Daylight and a single storey rear addition next door

For a single storey rear extension, the thing that decides a daylight objection is almost never the overall size of the extension. It is the depth at the boundary measured against how close the neighbour's nearest window sits to that boundary. A 4m deep addition next to a blank flank wall is invisible in daylight terms. The same addition next to a kitchen window 900mm from the party line will draw an objection, and in many authorities it will fail the council's own test on the officer's first pass.

Two geometric checks do most of the work. Councils apply a 45 degree test in plan for side-by-side situations, and BRE guidance uses a 25 degree check in section where the obstruction sits in front of a window.

How a low addition takes light from next door

A rear extension along a shared boundary does not block the sky above the neighbour's window. It blocks the slice of sky the window sees sideways and downwards, which is the part that lights the back of the room.

That is why the effect is worst on ground floor windows serving deep rooms, and why kitchens at the back of terraces are the usual flashpoint. The window sill is low, the room is long, and the light reaching the far wall arrives at a shallow angle across the garden. A wall built out along the boundary removes exactly that light path.

First floor windows are rarely affected by a single storey addition. Neither are windows set more than about 2m back from the boundary, unless the extension is unusually deep.

The 45 degree test in plan

Most householder design guides use a version of the same test. Draw a line at 45 degrees in plan from the mid-point of the nearest habitable room window in the neighbouring rear elevation, measured towards the boundary. If the proposed extension crosses that line, the authority treats loss of light as likely and looks for a reduction.

The detail varies between authorities and the variations matter.

• Some measure from the mid-point of the window, some from the nearest edge
• Some apply the test only to habitable rooms, excluding kitchens, utilities and bathrooms
• Some run a second line at 45 degrees in section from the window head
• Some apply a shallower angle where the extension is taller than 3m to the eaves

Read the council's householder design guide or supplementary planning document before you draw anything, because a scheme that passes in one borough fails across the street in another. Where no local test is published, the 45 degree plan line is the safest working assumption.

The 25 degree check in section

BRE guidance on site layout planning for daylight and sunlight gives an initial screening test for diffuse daylight. Where the obstruction subtends an angle of less than 25 degrees to the horizontal, measured from the centre of the lowest affected window, diffuse daylight is unlikely to be materially affected and no further calculation is needed.

That check is useful for rear additions facing a window across a garden, such as an outbuilding or an extension on the plot behind. It is less useful for the side-by-side case, because the geometry is lateral rather than frontal, which is why councils rely on the plan test instead.

Where the screening test is failed, the full assessment runs on three measures.

Those figures come from guidance rather than statute, and BRE states that they are advisory and should be applied flexibly. The baseline you compare against also matters a great deal, and the choice of baseline is where most disputes actually live. Our note on the mirror image baseline and existing obstructions covers how that choice is made.

Depth, height and boundary distance working together

Three dimensions control the outcome, and they trade against each other.

Depth is the projection beyond the neighbour's rear wall, not the length of your extension. An addition that runs 5m back but stops level with the neighbour's rear elevation takes no light at all. Height at the boundary is the eaves or parapet height where the wall meets the party line, not the ridge in the middle. Separation is the distance from the boundary wall to the neighbour's nearest window.

Reduce any one of the three and the objection usually weakens. The easiest to change is normally height at the boundary, because a mono-pitch roof falling towards the party wall can drop 500mm off the critical dimension without losing any floor area.

Illustrative example: a 3m deep flat roofed addition at 3m high, built hard against a boundary 1m from the neighbour's kitchen window, crosses the 45 degree plan line comfortably. Pull the last metre of the flank in by 700mm, drop the eaves at the boundary to 2.4m with a slope rising away from it, and the same floor area can sit outside the line. That is a drawing revision, not a redesign.

The neighbour's conservatory

Conservatories complicate the argument in a way that favours the applicant more often than neighbours expect. BRE guidance does not treat a conservatory as a habitable room for these purposes, and many council design guides exclude it from the 45 degree test.

Two practical points follow. A conservatory attached to the neighbour's rear wall already reduces light to the room behind it, so the existing baseline is lower than it looks. And where the neighbour's habitable room is now lit only through the conservatory, the authority will look at the effect on that inner room rather than on the glazed structure.

None of that makes a conservatory irrelevant. Officers do weigh the loss of amenity to a garden room that is used daily, and a scheme that leaves a neighbour's only outdoor-facing living space in permanent shade will attract a refusal on general amenity grounds, whatever the daylight numbers say.

Roof form and what it buys you

A flat roof at maximum height along the boundary is the worst case for the neighbour and the easiest form to build. A mono-pitch falling towards the boundary is the best compromise, keeping internal height where you want it and reducing the obstruction where it matters. A pitched roof with a ridge running down the middle usually sits between the two.

Rooflights and lanterns do nothing for the neighbour, and a glazed roof does not help either. Glass blocks diffuse daylight from the neighbour's point of view almost as effectively as tiles.

Changes that settle most objections

1. Reduce the projection beyond the neighbour's rear wall by 500mm to 1m
2. Step the flank wall in from the boundary for the last part of the projection
3. Lower the eaves or parapet at the boundary and slope the roof away from it
4. Move an obscured flank window to avoid inviting an overlooking objection in exchange
5. Offer a condition restricting roof terrace use where the flat roof is accessible

Offering one of these before determination costs far less than an appeal, and case officers tend to remember an applicant who amended without being told twice.

What to submit with a householder application

Existing and proposed plans and elevations are the minimum. What turns a contested application is the extra drawing: a plan showing the neighbour's rear windows, their use, their distance from the boundary, and the 45 degree line drawn from each, with the proposal shown against it. Add a section through the boundary showing the eaves height and the neighbour's window sill and head levels.

Where the numbers are close, or where the neighbour has already objected, a Daylight and Sunlight Assessment prepared by a suitably qualified author carries much more weight than an assertion in a covering letter. Councils usually expect that work to be signed by someone competent in the method, and professional review is strongly advised. Where the concern is sunlight to a garden or a patio rather than to a window, an Overshadowing Study is the right document instead. The assumptions made about how each room is used can change the result, as room use assumptions explains.

Rights of light is a separate civil matter with its own tests and remedies. A planning permission does not resolve it, and a daylight objection dismissed by the council does not prevent a private claim.

This is general guidance, not legal advice; check the position with your local planning authority or a qualified professional before relying on it.

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