Privacy distances: where the 21 metre rule comes from

The 21 metre privacy distance is a convention, not a rule. No national planning policy contains it, no statute sets it, and no appeal inspector is bound by it. It survives because most councils repeat it in a householder design guide, and because it produces a workable answer on the suburban plots it was designed around.

Knowing where it came from tells you when it applies and when a council is applying it to a situation it was never meant for.

The origin of the 21 metre convention

The figure is usually traced to interwar housing standards, and to the Tudor Walters report of 1918 and the housing manuals that followed it. Those documents recommended roughly 70 feet between the rear faces of facing terraces, which converts to about 21.3 metres.

The reasoning was as much about daylight as about privacy. A two-storey house is around 10.5 metres to the ridge, and at 10.5 metres away the facade subtends an angle of 45 degrees from the ground floor window opposite. Double it and you get facing houses that each keep a 45 degree slice of open sky. Twenty-one metres.

That genealogy matters. The number was derived for two-storey houses, on level ground, facing each other squarely, with rear gardens between. Every departure from those conditions weakens it, in either direction.

How councils apply separation distances today

Most design guides now set out a small family of distances rather than one:

• around 21 metres between facing habitable room windows at first floor level
• around 12 metres, sometimes 10.5 or 13.5, between a habitable room window and a two-storey blank flank wall
• shorter distances for single-storey relationships, often half the two-storey figure
• an increase where the site slopes, because a house on higher ground looks down into gardens and rooms rather than across

Habitable room means a living room, dining room, bedroom or kitchen used as a dining space. Bathrooms, landings, hallways, utility rooms and en-suites are not habitable and do not attract the same protection.

Distances are measured horizontally between the window faces, not between building lines or boundaries. That distinction decides marginal cases where a bay or a projecting extension sits forward of the main wall.

None of this is policy in the National Planning Policy Framework. It sits in supplementary guidance, which is a material consideration and is applied consistently by most case officers, so treat it as the working standard while remembering it can be argued with.

Oblique angles and screening

The full distance is designed for windows facing each other squarely. Where the relationship is oblique, most guides accept less, and some define the reduction with a cone drawn at 45 or 60 degrees from the centre of the window.

Draw the sightline. A first floor window looking at 60 degrees across a boundary sees a strip of the neighbour's garden and the corner of their elevation, and a plan drawing with the cone marked resolves the argument faster than a paragraph of text.

Boundary screening only answers part of the problem. A 1.8 metre close-boarded fence deals with ground floor windows and eye-level views across a garden. It does nothing about a first floor window, and planting is seasonal, removable and slow, so officers give it limited weight as mitigation.

Level differences work in both directions. A proposal on ground 1.5 metres higher than its neighbour needs more separation. A proposal set into a slope, with a first floor sitting close to the neighbour's ground level, sometimes needs less than the guide suggests.

Obscure glazing and fixed lights

The standard mitigation is a condition requiring obscure glazing and a restriction on opening. Typical wording asks for glass obscured to a stated privacy level, non-opening below 1.7 metres above internal floor level, and retained in that condition permanently.

The 1.7 metre figure is the point at which a standing adult cannot see out horizontally. Below it the glass is obscured. Above it a top-hung opening light gives ventilation without a view down.

Two limits are worth knowing. Councils generally will not accept obscure glazing on the only window of a habitable room, because it solves the neighbour's problem by creating one for the future occupier. And a condition of this kind removes the permitted development right to change the window later, which is the point of it. Applications to vary such a condition are refused routinely.

Where the affected window belongs to the neighbour rather than the applicant, obscure glazing is not available at all. You cannot condition works to someone else's building.

Balconies, terraces and roof gardens

Balconies are treated more severely than windows, and reasonably so. A window is used in passing. A terrace is occupied, at leisure, for hours, with an unobstructed downward view into gardens.

Expect officers to look for a privacy screen of 1.7 to 1.8 metres on the flank returns, a setback from the boundary, a reduced depth so the useable area sits away from the edge, or a Juliet balcony instead of a projecting one.

Converting a flat roof to a terrace is not permitted development, so it needs an application in its own right, and consent for a rear extension often carries a condition preventing the roof being used as a balcony. Read the conditions on a neighbour's permission before assuming a terrace is available.

Illustrative example: a two-storey rear extension on a 1930s semi with 17 metres to the rear elevation opposite. The council's guide asks for 21 metres. The applicant reorientates the first floor bedroom window to the side, sets a bathroom window with an obscure glazed and fixed-below-1.7-metre condition on the rear, and offers a 1.8 metre screen on the flank of the small terrace. The distance is unchanged. The overlooking is not.

Urban sites where the distance cannot be met

In town centres, on backland plots and in most of inner London, 21 metres is unachievable and irrelevant. The design-led approach that has replaced it asks a different question: is there actual overlooking of a habitable room or a private amenity space, and can it be designed out.

The moves that work:

1. offset windows so they do not align across the gap
2. angle bays, oriels or window reveals away from the sensitive direction
3. use fixed obscure panels in the lower part of a window and clear glass above
4. put circulation, bathrooms and kitchens on the constrained elevation and habitable rooms elsewhere
5. introduce a winter garden or a deep reveal that limits the viewing angle
6. put screen fins or brise soleil on the elevation where they suit the architecture

Support the argument with drawings, not adjectives. Sightline sections and plan cones showing what can be seen from where are what an officer can rely on in a report.

Making the privacy case

Deal with the numeric guide directly in the design and access statement. State the council's own figure, state the distance achieved, and where it falls short, say by how much and explain what the design does instead.

Include the drawings that carry the point: a plan with the sightline cones, a section through the two properties showing eye levels and the boundary treatment, and a window schedule identifying each rear opening by room type and the mitigation applied. Where daylight is also raised, keep it separate and answer it with a daylight and sunlight assessment rather than mixing the two arguments. Garden effects belong in an overshadowing study.

And offer the condition yourself. An applicant who proposes the obscure glazing condition in the covering statement gives the officer a route to approval that does not require a negotiation at week six.

This is general guidance, not legal advice. Check the position with your local planning authority or a qualified professional before relying on it. PlanScope provides AI-assisted draft reports for planning applications. Reports should be reviewed before submission. We do not guarantee planning approval. Enter your postcode on PlanScope to see which documents your extension is likely to need.