Trimble Architects

What are the rules on planning permission for extensions? If you want to extend your house, the answer depends on the size, position and design of the extension, the type of property you own and whether permitted development rights apply. 

Some house extensions can be completed as permitted development without making a full planning permission application, while other projects need approval from the local planning authority before work can start.

This article explains the main development rules for extensions in England, including single storey rear extensions, side extensions, two storey extensions, loft conversion work, boundaries, designated land and Building Regulations. 

Planning rules can change and individual properties may be subject to extra restrictions, so it is important to check the position for your site before beginning a project.

Permitted development rights allow many homeowners to extend an existing house without applying for full planning permission. However, permitted development does not mean that there are no rules. The extension must remain within strict limits covering its size, height, position, materials and relationship with the original house.

For a standard single storey rear extension, permitted development can normally allow an extension of up to three metres beyond the original rear wall for an attached house and four metres for a detached house. Larger single storey rear extensions can potentially extend up to six metres for an attached house or eight metres for a detached house through the larger home extension prior approval process.

This does not give every property an automatic right to extend by six or eight metres. Larger single storey rear extensions are subject to prior approval and the neighbour consultation process. Other permitted development rules apply at the same time, including height, land coverage and location limits.

The maximum height of a single storey rear extension under these rights is generally four metres. Where part of the extension is within two metres of the boundary, the eaves height is normally restricted to three metres. The development must also comply with other conditions relating to the roof, materials and appearance.

Permitted development rights apply to houses rather than every type of home. Flats and maisonettes do not benefit from the same household permitted development rights, so changes to flats and maisonettes can require planning permission. Rights can also have been removed by a condition attached to an earlier planning permission or by an Article 4 direction made by the local planning authority.

If there is any doubt about whether your extension is permitted development, you can apply to the local authority for a lawful development certificate. A lawful development certificate can provide formal confirmation that the planned work is lawful for planning purposes.

Although planning permission may not be needed, Building Regulations can still apply. These are separate development rules covering matters such as structural safety, insulation, ventilation, fire safety and energy performance. You should establish which approvals are required before you start building work.

WHAT IS THE DIFFERENCE BETWEEN LISTED BUILDING CONSENT AND PLANNING PERMISSION?

Planning permission will normally be needed when an extension goes beyond the limits or conditions of permitted development. This could happen because the extension is too large, too high, positioned too far forward, uses development rights that have already been used by previous extensions or does not meet the rules for the site.

An extension that extends forward of the principal elevation of the house and fronts a highway will not normally fall within the standard permitted development rules. Planning permission may therefore be required. Similar restrictions can apply where an extension changes a side elevation that fronts a highway.

Planning permission can also be needed where permitted development rights have been removed. This sometimes happens through a planning condition on the original house or through an Article 4 direction. Before relying on permitted development, it is sensible to check the planning history of the property and speak to the local planning authority.

Planning permission is also more likely to be needed where a house is on designated land. Additional limits apply in a conservation area, National Park, World Heritage Site and an area of outstanding natural beauty. Sites of special scientific interest can also be subject to extra restrictions under the development rules.

Planning application fees in England changed on 1 April 2026. At that date, the statutory fee for a householder application covering the enlargement, improvement or alteration of a single dwellinghouse is £548. Fees can change again, so the Planning Portal fee calculator or your local planning authority should be checked when you are ready to apply.

The fee paid with the planning application is only one part of the possible project cost. You may also need to allow for drawings, surveys, structural calculations, planning advice, Building Regulations work and professional services. Some local authorities also charge separately for pre-application planning advice.

A prior approval application for certain householder development, including a larger home extension, has a statutory fee of £249 from 1 April 2026. Where a proposed lawful development certificate is required, its statutory fee is normally half the equivalent full planning application fee.

If an online application attracts the Planning Portal service charge, this is separate from the statutory planning fee. The Planning Portal currently applies a service charge to qualifying applications submitted through its online system. Check the amount at the point of submission because these charges may change.

It is important to distinguish current fees from proposed future changes. Planning charges are subject to legislation and can be increased, so always find the current amount rather than relying on an older article or previous project.

Two storey extensions are subject to tighter permitted development rules than most single storey extensions. Where a rear extension has more than one storey, it must normally extend no more than three metres beyond the rear wall of the original house if it is to rely on the relevant permitted development right.

There are also controls over height, eaves and the roof pitch. Where practicable, the roof pitch of an extension with more than one storey should match the roof pitch of the existing house. Materials used on the exterior should also be of a similar appearance.

Two storey side extensions do not normally qualify under the same permitted development provision as a simple single storey side extension. If your project involves adding a second storey or extending sideways over more than one storey, professional planning advice can be useful before plans are finalised.

On designated land, the rules are more restrictive. An extension from a rear wall that results in an enlarged part of the house having more than one storey is not permitted development on Article 2(3) land. Planning permission will therefore be needed in those circumstances.

