Justifying a separation distance in a dense context

There is no national separation distance between windows in the English planning system. The 21 metre rule that officers quote, and the 13.5 metres to a blank flank wall that goes with it, come from local design guidance rather than from statute or the National Planning Policy Framework. That matters when you are working on a tight urban site, because a separation distance overlooking justification is an argument you are allowed to win on the merits rather than a threshold you have failed.

What the decision maker actually has to be satisfied about is whether the proposal gives a high standard of amenity to existing and future occupiers. Privacy is part of that. A number is one way of testing it, and on a dense site it is usually the wrong way.

Where the numbers came from

The figures in most householder design guides descend from interwar housing manuals, when the typical arrangement was a pair of two storey semi-detached houses with rear gardens of similar depth. Twenty one metres between facing rear elevations, and roughly half that to a side wall with no windows, described the pattern those manuals were producing. They were layout conventions for new suburbs.

They survive because they are easy to apply. An officer with 40 applications on the list can measure a drawing in 20 seconds, and a number in an adopted supplementary planning document carries weight as a material consideration even though it is not policy.

So treat the local standard seriously. Find the document, quote the wording, and check whether it is expressed as a requirement or as guidance to be applied with judgement, because most are written as the latter and then applied as the former.

Why a dense context changes the test

In a Victorian terrace street the existing back-to-back distance is frequently 15 to 18 metres, and nobody suggests demolishing the street. Where the prevailing pattern is already tighter than the guide figure, applying the figure to new development would prevent any development at all, which is not what the guidance is for.

The National Design Guide and the Framework both push towards making efficient use of land in locations well served by public transport. The London Plan 2021 takes the same line through its design-led approach to optimising site capacity in Policy D3, with housing quality and privacy dealt with in Policy D6, and the accompanying guidance is explicit that rigid separation distances are not the right tool in dense urban settings.

The argument to make is therefore about the actual relationship rather than the abstract distance. Two windows 14 metres apart with a mature street tree, an oblique angle and no direct sightline between habitable rooms may deliver more privacy than two windows 22 metres apart facing each other squarely across flat lawns.

Oblique angles and partial overlooking

Overlooking is directional. A window set at 30 degrees to the neighbouring elevation gives a view of the neighbour's boundary and a glancing view of their window, not a view into the room. The standard way to show this is a sightline drawing: plan the cone of view from the centre of each new window at a realistic eye position, and mark where it meets the neighbouring elevation and garden.

Three refinements strengthen the drawing.

• Show the cone from the seated eye position as well as standing, because the two produce different results at close range.
• Distinguish views of a garden from views into a habitable room, and label which rooms the neighbour uses as bedrooms and which as bathrooms or landings.
• Show the first 3 metres of the neighbour's garden separately. The zone immediately behind the back door is the part people actually care about, and that is where a case officer's attention goes.

Offsetting windows in plan is the least invasive intervention available. Moving a window 900mm along an elevation, or splitting one large opening into two narrower ones either side of a party line, can remove the direct relationship without changing the internal layout at all.

Screening, obscure glazing and where they stop working

Obscure glazing with a restricted opening is the standard fix, and it is overused. It solves the neighbour's problem by degrading the new dwelling's own living conditions, which is the other half of the amenity test. A case officer who accepts obscure glazing to every habitable window on one elevation has approved a flat with no outlook.

Use it where the room has another window, where the affected opening serves a bathroom, stair or utility space, or where the view being blocked is narrow in any case. Put it in a condition that names the window on the approved drawing and requires the glazing to be retained, because a condition worded as "obscure glazing shall be installed" is unenforceable once the occupier changes it.

Screens work better than planting. A 1.8 metre privacy screen on a balcony edge is a permanent physical thing that can be drawn, conditioned and inspected. A line of hornbeam is deciduous, someone owns it, and it can be removed the week after the decision. Where you rely on planting, say what happens if it dies.

Winter gardens and angled reveals are the more expensive options, and they change the elevation, so bring them in at design stage rather than as a late fix.

Level differences and roof terraces

Height defeats distance. A roof terrace at second floor level overlooking a garden 20 metres away can be more intrusive than a ground floor window 10 metres away, because the raised position sees over fences and into the whole garden rather than the strip nearest the house.

The same applies to sites that fall. A new house on ground 1.5 metres higher than its neighbour has windows that look down, and measuring the plan distance tells the reader nothing about that. Sections are the only honest way to present it, which is why a case officer opens the section sheet first, a habit we describe in what a case officer checks in a daylight report first.

Terraces and balconies are also where conditions concentrate. Expect proposed restrictions on the use of a flat roof as amenity space, and deal with it in the design rather than hoping it is not noticed.

Presenting the judgement so it can be adopted

The officer has to write a paragraph justifying the decision. Give them the material to write it.

1. Quote the local standard and its status, naming the document and the date it was adopted.
2. Set out the prevailing pattern in the street, with a measured plan of existing back-to-back and window-to-flank distances on neighbouring plots.
3. Give a schedule of every new window within the relevant distance, its room use, its distance to the nearest neighbouring window, and the angle between them.
4. Show the sightline plans and the sections together on one sheet, at a stated scale.
5. List the mitigation, distinguishing what is physical and permanent from what depends on maintenance.
6. State the residual effect in plain words and explain why it is acceptable in this context.

That schedule is the document that changes minds, because it converts an impression into a table. Where the numbers are tight, an honest sentence acknowledging a modest degree of mutual overlooking, in a context where mutual overlooking is the established pattern, reads better than a claim of no impact that the drawings contradict.

Where the relationship also raises daylight questions, a numerical Daylight and Sunlight Assessment against the Building Research Establishment guidance answers a different point, and the two should not be confused. Privacy is qualitative and contextual. Daylight is measured. An Overshadowing Study covers the garden and amenity space question, and internal conditions for the new units sit alongside it, as we cover in daylight inside new flats compared with neighbours.

One honest uncertainty. Inspectors do sometimes uphold a rigid application of a local separation standard, particularly in suburban contexts where the pattern is consistent and the departure is large. The argument for flexibility is strongest where the existing grain is already tight and weakest where you are the first to break a uniform pattern.

Browse the PlanScope report index to see which amenity and daylight reports a constrained infill site is likely to need. 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 rather than legal advice, so check the relevant local design guidance with your local planning authority before relying on it.