covered by development, materials and the property’s location.
Key Permitted Development Rules for Side Extensions
Maximum Width
The width of a side extension must not exceed half the width of the original house, measured at its widest point. This is based on the original dwellinghouse, rather than the current building, if previous extensions have already been added. For example, if the original house is 8 metres wide at its widest point, a side extension can be no wider than 4 metres under these permitted development rules. Previous extensions can affect the calculation, particularly where a new extension will be joined to an existing enlargement. The total enlargement must remain within the applicable limits.
Maximum Height
A permitted development side extension must be single storey and must not exceed 4 metres in height. There is also a general restriction where an extension comes within 2 metres of the property’s boundary. In this situation, the eaves height must not exceed 3 metres. These height restrictions should be considered carefully when designing the roof, particularly on properties with sloping ground or where the extension will be close to a neighbouring boundary.
Where Can a Side Extension Be Built?
Permitted development rules restrict extensions from projecting forward of the principal elevation of the original house. Similar restrictions apply where a side elevation fronts a highway. This means that a proposed side extension cannot simply be positioned anywhere alongside the property. The relationship between the original house, its principal elevation, side elevation, highway and boundaries needs to be considered.
Where a proposed extension occupies the area between the side and rear of a property, both the side-extension and rear-extension restrictions may apply. This can affect the permitted width, height and depth of the overall enlargement.
Other Conditions for Side Extensions
Meeting the half-width and 4-metre height limits does not automatically make a side extension permitted development. Other Class A conditions also need to be satisfied. For example, the total area of ground covered by buildings within the curtilage, excluding the original house, must not exceed 50% of the total area of the curtilage. This calculation can include existing extensions and other buildings such as garages and outbuildings.
The extension must also not be higher than the highest part of the existing roof, and the materials used for exterior work should generally have a similar appearance to those of the existing house. Permitted development rights also do not allow the construction of verandas, balconies or raised platforms as part of the extension.
Can a Two-Storey Side Extension Be Permitted Development?
No. A side extension that extends beyond a side elevation of the original house cannot have more than one storey under the standard householder permitted development rules. If you want to create a two-storey side extension, a planning application will generally be required.
This can be particularly relevant where homeowners want to enlarge a property at both ground and first-floor levels. Although some rear extensions have different permitted development provisions, these do not provide a permitted development route for a two-storey side extension.
Side Extensions on Semi-Detached and Detached Houses
Side extensions can be considered for both semi-detached and detached houses, provided the relevant permitted development requirements are met. However, the design possibilities can vary significantly between properties. A detached house may have more space around its side elevations, while a semi-detached property may have a narrower gap between the house and boundary. The half-width rule still applies, so the size of the original house must be established before determining the maximum permitted width.
Side Extensions on Designated Land
Additional restrictions apply to side extensions in certain protected areas. On Article 2(3) designated land, which includes areas such as conservation areas, National Parks, the Broads, Areas of Outstanding Natural Beauty and World Heritage Sites, side extensions are not permitted development under the standard Class A rights. This means that a side extension in one of these areas will generally require a householder planning application. An Article 4 Direction can also remove or restrict permitted development rights in particular locations. Planning conditions attached to an earlier permission may have a similar effect.
Do Side Extensions Need Building Regulations Approval?
Planning permission and building regulations are separate requirements. Even if your side extension qualifies as permitted development, the construction will normally need to comply with applicable building regulations. These requirements can cover structural stability, foundations, insulation, ventilation, fire safety, drainage and other technical matters. It is therefore important to consider both planning and building control requirements when developing the design.
Common Side Extension Planning Mistakes
A common mistake is assuming that a side extension is permitted simply because it is relatively small. The permitted development rules contain several requirements that must be satisfied together. Other common issues include:
- Measuring the extension against the wrong part of the original house.
- Exceeding half the width of the original dwelling.
- Designing a two-storey side extension.
- Exceeding the 4-metre height limit.
- Ignoring the 3-metre eaves restriction near boundaries.
- Building forward of the principal elevation.
- Failing to account for previous extensions.
- Overlooking a conservation area or Article 4 restrictions.
- Assuming permitted development removes the need for building regulations approval.
Getting the measurements or planning position wrong can result in enforcement problems and potentially expensive alterations.
Should You Apply for a Lawful Development Certificate?
If your proposed side extension appears to comply with permitted development requirements, you can apply for a Lawful Development Certificate to obtain formal confirmation from the local planning authority.
An LDC is not planning permission. Instead, it confirms that the proposed or existing development is lawful for planning purposes. Although an LDC is not always compulsory, it can provide useful evidence of the planning status of the extension, particularly when selling or remortgaging the property.
How Studio Charrette Can Help
Studio Charrette can help homeowners establish whether a proposed side extension is likely to qualify for permitted development or whether a planning application is required. Our team can assist with planning assessments, architectural design, drawings and planning applications, helping you understand the restrictions that apply to your property before construction begins.
For projects with unusual layouts, previous extensions, boundary constraints or potential planning restrictions, professional advice can help identify issues at an early stage and avoid unnecessary costs.
Planning Your Side Extension
A side extension can often be developed under permitted development rights, but only where the relevant conditions are satisfied. The most important requirements are that it is single storey, no more than 4 metres high and no wider than half the width of the original house. Other factors, including boundary distances, existing extensions, site coverage, the position of the principal elevation and the property’s location, can also affect whether permitted development applies.
Before starting work, it is advisable to check the planning position carefully and ensure that the proposed design complies with all relevant requirements. Where the project falls outside permitted development rights, a householder planning application may provide the appropriate route. If you need advice on your proposed side extension, contact Studio Charrette for professional planning and architectural support.
Frequently Asked Questions
Can I build a side extension without planning permission?
Yes, a side extension may be permitted development if it satisfies the relevant Class A conditions. It must generally be single storey, no more than 4 metres high and no wider than half the width of the original house.
How wide can a permitted development side extension be?
A side extension can be no wider than half the width of the original house, measured at its widest point. Previous enlargements may also need to be considered where they form part of the total enlargement.
Can a side extension be two storeys?
No. A side extension extending beyond a side elevation cannot have more than one storey under the standard householder permitted development rules. A two-storey side extension will generally require planning permission.
How high can a side extension be?
A permitted development side extension can be a maximum of 4 metres high. Where the extension is within 2 metres of the boundary, its eaves height must not exceed 3 metres.
Can I build a side extension in a conservation area?
Standard permitted development rights do not allow side extensions on Article 2(3) designated land, which includes conservation areas and several other protected areas. Planning permission will generally be required.
