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6 Steps to Run the 45 Degree Check for UK Homeowners: Practical Fixes

October 4, 2026
6 Steps to Run the 45 Degree Check for UK Homeowners: Practical Fixes

The 45 degree test is a screening tool that councils use to check whether an extension will block daylight to a neighbour's window: it is a guideline, not law, so a breach makes refusal more likely but never automatic. The best first step is to draw the 45 degree lines on a measured plan and elevation, or check your council's design guidance, before you commit to a layout. The main references behind the method are BR 209 and Gov.

TL;DR:

  • The 45 degree rule mainly applies to principal habitable room windows and excludes non-living spaces like bathrooms and hallways.
  • Variations exist between councils on check angles and whether both plan and elevation tests are required, so local guidance is essential.
  • Fixing a failed 45 degree check can often be done by reducing extension depth, lowering eaves, or repositioning windows, without major redesigns.
  • Complex urban plots and rooms with windows on multiple elevations limit the accuracy of the geometric test, necessitating full daylight assessments.
  • A formal daylight and sunlight assessment is recommended if the scheme breaches both plan and elevation checks significantly or faces previous objections.
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Table of Contents

What the 45 degree rule actually measures

The 45 degree rule grew out of daylight and sunlight guidance developed by the Building Research Establishment, now published as BR 209. Councils use it as a quick way to flag whether a proposed extension might cut off too much light or outlook from a neighbour's habitable room window, long before anyone needs a detailed technical study.

The method works in two stages. In plan view, a line is drawn at 45 degrees from the midpoint of the neighbour's window, and if your extension crosses that line, it is flagged for a closer look. A second check, done in elevation, looks at the same relationship from the side, sometimes using a 45 degree line and sometimes a 25 degree line measured from the cill, depending on the council.

BR 209 (2022) is explicit that this is advisory guidance, not a legal threshold, and its authors recommend treating the 45 degree test as an early filter rather than a final verdict. Local planning authorities do not apply it identically:

  • Some councils use only the plan test, others require both plan and elevation checks.
  • The angle itself can shift, with 25 degrees from the cill appearing in several local technical notes.
  • Mitigating factors such as screening, orientation and site levels are weighed differently from one authority to the next.

That variation is exactly why checking your own council's supplementary planning document matters more than relying on a generic rule of thumb.

When the 45 degree rule applies to your extension

Not every window triggers the test, and not every extension needs to worry about it. The 45 degree check generally applies to principal habitable room windows, meaning living rooms, kitchens and bedrooms where people spend significant time, including patio doors that serve as a room's main source of light.

Several situations take the test out of play or reduce its weight:

  • Windows to bathrooms, hallways, stores and other non-habitable spaces are usually excluded.
  • A neighbour's window that already faces directly opposite your proposed wall, rather than at an angle, often falls outside the standard assessment.
  • Rooms that already have a second window on a different elevation are judged less strictly, since daylight loss is less severe.

Orientation changes the practical impact too. A north-facing window relies more heavily on diffuse sky light, so a projecting extension can feel more significant there than on a south-facing elevation with stronger direct sun. Existing trees, boundary walls and changes in ground level between properties also affect the real-world outcome, even when the geometry on paper looks identical to a textbook example.

Given how much this varies by location, the practical move is to find your local authority's SPD or residential design guide and read the worked examples it gives, rather than assuming a single national rule applies everywhere.

How to run the 45 degree check step by step

You do not need specialist software to get a useful first read on your own design. A measured plan, a scale rule and a pencil are enough to flag an obvious problem before you pay for formal drawings.

  1. Identify the neighbour's nearest habitable room window and mark its midpoint on your site plan.
  2. From that midpoint, draw a line at 45 degrees towards your proposed extension.
  3. Check whether any part of your new wall crosses that line: if it does, the plan test has flagged a risk.
  4. Repeat the exercise in elevation, drawing a 45 degree line (or 25 degrees from the cill, where your council specifies that version) from the same window.
  5. Note whether your roofline, eaves or wall height crosses the elevation line as well as the plan line.
  6. Where patio doors are involved, use the standard 1.6 metre reference height for the opening when drawing the elevation line, since this affects where the angle starts.

Intrusion into both the plan and elevation lines together signals a higher risk than breaching just one. A small nick on the plan line with a clean elevation result is a very different conversation with a planning officer than a design that fails both tests.

Pro Tip: Stand at the neighbour's window, or as close to it as you reasonably can from a public footpath or shared access, and look at the proposed extension's outline marked out with canes on site. It often reveals problems that a flat drawing hides.

Extension outline marked with garden canes

Work from properly measured drawings, not estimated dimensions. A few hundred millimetres of error on a boundary distance can turn a pass into a fail once you are drawing lines at 45 degrees across a garden width.

Where the 45 degree test falls short

The 45 degree rule is a geometry check, and geometry alone does not capture everything that affects how much daylight a room actually receives. It ignores reflected light bouncing off other surfaces, treats every window as equally important regardless of how a room is actually used, and struggles with sites that have sloping gardens or multiple windows at different heights.

Its limitations show up most clearly on complex urban plots:

  • Terraced housing with narrow gaps between properties often fails the test on paper even where real daylight loss is modest.
  • Rooms with windows on two elevations are poorly represented by a single-window check.
  • Commercial and non-residential buildings sit outside the residential assumptions the test was built around.
  • Reflected light from pale walls, glazing and paved surfaces is not accounted for at all.