A side extension can sometimes be completed under permitted development, but its dimensions are restricted. The side extension must normally be single storey, must not exceed four metres in height and must not be wider than half the width of the original house.

The location of the side elevation matters. An extension cannot normally be treated as permitted development if it extends beyond a wall forming the principal elevation of the original house. Further restrictions can apply where the relevant elevation fronts a highway.

On designated land, extensions beyond a wall forming a side elevation of the original house do not benefit from the standard Class A permitted development right. This can make planning permission necessary for a side extension in a conservation area or other protected area even where a similar project elsewhere might be permitted.

Yes. Previous extensions can affect what is still allowed under permitted development rights. Many limits are measured by comparing the existing proposal with the original house rather than simply measuring the new extension on its own.

For example, where a proposed rear extension joins an earlier extension, the permitted development limits can apply to the total enlargement. This means that a project that appears small when viewed on its own can exceed the allowed development limits once earlier changes are taken into account.

This is one reason why it is useful to find the planning history of a house before designing a new extension. Old planning decisions, drawings and lawful development certificates can help establish what changes have already been made.

The neighbour consultation process applies to certain larger single storey rear extensions under permitted development. It allows an attached house to potentially extend up to six metres beyond the original rear wall and a detached house to potentially extend up to eight metres, provided the project complies with the other permitted development rules.

The homeowner must notify the local planning authority before building work begins. The authority then tells adjoining neighbours about the proposal and gives them an opportunity to comment on its effect on their home.

If an adjoining neighbour objects on amenity grounds, the local planning authority considers the effect of the project and decides whether prior approval should be given. A neighbour's objection does not automatically mean that the extension is refused, but the prior approval process must be completed correctly before work begins.

This process is different from a normal planning permission application. It is a form of prior approval under permitted development rather than full planning permission.

An extension can sometimes be built up to or close to a boundary, provided the planning, Building Regulations and property law requirements are met. There is not one planning rule that requires every extension to stand a fixed distance from every boundary.

The height of the building can become important close to the boundary. Party wall legislation may also apply to certain work on or close to a shared structure. The exact ownership and position of the boundary should therefore be established before construction starts.

Permitted development rules are more restrictive on certain designated land. Article 2(3) land includes conservation areas, National Parks, the Broads, areas of outstanding natural beauty and World Heritage Sites.

For a house on this type of designated land, planning permission may be needed for work that could have been permitted development elsewhere. Side extensions are more restricted, and an extension from a rear wall with more than one storey will not normally qualify as permitted development under the standard householder right.

There are also restrictions on exterior cladding. Cladding part of a house with stone, artificial stone, pebble dash, render, timber, plastic or tiles is not permitted development under the relevant right on Article 2(3) land.

Roof extensions for a loft conversion are also more restricted on designated land. If the property is a listed building, separate listed building consent may be required for work affecting its special character, even where another form of planning approval is in place.

A site of special scientific interest is subject to some additional permitted development restrictions as well. If the house or land has a protected status, consult the local planning authority before assuming that standard development rights apply.

A loft conversion can sometimes be completed as permitted development, but the rules for roof extensions are different from those applying to a ground floor extension. The amount of additional roof space, the position of the extension, its height and its relationship with the existing roof all matter.

A dormer loft conversion project must stay within the relevant permitted development limits if planning permission is to be avoided. Among other requirements, an addition should not normally extend higher than the highest part of the existing roof and materials should have a similar appearance to those used on the existing house.

Roof extensions are more restricted on Article 2(3) designated land. Planning permission may also be needed where development rights have been removed or the house does not benefit from the standard permitted development rights.

When choosing roof tiles and other external materials, consider both the planning conditions and the appearance of the existing building. A design with a similar appearance can make the changes less visually intrusive.

Balconies, verandas and certain raised platforms are treated differently from a normal extension. The standard householder permitted development rights do not allow every form of balcony or veranda to be added without planning permission.

Raised platforms can also fall outside permitted development when they exceed the allowed height. Garden decking may qualify under separate development rights where the platform is no more than 0.3 metres high and the other conditions are met.

If your extension design includes balconies, verandas or a significant raised platform, check whether planning permission is needed rather than assuming they are covered by the approval for the main building.

Garages, sheds and other outbuildings can affect the amount of land available for an extension under permitted development. One of the main limits is that the total area of ground covered by buildings within the land around the house, excluding the original house itself, must not exceed half of that land.

This means an existing garage, shed, previous extension or other building may need to be included when checking whether the 50% limit has been met. The floor space of each structure is not considered in isolation.

Outbuildings also have their own development rules covering matters such as height, location and use. A building intended for a use that is not incidental to the enjoyment of the house may need separate planning permission.

Windows are an important part of any extension because their position can affect appearance, light and privacy. Replacing existing windows with windows of a similar appearance will not usually require a separate planning permission application unless special restrictions apply to the property.

