Extension height limits and the 45 degree rule
A single-storey rear extension built under permitted development can be up to 4 metres high, dropping to 3 metres at the eaves once any part comes within 2 metres of a boundary. The 45 degree rule is a separate thing entirely — a daylight test your council uses to judge whether an extension robs a neighbour of light. Get both right and most extensions sail through.
Two rules that get muddled
Height limits and the 45 degree rule solve different problems, and people run them together all the time. Sorting out which one you are dealing with saves a lot of wasted drawing time.
The height limits are hard numbers written into permitted development law. Stay inside them and, for a great many houses, you can build without applying for planning permission at all. Break one and you need a full householder application — the limit itself is not negotiable.
The 45 degree rule is different. It is a rule of thumb, not a law, and it only comes into play when you are making a planning application in the first place. Officers use it to check whether your extension would block a meaningful amount of daylight to next door. You can be well inside the height limits and still trip the 45 degree line; you can also clear the 45 degree line comfortably and still need permission for some other reason, like depth.
Permitted development height limits
These are the figures that decide whether your extension needs planning permission on height grounds. They apply to houses in England — not flats, not maisonettes, and not the tighter regimes on designated land.
| What is measured | Limit |
|---|---|
| Single-storey rear extension, overall height | 4m maximum |
| Eaves height where any part is within 2m of a boundary | 3m maximum |
| Eaves height (general) | no higher than the existing eaves |
| Overall height of any extension | no higher than the highest part of the existing roof |
| Two-storey rear extension | within 3m of the original rear wall, and at least 7m from the rear boundary |
| Side extension | single storey, 4m high maximum, half the width of the original house |
Source: Planning Portal, permitted development for extensions; GOV.UK technical guidance on permitted development rights for householders (England). Heights are measured from the highest adjacent ground level.
Two points catch people out. Height is measured from the highest point of the ground next to the extension, so a sloping garden can quietly eat into your allowance. And the 3m eaves limit bites on far more builds than owners expect, because a typical rear extension on a terrace or semi sits close to at least one boundary. If yours does, the flat-roof eaves line has to stay at or below 3 metres for the whole stretch within 2 metres of that boundary.
A two-storey extension has no separate height figure in the rules — it simply cannot rise above the existing ridge, and its roof pitch should match the house as far as practicable. For the depth side of the two-storey rules, see two-storey extensions, and for the full set of permitted development limits by property type, do I need planning permission for an extension covers depth and coverage too.
What the 45 degree rule actually measures
The 45 degree rule is a quick way for a planning officer to judge whether your extension would take too much daylight from a neighbour. It comes from the Building Research Establishment’s guide BR 209: Site layout planning for daylight and sunlight (2022 edition), the document councils lean on for daylight questions.
It is drawn twice. In plan — looking straight down at the site — a line is taken at 45 degrees from the middle of the neighbour’s nearest window, running out towards your boundary. In section, looking side-on, a second 45 degree line rises from the window. For a single-storey extension that line starts at the window sill of the affected ground-floor room. For a two-storey extension, the assessment moves to the relevant upper window and is usually taken from a point partway up it.
If your proposed extension crosses both the plan line and the section line, the council is likely to treat it as causing real harm to daylight, and that becomes a reason to refuse. Cross neither and daylight is rarely an issue. Cross one but not the other and it comes down to officer judgement and the specifics of the room.
The rule only protects habitable rooms: living rooms, kitchens, bedrooms, studies and the like. A bathroom, hallway, landing or utility window does not count, so an extension that looms over next door’s frosted bathroom window is not caught by this test at all.
If you cross the line
Crossing the 45 degree line is not an automatic no. It is a flag. What usually happens next is the council asks for a proper daylight and sunlight assessment under BR 209, which measures the light a window actually receives rather than relying on a line on a drawing.
The headline test there is the Vertical Sky Component. A neighbour’s window is normally considered still adequately lit if it keeps at least 27% VSC, or at least 0.8 times whatever it had before your extension went up. Meet that and the daylight objection tends to fall away even where the 45 degree line was clipped. There is also a lesser-known 25 degree rule, taken from 2 metres above ground for windows that directly face the extension, but councils reference it far less often.
The practical takeaway: if a neighbour’s habitable-room window is close to your boundary and you are planning something tall or deep, sketch the 45 degree lines before you pay for full drawings. It is the cheapest way to find out whether you have a fight on your hands.
Checking your own plans first
You do not need a surveyor to get a feel for whether height will be a problem. A tape measure, a scale drawing and half an hour will tell you most of it.
- Measure from the highest ground point beside the extension up to the proposed flat-roof or eaves level — that is the figure the 3m and 4m limits apply to.
- Mark where your extension sits within 2 metres of each boundary, and check the eaves stay at or below 3 metres along that whole run.
- On a plan of both properties, find the nearest habitable-room window next door and draw a 45 degree line out from its centre. See whether your outline crosses it.
- Repeat the 45 degree line side-on, starting at that window’s sill, to test the height as well as the footprint.
If you clear the height limits and stay behind the 45 degree lines, you are in strong shape. If not, a designer can often adjust the roof form — a lower pitch, a hip instead of a gable, a set-back at the boundary — to bring you back inside without losing much floor space.
Frequently asked
Get quotes from extension builders near you
Tell us what you are planning and we will match you with up to three builders who cover your area. Free, no obligation, and you decide whether to go ahead.
Get free quotes