45 degree rule planning: what you need to know about natural light and your development

Councils use the 45 degree rule during the planning process. It’s a way of checking whether an extension will block too much natural light from neighbouring properties.

In this article, you’ll learn about the 45 degree rule, how councils use it in planning permission and how it can affect your development project.

The 45 degree rule, your extension and the local council

The intention of the 45 degree rule is to strike a fair balance between your desire to extend and your neighbour’s right to natural light.

When you submit a proposed extension design, the council uses it to judge its impact on the daylight in rooms in your neighbours’ houses. They do the same when assessing the impact of new developments, too.

The council could decide to reject your build project if it significantly reduces light to habitable spaces next door.

Which windows does the 45 degree rule cover?

Not every window on a neighbouring property gets protection from the 45 degree rule.

It only applies to the nearest window in a neighbour’s property for a habitable room they use regularly. These include living rooms, bedrooms, kitchens and dining rooms. They don’t include utility rooms, bathrooms, hallways or storage spaces. Blocking light to hallways or bathrooms rarely causes planning concerns.

Your planning officer will identify which windows qualify for assessment during their review.

The 45 degree rule does not consider all windows

The 45 degree rule is there to help strike a fair balance (January 2022) between your desire to extend and your neighbour’s right to natural light.

When you submit a proposed extension to an existing building, the council uses the rule when deciding whether to grant approval. They do the same when assessing the impact of new developments, too.

The council could reject your build project if it significantly reduces light to habitable spaces next door.

Not every window on a neighbouring property gets protection from the 45 degree rule. It only applies to the nearest window in a neighbour’s property for a habitable room they use regularly.

Here are the windows they’re likely to consider and not:

Assessed	Not likely to be assessed
Living rooms	Utility rooms
Bedrooms	Bathrooms
Dining rooms	Hallways
Reception rooms	Storage spaces

So, blocking light to your neighbours’ hallways or bathrooms is highly unlikely to cause any planning concerns. Some councils treat large glazed doors (for example French or patio doors) as windows for these checks.

Your planning officer will identify which windows qualify for assessment during their review.

Gardens and amenity space

Many councils also apply a natural light test to garden and amenity space as well as windows. They’ll test private rear gardens, usable side strips, patios, roof terraces and balconies. For flats and small developments, they may also test shared courtyards. Planning officers generally disregard front drives unless they’re used as seating or play space.

They mark the garden boundary on a scaled plan and then model the position of the sun as on 21 March. They compare the “before” and “after” including shaded areas and fixed features like walls, fences and outbuildings.

You’ll get a pass if at least half gets two hours, or if the sunlit area stays at least 80% of what it was before the build. If not, the officer may ask for a study or for you to change your plans.

The 45 degree rule and your local council

The 45 degree rule involves drawing lines from both horizontal and vertical perspectives. If your plans breach the rule in both directions, your extension is likely to cause significant daylight reduction. As this is widely used local guidance, the council may refuse your application.

Here’s how the two tests work:

The Horizontal Plane Test

The horizontal plane test assesses how far your extension projects into the outlook from your neighbour’s window.

The planning officer measures from your neighbour’s nearest habitable room window at a 45 degree angle. The line extends diagonally across your property boundary towards your extension or new building.

This is how you can try it for yourself:

Identify the nearest habitable room: This will usually be a lounge, dining room, bedroom or kitchen with a regular outlook.
Draw the 45 degree line on your plan: Measure from the centre point of your neighbour’s nearest habitable room window at a 45 degree angle.
Assess the footprint: If the line crosses the angle line from a bird’s eye view crosses your extension, it may fail the horizontal test.

If the line does cross, get a second opinion from an architect and change your design, if necessary.

The Vertical Plane Assessment

The vertical plane test examines your extension’s height impact.

Draw a 45 degree line upward from the cill towards your proposed development. The cill is the bottom ledge of the window where the frame meets the wall.

If the height of your extension is below the angled line, this will be acceptable in most cases. If it isn’t, the council may consider it overbearing or harmful to your neighbour’s light and outlook. You may need to reduce the height or change the roof shape.

How to measure from your neighbour's window

In all 45 degree rule methods, you choose the nearest window in the nearest habitable room in your neighbour’s property.

Some councils measure from the centre of the nearest habitable room window. Other authorities use the closest edge of the windowpane. The centre point approach is more common.

Check your council’s SPD to clarify the exact measurement point before finalising your plans.

The 45 degree rule and your development

Different types of extensions lead to different types of planning authority assessment.