When a scheme clearly fails the screening test, or sits on a sensitive boundary with a long history of neighbour objections, councils increasingly expect a proper daylight and sunlight assessment carried out to BR 209 methodology, with calculations rather than lines on a drawing. Commissioning one makes sense once your design breaches both the plan and elevation tests by a meaningful margin, or when a previous application on the same site was refused on daylight grounds.

Practical redesign fixes if you cross the 45 degree line

Failing the screening test on a first sketch is common, and it usually points to a specific, fixable design choice rather than a dead end; consulting with Luxury Interior Design London & Bespoke Customisation can help with styling and finishing options after your extension design is set. Several adjustments tend to bring a scheme back within acceptable limits without gutting the brief.

  • Reduce the depth of the projection, particularly on a single-storey rear extension.
  • Set the extension back further from the shared boundary.
  • Lower the eaves height or switch to a hipped or lean-to roof instead of a flat or gable form.
  • Reposition windows and use rooflights on the elevation facing away from the sensitive boundary.

Conservatories and fully glazed garden rooms are treated more leniently by some councils, since more light passes through glass than solid masonry, though this depends entirely on local policy and is worth confirming before you design around the assumption, as North Somerset's residential design guide notes.

Existing screening, such as a mature hedge or an outbuilding that already limits a neighbour's view, can sometimes be formalised through a planning condition requiring it to be retained, which strengthens your case without changing your design at all.

Pro Tip: Sketch two or three roof options at the same footprint before choosing one. A hipped roof or a lower ridge line often solves a 45 degree problem more cheaply than reducing the floor area.

A side-return extension in Dudley used exactly this logic, trading a flat roof for a stepped profile with a set-back upper section to keep both the plan and elevation tests clear. You can see the finished layout in the Dudley side-return project.

How councils weigh the test against planning law and timescales

The 45 degree test never decides a planning application on its own. Under section 70 of the Town and Country Planning Act 1990, local planning authorities must have regard to the development plan and all other material considerations when deciding an application, and the 45 degree result is one material consideration among several, alongside things like street scene, precedent and overshadowing.

Many smaller extensions never need a full application at all, since they fall under permitted development rights. GOV.UK's permitted development technical guidance sets limits on rear and side projection depth, eaves height near boundaries and overall coverage, with different rules in designated areas such as conservation areas.

  • Permitted development has fixed limits on depth, height and coverage that must be checked before assuming no application is needed.
  • Where an application is required, GOV.UK guidance sets the standard decision period for ordinary householder applications at 8 weeks.
  • Larger or more contentious schemes can take longer where the council and applicant agree an extended timeframe, or where additional information is requested.

Extension Kings case studies: 45 degree issues on real projects

Several West Midlands projects have needed direct 45 degree adjustments. A Walsall rear extension with a utility room was redesigned after measured drawings showed the original depth crossed both plan and elevation lines to a neighbouring kitchen window, resolved by trimming the projection and lowering the roof pitch. A Sutton Coldfield wrap-around extension used a set-back return and rooflights to keep a side elevation clear of a neighbour's bedroom window. The Dudley side-return project combined a stepped roof with careful window placement. In each case, measured surveys, council pre-application checks and design revisions ran as one coordinated process rather than separate stages.

A screening tool, not a verdict

The 45 degree rule gets treated like a pass or fail exam far too often, when it is really a prompt to look harder at a design before you commit to it. Most schemes that fail the first sketch pass comfortably once the roof form or setback changes.

Get a measured drawing done early, talk to the neighbour whose window is affected if the relationship is good, and bring in professional help once the geometry gets genuinely tight. For readers who would rather have someone else run these checks, the next section covers a managed alternative.

— Raja

How Extension Kings handles planning and design together

A managed design-to-build process can cover single-storey, double-storey, kitchen, rear, side-return, wrap-around and over-garage extensions, from the first measured survey through to planning liaison and handover.

Extensionkings

Working with a coordinated team rather than separate designers, planners and builders helps address the 45 degree check, neighbour considerations, and budget together early in the process.

  • Measured drawings and 45 degree checks can be integrated into the early design stage, rather than added later.
  • Planning applications and any necessary technical reports are coordinated with the local council.
  • Budgets may be set before site surveys, with staged payments tied to project milestones.

If you are planning a rear, side-return or wrap-around extension and want the planning side handled alongside the design, get in touch with Extension Kings to start the conversation.

FAQ

What is the 45 degree rule for house extensions in the UK?

It is a daylight screening test, based on BRE guidance, where a 45 degree line is drawn from a neighbour's window to check whether a proposed extension might block too much light. Crossing the line flags a risk for planning officers to examine, rather than confirming an automatic refusal.

How can you get around the 45 degree rule?

There is no way to bypass the test, but a failed check is usually fixed through design changes such as reducing the extension's depth, setting it back from the boundary, lowering the eaves or changing the roof form. Repositioning windows and using rooflights on the affected elevation are common adjustments too.

What are the rules for extensions in the UK?

Extensions are governed by permitted development limits on depth, height and coverage, set out in GOV.UK's technical guidance, and by full planning applications where those limits are exceeded. Under section 70 of the Town and Country Planning Act 1990, councils weigh the development plan and material considerations such as the 45 degree test when deciding applications.

How long does a householder planning application take to decide?

GOV.UK guidance sets the standard decision period for ordinary householder applications, including most extensions, at 8 weeks. Larger or contested schemes can take longer where more information is needed or where the council and applicant agree an extended timeframe.

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