Overlooking and the 21 metre separation convention
Separation distances protect privacy, not sightlines on a plan. How officers apply the 21 metre convention, when a shorter distance works and what to draw.
Twenty-one metres between facing habitable room windows is a convention, not a rule. It appears in dozens of council design guides and supplementary planning documents, it is applied loosely by officers and strictly by objectors, and it has no basis in national planning policy at all. Understanding where it came from and what it is actually protecting is what lets an architect defend a scheme at 15 metres and lose one at 24.
The interest being protected is privacy: the ability to use a living room or bedroom without being overlooked from another dwelling. Everything else about the convention is a proxy for that.
Where the 21 metre convention came from
The figure descends from post-war residential layout guidance, where roughly 70 feet between facing two storey houses was treated as enough to stop direct overlooking across back gardens. Councils carried it into their own design guides, often adding a companion figure of about 10 to 12 metres from a habitable window to a boundary or to a blank flank wall.
The numbers still work for the case they were drawn for: two storey suburban houses, facing each other across rear gardens, on level ground. Almost every dispute happens when the convention is applied outside that case.
Check the local guide before designing to any figure. Separation distances vary between authorities, some measure to the boundary and some window to window, and a few express the standard as an angle rather than a distance. The adopted supplementary planning document is what the officer will quote in the report.
Window to window and window to boundary distances
Two different measurements get confused. Window to window is the distance between facing habitable room windows in separate dwellings. Window to boundary is the distance from a habitable window to the site boundary, used to protect the development potential of the adjoining plot as much as current privacy.
Habitable rooms are normally living rooms, dining rooms, bedrooms and kitchens where a kitchen is the main living space. Bathrooms, hallways, landings and utility rooms are not, which is why obscure glazed and restricted opening windows to those rooms solve so many layouts. A secondary window to a room that already has a primary window elsewhere carries less weight.
Ground floor relationships are usually protected by boundary treatment rather than distance, because a 1.8 metre fence removes direct overlooking between ground floor rooms. The convention does its real work at first floor and above.
Oblique angles and screening
Direct facing windows are the strict case. Where windows face at an angle, the overlooking is oblique and the effective distance is longer than the plan dimension suggests. Many design guides accept a reduced separation where the angle between the two window planes exceeds a stated figure, and officers will usually accept a well drawn diagram showing the cone of view.
Screening has to be permanent to count. A fence, a wall, an obscure glazed screen fixed to the building, or a window designed with fixed louvres will be accepted. Planting will not, because it can be removed, and neither will a promise to fit obscure film. Where privacy depends on a screen, expect a condition requiring it to be installed before occupation and retained.
| Relationship | What usually satisfies an officer |
|---|---|
| Facing habitable windows, first floor | The local standard distance, or a lesser distance with an oblique angle diagram |
| Habitable window to boundary | The local figure, commonly around 10 to 12 metres, or screening plus level change |
| Window to blank flank wall | A shorter distance, since there is no reciprocal view |
| Secondary window to a dual aspect room | Obscure glazing and restricted opening, secured by condition |
| Roof terrace or balcony | Fixed privacy screens to the flanks, set back from the edge |
Upper floor windows and roof terraces
A dormer or a rooflight in a loft conversion changes the relationship above the level a fence can protect, which is why so many householder refusals turn on a rear dormer window rather than on the extension beneath it. A rooflight set at a high cill level, above 1.7 metres from the finished floor, is the standard answer and is easy to condition.
Roof terraces are harder. A terrace creates a wide angle of view from a position people occupy for long periods, and the usual solution is a set back from the parapet combined with fixed screens to the sides. Where a terrace also raises sunlight and comfort questions, our note on roof terraces, sunlight and wind comfort covers how the assessments interact.
Dense urban sites where the convention fails
Apply 21 metres in a town centre and you cannot build a courtyard block. Most urban authorities accept this and their design guidance says so, allowing reduced separation where the design manages privacy through window placement, floor levels, winter gardens, angled bays or internal layouts that put circulation on the exposed side.
The argument that works is a design argument, not a plea for flexibility. Show the sections. Show which rooms face which, at what level, with what treatment. Point to the existing pattern of the street, because a proposal that matches the separation of the Victorian terraces around it is hard to refuse on privacy grounds.
Where the baseline is already tight, daylight becomes the harder test rather than privacy, and the evidence needed is set out in our note on daylight assessment in low baseline conditions. A daylight and sunlight assessment answers a different question from an overlooking analysis, and submitting one does not cover the other.
Design responses that satisfy an officer
The responses that succeed are physical and permanent. Offset the windows so the views cross rather than meet. Raise cill levels on the sensitive elevation. Use obscure glazing with restricted opening for secondary rooms. Angle a bay away from the shared boundary. Drop a floor level so that first floor windows sit lower than the facing ones. Put stairs, stores and bathrooms on the elevation facing the neighbour.
What does not succeed is a note on the drawing saying that planting will screen the view, or an assertion that the neighbours do not mind. Objections are made by the occupiers of the moment, and the officer is assessing the relationship for the life of the building.
Recording the assessment on drawings
Put the evidence on the drawings rather than in the text. A site plan with the separation distances dimensioned, window to window and window to boundary, marked against the local standard. A section through the critical relationship showing floor levels and sight lines. A plan showing the angle of view where an oblique relationship is relied on. A window schedule identifying which openings are obscure glazed and restricted, ready to be secured by condition.
Then explain the departures in the planning statement in one short section, naming the adopted standard, stating the distance achieved and setting out the design measures that make the relationship acceptable. Officers write their reports from what you give them. A dimensioned drawing and three sentences of justification gets copied across; an unexplained 16 metres gets refused.
PlanScope drafts the planning statement and design and access statement from your answers and flags where measurements are missing. 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. Separation standards are set locally, so check the adopted design guidance with your local planning authority before relying on any figure. Enter your postcode on PlanScope to see which supporting documents your scheme needs.
Frequently asked questions
Is the 21 metre rule national planning policy?
No. It is a convention carried into local design guides and supplementary planning documents, with the figure and the method of measurement varying between authorities. National policy deals with amenity in general terms, so the document to check is the adopted local design guidance rather than the National Planning Policy Framework.
Which rooms count as habitable for overlooking purposes?
Usually living rooms, dining rooms, bedrooms and kitchens used as the main living space. Bathrooms, cloakrooms, halls, landings, stores and utility rooms are not treated as habitable, which is why obscure glazed and restricted opening windows serving those rooms resolve many tight relationships.
Can planting be used as privacy screening?
Not on its own. Officers want screening that is permanent and within the applicant's control, such as a wall, a fence, a fixed obscure glazed screen or built-in louvres. Planting can be removed or can die, so it is treated as a supporting measure rather than the mitigation that makes a relationship acceptable.
How are oblique window relationships assessed?
By looking at the angle between the two window planes and the resulting cone of view rather than the straight line distance. Many design guides accept reduced separation where the relationship is sufficiently oblique. Draw the angle on the plan, because an officer will not measure it for you.
Does a roof terrace need more separation than a window?
In practice yes. A terrace gives a wide angle of view from a position people occupy for long periods, so authorities expect set backs from the edge and fixed privacy screens to the flanks. Balustrade design and the position of doors onto the terrace both affect how much overlooking is possible.
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.