45 Degree Rule Calculator UK (Free) — Extension Daylight…

45 degree rule calculator — UK extension & loss of light test

Free 45 degree rule calculator for UK planning objections. Test whether a neighbour's extension may breach daylight lines from the centre of your nearest habitable room window. If the proposed build crosses a 45° line in plan view, loss of light may be a material planning consideration (check your council's design guide or SPD).

Not legal advice. Planning Guard is a planning tool to help you explore material planning issues and draft letters — not a solicitor or planning consultant. See Terms.

Distance from centre of your window to the shared boundary (metres)

6.0 m

Depth of the proposed extension past the boundary (metres)

4.0 m

Plan view (not to scale for every site) — blue = neighbour window, shaded block = extension

Likely Acceptable: You may need to find other material grounds to object.

Need help finding other material grounds?

The 45° test is only one angle. Run our free material-grounds scan to see what else may count in your case.

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Read the full guide: loss of light & overshadowing in UK planning

Rule of thumb used here: if extension depth exceeds the window-to-boundary distance, the 45° envelope is breached in plan. Always check validated drawings and any local design / SPD standards.

How the 45 degree rule works for extensions & neighbour objections

The 45 degree rule is one of the most searched planning topics when a neighbour applies to build a rear or side extension. Councils use it as a quick loss of light screening test before deciding whether a fuller daylight assessment is needed. This free 45 degree rule calculator lets you test the plan view in seconds — then move on to a structured planning objection letter.

How to use this daylight calculator

Open the validated floor plans on your council's planning portal and find the nearest habitable room window that could be affected (living room, kitchen, bedroom — not bathrooms or WCs unless policy says otherwise).

Measure from the centre of that window to the shared boundary, at right angles. Enter the distance in metres in the first slider.

Measure how far the proposed extension projects past the boundary. Enter that depth in the second slider.

If depth exceeds distance, the 45° envelope is breached in plan — a common starting point for an overshadowing planning objection. Read our loss of light guide for policy citations and evidence.

45 degree rule vs 25 degree rule

Many authorities use the 45 degree rule for single-storey extensions and general plan-view screening. A stricter 25 degree rule sometimes applies to two-storey or flank-side development. Rules are set out in local design guides and SPDs — there is no single national statute. Check your council hub in our UK council directory for portal links and objection help.

Important things to know

Plan view only: two-storey schemes need a vertical elevation check from window head height as well.

Policy, not automatic refusal: officers weigh loss of light with orientation, screening, and room use.

Check every affected neighbour: run the test for each window that could be impacted, not just one boundary.

Council variation: some LPAs measure from the nearest point of the extension, not the farthest edge — confirm in the adopted SPD.

Breached the line? Object with evidence

Loss of light is a recognised material planning consideration when you can tie harm to policy and drawings. See planning objection examples for wording, then run our free scan before you pay for a letter draft.

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Frequently asked questions

What is the 45 degree rule in UK planning?

The 45 degree rule is a design guideline used by many local planning authorities to screen whether an extension or new building may cause unacceptable loss of daylight to a neighbour's habitable room. In plan view, a 45-degree line is drawn from the centre of the affected window. If the proposed structure crosses that line, officers may treat it as a likely overshadowing impact — subject to local policy and site context.

Is the 45 degree rule a law?

No. It is planning policy guidance in many council design guides and supplementary planning documents (SPDs), not a statute. Breaching the line does not automatically mean refusal — but it is a strong signal that loss of light may be a material planning consideration worth addressing in an objection or assessment.

Does the 45 degree rule apply to permitted development?

Generally, the 45 degree rule is used when full planning permission is required. Permitted development rights are assessed under different tests. If a neighbour's scheme needs planning permission, the rule may still be relevant to your objection.

What if my neighbour's extension breaches the 45 degree rule?

A breach can support a loss-of-light or residential amenity objection — but you should cite your council's adopted policy (SPD or design guide), reference validated drawings, and explain the harm in planning terms. Use our free scan to check whether your wider objection is on solid material grounds before you submit.

What is the difference between the 45 degree and 25 degree rule?

Both are geometric daylight screening tests. The 45 degree rule is common for single-storey and general plan-view checks. Some councils apply a 25 degree rule for two-storey or flank-side development. Always check your local authority's design guidance — rules vary by council.

Does this calculator work for two-storey extensions?

This tool checks the horizontal (plan) view only. Two-storey extensions often need a separate elevation check from the window head height using the same 45-degree principle vertically. Use validated drawings for both tests.

How do I measure the distance from my neighbour's window to the boundary?

Measure at right angles from the centre of the nearest habitable room window to the shared boundary. You can estimate from your garden or from scaled floor plans on the planning application. Enter that distance and the extension depth past the boundary in the sliders above.

How do I object to a planning application for loss of light?

Find the application on your council's planning portal, note the consultation deadline, and submit a representation focused on material planning considerations. Link loss of light to local policy where you can. Planning Guard offers a free material-grounds scan and an editable objection letter draft from £4.99 — not legal advice.