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What is the 45-degree planning rule, and how can it affect my extension?

Whether you're planning an extension yourself or are concerned about your neighbour's new planning application, our simple guide to the 45-degree rule can help you understand how this rule can restrict developments and how it can affect both you and your neighbours.

What is the 45-degree rule?

Most councils apply a 45-degree rule when it comes to determining the maximum size of proposed developments and extensions. This is in an effort to guarantee that new proposals aren't granted permission when they would obstruct views or access to daylight for neighbouring residents. It seeks to strike a balance between the desires of property owners to construct and upgrade their properties on the one hand and the 'Right to Light' (‘RTL’) of people residing in neighbouring properties on the other.

To avoid falling foul of the rule, projects should be designed in such a way that any new building mass does not protrude through an imaginary 45-degree plane, starting at the window of the nearest neighbouring 'habitable room'. The term, in this case, refers to any living room, bedroom, kitchen, and conservatory but does not include bathrooms, utility rooms, hallways, landings, or garages.

The 45-degree rule and the Permitted Development trap

Many people who construct extensions under Permitted Development (PD) Rights fail to realise that 'Right to Light' easements are considered independently of Lawful Development and Planning Assessments. Beware, you can construct a project that is considered PD, only to receive a letter from a neighbour's lawyer demanding compensation, alteration or demolition. Upon receiving this, you would need to prove your neighbour's rights have not been breached through a 'Right to Light' assessment, settle the claim, or adjust your project to ensure any breach is remedied. Holtbeck Architects advises you to consider your project carefully and not rely on untrained design work or advice. We have seen countless PD projects breach this rule. So many that we could probably start a lucrative legal business advising neighbours on how to cash in on neighbours' and builders' nativity when it comes to the importance of good design practice. As with many laws, ignorance is not an excuse. If in doubt, please get in contact; one of our team here will be happy to help. Please also see our post on 'RTL' on our dedicated post on the topic here.

Whilst the 45-degree rule is a good way to avoid falling foul of 'RTL' laws, it doesn't cover all bases, and for contentious projects, we would recommend specialist input from a 'Right-to-Light' surveyor. These surveyors will assess the project and its surroundings and provide a report confirming its compliance with 'RTL' legislation prior to breaking ground on site and prior to any nasty legal rumblings. Holtbeck Architects also offers a specialist PD design service, where we will ensure that your relationship with your neighbour has the greatest chance of remaining amicable, that your extension is designed in a lawful manner, that you obtain a 'Lawful Development Certificate' from your local council and that you don't fall prey to the Permitted Development Right to Light trap.


The 45-degree rule can help you get off on the right foot when assessing the impact of 'RTL' on your project, and it’s worthwhile to be familiar with it before you begin. Doing so will ensure that you avoid disappointment when it alters or reduces the scale of your design ambition or creates future complications in the form of expensive alterations, financial settlements, or demolition.

At Holtbeck Architects, we always consider 'RTL' at an early stage in our projects. Our team is available to answer any queries you might have about the 45-degree rule or any of the other topics covered in this post; if you have any, please feel free to get in touch. You can reach us on our contact page, send us a message through our live chat feature, or find answers on our FAQ Blog or in our Terms Glossary.

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