New windows in an extension need to comply with any conditions attached to the planning permission or permitted development right. Upper floor windows in a side elevation can be subject to conditions concerning obscured glazing and whether they can open.

Building Regulations also apply to the thermal and safety performance of glazing. Modern double glazing can improve insulation, reduce heat loss and help make the new floor space more comfortable throughout the year.

Where a project is on designated land, involves a listed building or makes major changes to an important elevation, the design and materials used for windows may require additional consideration.

An extension gives homeowners an opportunity to improve the energy performance of the building. Insulation, windows, glazed doors, heating and ventilation should all be considered as part of the project rather than after construction has started.

Building Regulations set standards for the thermal performance of new building work. This can include walls, floors, the roof, doors and windows. A well-designed extension can reduce unwanted heat loss while still providing useful natural light and ventilation.

Planning and Building Regulations have different purposes. Receiving planning permission does not mean that the construction automatically complies with Building Regulations, and Building Regulations approval does not replace any planning permission that is needed.

When planning the extension, consider how the position and size of windows will affect daylight and comfort in the new floor space. Large areas of glass can provide more light, but their orientation and energy performance should be considered carefully.

Good-quality glazing can reduce outside noise and unwanted heat loss. The design should also take account of privacy, particularly where new windows face neighbouring homes.

Windows should form part of the overall design rather than being considered separately. Their size, materials and appearance can make a significant difference to how well the extension works with the existing house.

The normal target period for deciding many householder planning applications is eight weeks, although some applications take longer or are dealt with within an agreed extended period.

The latest published government statistics available at the time of writing show that local planning authorities in England decided 151,900 householder development applications in the year ending March 2026. Around 90% were granted, while 93% were decided within eight weeks or an agreed period.

These figures cover householder development that required a planning decision, such as extensions, conservatories and loft conversion projects. They do not mean that every proposed extension has a 90% chance of approval, because every planning application is assessed according to the site, design, local planning policies and other relevant circumstances.

A clear application with suitable drawings and supporting information can reduce the risk of delays caused by missing documents. Before making an application, find out which validation requirements apply to your local planning authority.

A local planning authority can use an Article 4 direction to remove particular permitted development rights from a defined area or property. If the right needed for your project has been removed, you may need to apply for planning permission even though the same extension would normally be permitted elsewhere.

Article 4 directions are particularly important in some conservation areas and parts of London where councils want greater control over changes to buildings. Restrictions may also have been placed on a house through a condition attached to an earlier planning decision.

Before relying on permitted development, check both the planning history of the property and any local restrictions. This can prevent plans being prepared for an extension that cannot use the expected development rights.

The standard householder permitted development rights described in this article mainly apply to houses. Flats and maisonettes do not have the same rights to make extensions and alterations.

If you own a flat or maisonette and want to extend the building, change its roof or make significant external changes, planning permission may be needed. Other approvals from a freeholder, landlord or management company may also be required depending on the property.

Do not assume that a project is permitted development simply because an identical change would be allowed on a detached or terraced house. The class of property is an important part of the planning assessment.

If you believe an extension is permitted development, you do not normally make a full planning permission application solely to use that right. However, obtaining a lawful development certificate can provide formal confirmation from the local authority that the planned development is lawful.

This can be useful when the permitted development rules are complex, when you intend to sell the house later or when a lender or buyer may want clear evidence of the planning position.

The application should include enough information for the authority to determine whether every relevant condition and limit has been met. Accurate plans and measurements are therefore important.

Before you start work, establish whether the proposed extension needs planning permission, qualifies as permitted development or needs prior approval. Check whether the house is on designated land, whether it is a listed building, whether an Article 4 restriction applies and whether previous extensions have affected the development rights that remain.

You should also consider Building Regulations, boundaries, party wall requirements, drainage and access. Planning permission deals with the acceptability of development, but it does not remove the need to comply with other legal requirements.

The measurements used in permitted development rules must be accurate. Small changes in height, width or depth can decide whether an extension is permitted. This is particularly important around the rear wall, side elevation, eaves, roof and boundary.

If the project does not meet permitted development rules, that does not necessarily mean the extension cannot be built. It means that a planning permission application may be needed so the local planning authority can consider the proposal through the normal planning process.

There is no general rule saying that every extension must be designed by an architect. However, professional help can be useful where planning rules are complex, the property is on designated land, the design involves two storey extensions, a side extension or major roof changes, or a planning permission application is needed.

An experienced professional can review the existing house, previous changes and site conditions before plans are prepared. They can also help make sure that drawings clearly show the dimensions and appearance required for the planning process.

For projects in London, local policies, conservation areas and Article 4 restrictions can add further considerations. Obtaining planning advice at an early stage can help identify these matters before significant time is spent developing the design.

If you want to find out whether you need planning permission for an extension building project, you can discuss the proposal with your local architect. If you are looking for planning advice in London, we can help you with planning permission advice.