WindsorPatania note: Councils apply the same 45 degree test to conservation areas and Article 4 extensions. You don’t need to apply for a different test in these circumstances.

Single storey extensions

Local planning authorities often apply stricter rules to double storey structures. Upper floors cause more difficulty because they sit above the line of sight and are more likely to block light to nearby windows.

Single storey ground floor extensions up to maximum height of three metres in eaves and up to four metres overall don’t receive as much scrutiny. Their smaller dimensions means there is less chance of overshadowing.

Double storey extension/second storey extensions

Two storey extensions on semi detached and terraced properties often struggle to meet the 45 degree rule.

This is because the extra height from a second storey creates more risk of blocking light from ground floor windows in neighbouring homes. On narrower plots, there’s less space to set it further away from your neighbour’s boundary.

In many cases, you’ll need to modify your plan to comply. Your planning authority may request professional assessments for this type of proposed building project.

Rear extensions and side extensions

Rear extensions commonly trigger 45 degree rule assessments by planning officers. That’s because they extend towards neighbouring gardens and rear windows.

The position of your rear wall can affect how neighbouring properties receive light. This is especially so if your existing house already sits near your neighbour’s windows.

You’ll also need to test side extensions, especially when they’re close to boundary lines.

Loft extensions

Loft extension projects need careful planning because they add height to your home.

For these types of extension, councils rely more on the 25 degree test to rear-facing windows for most rear dormers. They also use the BRE 209 garden sunlight check more than the side-window 45 degree rule.

That said, the 45 degree rule can still apply if the design adds bulk close to a neighbour’s side window. Examples of this could be a hip-to-gable change, raised ridge, or large mansard near the boundary.

On permitted rights development

Certain types of development don’t need approval from your local planning authority.

Permitted development rights under Schedule 2 Part 1 cover extensions within the curtilage of a dwelling house. Curtilage is the land immediately around your home that forms part of its residential use. This is usually your front, side and back garden.

Even under permitted development, you should factor in the 45 degree rule as guidance used in amenity assessments. If you don’t and neighbours complain about reduced light, the council may refuse a future application or request changes. In the worst cases, they may take enforcement action if you’ve breached planning limits.

WindsorPatania note: Extensions on terraced and semi detached houses often need modification. Detached houses are often easier because neighbour windows are further from the boundary.

What happens if your extension fails the test?

If your extension work isn’t deemed acceptable because your plan failed the 45 degree test, it’s not all over just yet.

Whether you’re building a new extension or a new house, there are other approaches property owners can take.

One way is to ask for a Vertical Sky Component analysis. This is a measurement of how much skylight reaches a specific window, based on its angle to the open sky and surrounding structures.

Alternatively, you could pay for an expert report. Assessors in this case will use No Sky Line (NSL) calculations. They measure how much of the room still has a view of the sky. If the post-build No Sky Line drops below 80% of what it was before, it’s taken as a sign of noticeable harm.

The report you get back might confirm the council’s belief that your proposal would block too much light. In this case, you’ll need to rethink and resubmit your plans.

Reducing the ceiling height, depth or width of your extension may bring it within acceptable limits. Stepping back the upper floor on a two storey extension helps clear the vertical angle.

Sometimes relocating the extension to a different part of your property avoids affected windows.

Neighbour complaints

Neighbours could object to your project if they believe it affects their access to natural light. Getting feedback from neighbouring properties is standard practice during the planning process.

In some cases, neighbours may pursue legal action if they can prove direct loss of light even after you gain planning approval.

The best approach is to talk about your plan with your neighbours before submitting your application. This can go a long way to preventing legal disputes.

Most extension projects proceed without legal challenges when you follow guidance.

WindsorPatania: managing the 45 degree rule planning process

he 45 degree rule is an important part of the planning process. Most councils follow BRE guidelines however many others follow their own rules.

Whatever rule applies in your borough, it’s best to prepare for such situations. Make sure your architect factors in the 45 degree rule in the planning and drawing stage. Consider hiring (or asking your architect) to hire an expert to carry out a test. If you show you’ve addressed this problem already, then your application is more likely to pass without delay or requests for redesign.

Approval times vary, but most straightforward applications receive decisions within eight weeks.

WindsorPatania is a national firm of leading architects with extensive experience. We have helped clients around the country on extension, (loft/basement) conversion and new build projects. From site layout planning for your proposed structure to achieve the planning decision you want, work with us.

We provide end-to-end architecture service for your build project, including: