Privacy and overlooking: where the distance rules came from

No national document sets an overlooking distance planning rule. There is no statutory separation distance, nothing in the National Planning Policy Framework published on 17 August 2026 that fixes a figure, and no British Standard. The 21 m back-to-back figure that case officers quote comes from local design guidance, and the number a particular council applies is whatever its own adopted document says.

Which is why arguing about whether 21 m is "the rule" wastes everyone's time. The question is what your authority's guide says, how rigidly it applies it, and whether the actual relationship between the two buildings justifies something different.

Why separation distances exist at all

Privacy between dwellings is a material consideration under general amenity policy, and the Framework expects development to provide a high standard of amenity for existing and future users. That is the whole of the national requirement. A distance standard is simply a proxy: a quick test that avoids assessing every window relationship from scratch.

Proxies are useful to a development management team deciding 2,000 applications a year, and they are the reason most authorities publish a householder design guide with figures in it. They are also why a scheme that fails the figure by half a metre sometimes gets refused when a scheme that passes it while looking directly into a bedroom from eight metres away at an angle gets approved.

The figure does work, broadly, for the situation it was designed for: two-storey houses facing each other across rear gardens on a conventional suburban layout. It works badly almost everywhere else.

Where the commonly quoted figures come from

The widely used numbers are 21 m between directly facing habitable room windows at first floor level and above, roughly half that where a habitable room window faces a blank elevation, and a shorter distance again from a rear elevation to the boundary itself. Variants appear in hundreds of local design guides and supplementary planning documents.

The provenance is imperial. Twenty-one metres is 70 feet, give or take a few centimetres, and the figure appears in mid-twentieth century housing design guidance that predates metrication. It was carried forward into local design guides through the 1970s and 1980s and has been copied between authorities ever since. No current national document re-states it, and no authority is obliged to use it.

That history matters for one practical reason. A figure with that pedigree is a convention rather than a standard, and conventions can be departed from with reasons. A standard adopted as policy in a local plan is harder to argue with, so check whether your council's figure sits in a development management policy or in a design guide, because the two carry different weight. The note on how local plan amenity policies use BRE 209 covers the same distinction for daylight.

How authorities apply them in practice

Four refinements show up in nearly every guide.

Only habitable rooms count. Bathrooms, landings, halls and utility rooms are not habitable, so a bathroom window at six metres from a neighbour's kitchen is normally acceptable where a bedroom window would not be.

Ground floor relationships are usually excluded, on the basis that a 1.8 m boundary fence or wall deals with them. The distances bite at first floor and above.

The distance is measured window to window, or window to elevation, on the shortest line between them rather than along the axis of either building. Measure it on a drawing rather than estimating from a site visit.

Oblique relationships attract a reduced figure. Most guides allow a reduction where the angle between the two windows exceeds a stated value, and some use a sliding scale. If your guide does not specify one, the argument still runs: at a sufficiently oblique angle the view from one window into another is of the side wall rather than the room.

Levels, terraces and roof additions

Changes in level defeat the convention completely. A first floor window on ground that sits 1.5 m above the neighbour's garden behaves like a second floor window, and a terrace at that level looks directly into the garden rather than across it. Where the site slopes, draw a section through the relationship. It is the only way to see what the plan hides.

Raised terraces and balconies are the single most common source of overlooking refusals on householder schemes, and permitted development law reflects that. A verandah, balcony or raised platform is excluded from the rights in Part 1 of the General Permitted Development Order 2015, with a raised platform defined as one more than 0.3 m above ground level. So a flat roof over a rear extension cannot lawfully be used as a terrace under permitted development, however tempting the parapet looks.

The same Order supplies the standard mitigation for side-facing windows. Upper-floor windows in a side elevation are permitted only where they are obscure-glazed and non-opening below 1.7 m above the floor of the room. That 1.7 m figure is worth remembering, because it is the dimension officers reach for when conditioning a window on a full application too.

Higher density and the design-led alternative

Applying a suburban separation distance to a dense urban site produces absurd results, and policy has moved on. The London Plan takes a design-led approach to site capacity and discourages the blanket application of fixed distances, asking instead for an assessment of the actual relationships created.

What replaces the figure is a room-by-room analysis. Which windows look into which. At what angle. Over what distance. With what mitigation. Whether the room overlooked is a primary habitable space or a secondary one. Whether the existing context already involves mutual overlooking at similar distances, which in a Victorian terrace it almost always does.

That analysis belongs in the Design and Access Statement, with a plan showing sight lines and a section showing levels. It is more work than quoting a number, and it is the only approach that works on a constrained site.

Arguing for a reduced distance

An illustrative example. A two-storey rear extension to a 1930s semi brings a new first floor bedroom window to 17 m from the facing window of the house behind, where the local guide asks for 21 m. Three arguments are available, and they are stronger together than separately.

The geometry. If the two windows are offset so the line between them crosses at 40 degrees rather than square on, the view is substantially of the neighbour's flank wall and roof. Draw it.

The window. Reposition the opening so the overlooking view is from a secondary window to a landing rather than from the bedroom, or raise the cill, or propose a high-level window with an opening section above 1.7 m. Offer the obscure glazing condition yourself rather than waiting to be asked.

The context. Establish the existing pattern. If every other house in the terrace has a first floor rear window at 17 to 18 m from the facing houses, the proposal creates no new relationship of a kind the street does not already have. Measure three or four neighbouring pairs and put the figures in a table.

What does not work is asserting that 21 m is only guidance and therefore need not be met. Officers hear that weekly. And it concedes the point without offering anything in its place.

Where the concern is light rather than privacy, the tests are different and the evidence is different, which is why the Daylight and Sunlight Assessment is a separate exercise from the privacy argument. The two get conflated in objections constantly, and the distinction between privacy standards and back-to-back daylight tests is set out in the note on back-to-back distances and privacy standards.

Note: a reduced distance accepted on one application does not create a precedent the authority is bound by, though it is a material consideration you can point to.

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