Overlooking distances and where the 21 metre rule began
The 21 metre standard began as a daylight figure in early housing manuals and became a privacy convention. That history is why it bends more than people expect.
The 21 metre overlooking rule is a local plan convention, not a national standard, and it describes the separation councils normally look for between facing habitable room windows across a back-to-back relationship. Roughly 12 metres is the figure usually applied where a habitable room window faces a blank flank wall or a boundary. Neither figure appears in national policy, and an inspector will treat both as guidelines to be applied with judgement rather than as rules to be met.
Knowing where the numbers came from explains why they bend more easily than most people expect.
Where the separation distances came from
The 21 metre figure is usually traced to the housing manuals and layout guidance of the early twentieth century, which recommended about 70 feet between facing rows of houses. The reason given at the time was light and air, at a period when the priority was to end back-to-back terraces and to get sunlight into the rear of a house. Twenty-one metres is that 70 feet converted, and it entered local plans as a privacy standard long after the original reasoning had been forgotten.
That history matters in two ways. It explains why the figure is generous for privacy purposes, since it was set for daylight at a time when windows were small and no daylight modelling existed. And it explains why councils apply it as a starting point rather than as an absolute, because the origin does not support treating it as a threshold.
Modern daylight questions are answered with the Building Research Establishment methodology rather than with a separation figure, as set out in the guidance on responding to a neighbour's daylight objection. The separation distance survives as a privacy convention.
Back to back, back to flank and oblique
Three relationships behave differently and are usually given different figures.
| Relationship | Common local plan figure | What it protects |
|---|---|---|
| Habitable window to habitable window, directly facing | About 21 metres | Mutual privacy between rooms |
| Habitable window to blank flank wall or boundary | About 12 metres | Outlook and a sense of enclosure |
| Habitable window to habitable window, oblique | Reduced, often to about 14 to 16 metres | Privacy where the view is angled |
| Window to private garden | Varies widely, often 10 to 12 metres | Privacy in the most used part of the garden |
The oblique case is the one that is argued most and understood least. Where two windows face each other at an angle, the effective intrusion falls away quickly, and many authorities apply a reduced figure or a sliding scale. Where the angle exceeds roughly 45 degrees, several local plans treat the relationship as not directly overlooking at all. Check the wording of the policy, because the angle at which the relaxation starts differs.
Why the figures differ between councils
Separation distances sit in local plan policy, in supplementary planning documents or in a householder design guide, and each authority sets its own. Urban authorities, and London boroughs in particular, commonly apply lower figures or none at all, because a 21 metre standard applied literally would prevent the terraced and mansion block patterns that already define the area.
Densely built historic centres are usually handled by a policy that asks whether the proposal causes unacceptable loss of privacy, judged against the existing pattern, rather than by a number. Suburban districts tend to keep the numbers.
Read the adopted policy and any design guide before you draw. A scheme designed against the wrong authority's figure is a common and avoidable problem for anyone working across a boundary.
Changes of level and how they alter the distance
Where the proposed building sits higher than the neighbour, the same plan separation gives more overlooking, because the view angles down into the garden and through the window. Where it sits lower, the reverse.
Most policies deal with this by requiring the separation to be increased where there is a change of level, sometimes by a stated amount per metre of level difference and sometimes by a general requirement to demonstrate no harm. Where the policy is silent, the answer is a section drawing. A cross-section through the two properties showing eye levels, sill heights and the sight line does more than any amount of text, and it is the single most useful drawing to include for a contested relationship.
Balconies and roof terraces change the analysis again, because the viewer is outside and can move, and the conditions councils impose on them are covered in the guidance on roof terraces and the conditions councils impose.
Screening, obscure glazing and set backs
Where the distance cannot be met, four mitigations are routinely accepted, in roughly this order of preference.
Reposition or reorientate the window so the relationship does not arise. Set the upper floor back from the boundary so the sight line is broken by the roof. Design the window as a high level light with a sill above standing eye height. Or glaze it obscurely with a restricted opening, which is the least satisfactory answer and the one officers reach for last.
Planting is not usually accepted on its own, because it can be removed and because it is seasonal. A permanent structure, a wall, a fin, a privacy screen fixed as part of the building, carries much more weight. Where obscure glazing is proposed, expect a condition requiring a stated obscuration level and retention in perpetuity.
Where separation distances are relaxed
Four situations regularly justify departure from the figure.
The existing pattern is already tighter, and the proposal is consistent with it. The relationship is oblique. The affected window serves a kitchen, bathroom, landing or utility rather than a main habitable room, which several policies exclude from the standard. Or the intervening land is not private, for example where a road, a railway or a public path runs between.
There is a fifth, less comfortable one. Where the applicant owns both properties, some authorities apply the standard anyway, on the basis that ownership can change. That is a reasonable position and worth anticipating rather than arguing.
Making the case when the distance is not met
Do three things in the supporting statement.
- State the actual distances, measured window to window and window to boundary, on a dimensioned plan, rather than describing them.
- Provide a section through the relationship showing eye level sight lines, with any level difference to scale.
- Identify what each affected window serves, so a kitchen window is not treated as a bedroom.
Then argue the policy on its own terms. Most separation policies are expressed as a normal expectation subject to design, and the sentence that follows the number is usually the one that helps you. Where the policy is worded as an absolute, the argument runs on the development plan as a whole and on whether the harm identified is material.
A well drafted Planning Statement sets out the measured position, the policy wording and the mitigation offered, in that order. Officers reading a case that starts with the measurements take it more seriously than one that starts with an assertion that privacy is protected. Where daylight is also in issue, a Daylight and Sunlight Assessment answers a separate question and should not be confused with the separation argument.
PlanScope provides AI-assisted draft reports for planning applications. Reports should be reviewed before submission. We do not guarantee planning approval. This is general guidance, not legal advice; check the position with your local planning authority or a qualified professional before relying on it.
Frequently asked questions
Is the 21 metre rule national policy?
No. It appears in local plan policies, supplementary planning documents and householder design guides, and each authority sets its own figure. National policy contains no separation distance. Inspectors treat local figures as guidelines applied with judgement rather than as thresholds that automatically determine an application.
What distance applies to a window facing a blank wall?
Around 12 metres is the figure most commonly used for a habitable room window facing a blank flank wall or a boundary, though it varies by authority. It protects outlook and guards against a sense of enclosure rather than privacy, so the arguments run differently from a window to window case.
Does a change in ground level affect the separation required?
Yes. A building on higher ground overlooks further into a garden and further through a window at the same plan separation. Many policies require the distance to be increased where levels differ. Provide a cross-section through both properties showing eye levels, sill heights and the sight line, drawn to scale.
Will planting count as screening?
Rarely on its own. Planting can be removed, takes time to establish and is seasonal, so councils prefer a permanent structure such as a wall, a fin or a fixed privacy screen forming part of the building. Where obscure glazing is proposed instead, expect a condition fixing the obscuration level and requiring retention.
Do bathrooms and kitchens count as habitable rooms?
Usually not for separation distance purposes. Many policies apply the standard only to main habitable rooms such as living rooms and bedrooms, excluding kitchens, bathrooms, landings and utility rooms. Identify what each affected window serves on the submitted plans, because officers will otherwise assume the worst case.
Sources and further reading
This article is general guidance about the planning system in England and, where stated, the rest of the UK. It is not legal or professional advice. Check your local planning authority’s validation requirements and take professional advice for your specific site.