Planning Permission for a Rear Extension: The Complete Guide

Rear extensions are one of the most popular ways to improve a home, create additional living space and add value. However, before starting work, it is important to understand whether planning permission is required, whether permitted development rights apply, and whether a Certificate of Lawfulness should be secured.

Do you need planning permission for a rear extension?

Before exploring the detail of planning permissions for rear extensions it is important to consider if your scheme requires planning permission at all.

You do not always need planning permission for a rear extension. Many homeowners assume that every rear extension requires a full planning application, but this is not necessarily the case. Some rear extensions can be built under permitted development rights, provided the proposal complies with the relevant limitations and conditions.

However, the rules are technical. The planning position will depend on the size, height and depth of the extension, the type of property, previous extensions, planning history, location constraints and whether permitted development rights have been removed.

When is full planning permission required?

Full planning permission will usually be required where the rear extension falls outside permitted development limits or where permitted development rights are not available.

A planning application will be required where:

• the extension projects too far beyond the original rear wall;
• the height or eaves height exceeds permitted development limits;
• the extension extends beyond a side wall of the property and is more than half the width of the original house;
• the property has already been significantly extended;
• permitted development rights have been removed by a planning condition;
• the property is listed or affected by heritage constraints;
• the proposal is within the Green Belt and raises issues of proportionality;
• the building being extended is a flat or maisonette, or change of use permitted development rights have been used;

Permitted development rights for rear extensions

Permitted development rights allow certain domestic extensions to be carried out without submitting a full planning application. For rear extensions, this can often provide a quicker and more certain route, but only where the proposal meets the relevant rules.

These rules commonly relate to projection from the original rear wall, overall height, eaves height, proximity to boundaries, materials, previous additions and whether the property is located within a restricted area.

Previous extensions are taken into account, as the requirements relate to the 'original dwelling' This means that an extension built many years ago, even if it was built by a previous owner, may still affect whether a new rear extension can be carried out under permitted development rights.

Planning Permission for Rear Extensions

If your proposal does not meet the requirements for permitted development you will need to apply for full planning permission for your works. This is also commonly referred to as a householder planning application.

As part of assessing your application, the Council will take into account their local planning policies and any supporting guidance. Some Councils have very descriptive and illustrative design guidance on the size and scale of extensions they will generally allow. Reviewing your plans against these documents is a valuable first step in your planning journey.

The key considerations for your application will depend on your site and proposal. However, we cover some of the main themes below.

Residential Amenity of Neighbouring Properties

If you need planning permission for your rear extension you will need to demonstrate that your proposal does not cause an unacceptable level of overlooking or a loss of light to neighbouring properties.

Council's often use the 45-degree and 25-degree rules when assessing whether a proposed extension would cause an unacceptable loss of daylight and sunlight to neighbouring properties. In applying the 45-degree rule, a line will be drawn from the midpoint of the nearest window on the neighbouring property at a 45-degree angle. If you extension breaches this line, the Council may look to refuse your application.

It is important to note that the 45-degree rule only applies to householder planning applications, or where an objection is received from a neighbour to a prior approval application.

The Scale and Mass of an Extension

As part of your proposal, the Council will judge if your extension is subservient in scale and massing to your existing property. This will include consideration of the size of the extension, as well as other factors such as its height and visibility. Paying careful attention to roof forms and materials can help reduce the scale and massing of an extension.

Heritage and Landscape Matters

If your property is located in a sensitive landscape or heritage location, special consideration will need to be given to these matters.

Green Belt

Addition considerations often apply in Green Belt locations. National planning policy requires that extensions to buildings does not result in disproportionate additions over and above the size of the original building. The impact on openness of the Green Belt is also a key considerations.

In such locations, a clear planning strategy can give your proposal the best chance of success. This can include using permitted development rights and prior approval applications to create a fallback position.

Prior approval for larger rear extensions

There is also a prior approval route for certain larger single-storey rear extensions. This is not the same as a standard planning application, and it involves neighbour consultation and assessment of the impact on adjoining properties.

As Cedar Planning has a separate blog on prior approval rear extensions, this guide only deals with the route briefly. In practice, prior approval can be useful where a homeowner wants a larger single-storey rear extension than would normally be allowed under the standard permitted development depth limits.

Why a Certificate of Lawfulness is strongly recommended

If you are relying on permitted development rights, a Certificate of Lawfulness provides formal confirmation from the Council that the proposed rear extension is lawful.

This can be valuable because it gives certainty before construction begins. It can also help avoid disputes with neighbours, enforcement issues, or complications when selling or refinancing the property.

A Certificate of Lawfulness can help confirm:

• whether permitted development rights are available;
• whether previous extensions affect the proposal;
• whether the proposal complies with the relevant size and height limits;
• whether restrictive planning conditions apply;
• whether the extension can lawfully proceed without full planning permission.

Using permitted development as a fallback position

Permitted development rights and prior approval can sometimes be used strategically as a fallback position. This can be particularly useful where a homeowner is seeking planning permission for a larger or more ambitious rear extension.

A fallback position may demonstrate that an alternative form of development could lawfully be implemented even if the Council refuses the preferred scheme. In some cases, this can carry weight in the planning balance, especially where the fallback scheme would have similar or comparable impacts.

This can be especially valuable in Green Belt locations, where the impact on openness and the scale of additions to the original dwelling can be central issues.

Common reasons rear extension applications are refused

Rear extension applications are often refused where the Council considers the proposal to be harmful to neighbouring amenity, poorly designed, excessive in scale, or inconsistent with local guidance.

• Overbearing impact: where the extension appears dominant or enclosing.
• Loss of light: where the extension affects neighbouring windows or gardens.
• Overlooking and privacy: particularly where raised terraces, balconies or upper-floor windows are proposed.
• Poor design: where the extension fails to respect the host dwelling.
• Green Belt harm: where additions are considered disproportionate.
• Heritage harm: where the property is listed or within a conservation area.

How Cedar Planning can help

Cedar Planning provides professional planning advice for rear extensions, householder planning applications, Certificates of Lawfulness, permitted development assessments, planning appeals and Green Belt extension strategy.

We can advise whether planning permission is required, whether permitted development rights apply, whether a Certificate of Lawfulness should be submitted, and how best to present your proposal to the Council.

Rear extension planning permission FAQs