Daylight for an infill extension on a terraced house

A terrace infill extension takes light from one window, and usually only one. That concentration is what makes these schemes contentious out of all proportion to their size. Fill the side return on a Victorian terrace and the wall you build stands within a metre or two of the neighbour's rear window, at right angles to it, for the full depth of the extension. Nothing else a householder builds does that.

Which is also why the outcome is predictable enough to test before you pay anyone. Two checks with a tape measure and a protractor will tell you whether you have a problem, and the answer changes the design rather than the argument.

Why terraces concentrate the problem

On a detached house, an extension casts its effect across a garden and the nearest neighbouring window is usually well away from the flank wall. On a terrace the geometry is fixed by the plot. Rear returns are narrow, boundary walls sit on or near the party line, and the original rear elevation typically has a kitchen or dining window looking straight down the return.

Build into that return and the new flank wall sits beside the neighbour's window at close range. The angle the wall subtends from that window rises steeply with depth, and it rises faster than most people expect, because the distance from window to wall is measured in metres while the depth runs to three or four.

Terraces also tend to come in matched pairs of returns, so your neighbour may already have built into theirs. That changes the baseline and it changes the answer.

The window that decides it

Identify it before anything else. Walk to the boundary, look at the neighbour's rear elevation and find the window closest to your proposed flank wall at ground floor level.

Then establish what room it serves. A habitable room, meaning a living room, dining room, kitchen used for dining or a bedroom, is protected in a way that a utility room, a bathroom or a hallway is not. A kitchen used only for cooking sits somewhere in between in most councils' design guides. Where the neighbour's rear extension already has a roof light serving the same room, the room has a second source of daylight, and that matters to the assessment even though it does not eliminate the effect on the window.

Check whether the window is the only one serving the room. A rear room with a side window or a patio door onto the garden is far less vulnerable than a single window in a deep plan room.

The two quick checks you can do yourself

The first is the 25 degree test taken from BRE guidance. Stand at the centre of the neighbour's affected window, at the level of the window's lowest point, and measure the angle up to the top of your proposed extension. If the obstruction subtends less than 25 degrees to the horizontal, daylight to that window is unlikely to be materially affected, and for a single storey extension that is often the end of the enquiry.

The second is the 45 degree test that most councils use in their householder design guidance. Draw a line in plan at 45 degrees from the centre of the neighbour's window, and a second line in section at 45 degrees from the mid-point of that window. If the extension breaks both lines, expect an objection to be taken seriously. Breaking one is common and often accepted.

Neither test is a rule of law. Councils apply the 45 degree approach with local variations, and the numbers in a published design guide are what an officer will quote back at you, so read the guide for your own authority before you settle the depth. Where the massing is marginal, obstruction angles and the early massing test sets out how the same geometry is handled on larger schemes.

If both checks fail and the depth cannot move, that is the point to commission a Daylight and Sunlight Assessment rather than argue by assertion. A numerical assessment reports Vertical Sky Component at the affected window before and after, against the BRE reference value of 27 per cent and the 0.8 ratio of former value, and it either supports the scheme or tells you to redesign while redesigning is still free.

Roof form and glazing that buy you depth

Depth is the variable everyone fixes first and it is usually the wrong one. Roof form matters more.

A flat roof at 2.5m to the underside with a parapet above reads as a much lower obstruction from the neighbour's window than a pitched roof at 4m, even though the footprint is identical. A mono-pitch falling away from the boundary does the same job while giving you head height on your own side. Where the neighbour's window is close to the boundary, setting the last metre or so of the extension down in height, or cutting a chamfer across the corner nearest that window, can bring a 45 degree section line back into compliance.

Glazing helps your own room rather than theirs. Roof lights and a glazed roof section over the return recover the daylight your own kitchen loses when the side return is enclosed, which is the objection that comes from inside the house rather than next door. A glazed link between the original rear wall and the new extension also keeps the original window reading as a window, which conservation officers notice in a conservation area.

None of this helps if the extension is too deep for the plot. Three metres from the original rear wall on a terrace is the permitted development limit for a reason.

Boundary height, eaves and the party wall line

Permitted development under Class A of Part 1 of Schedule 2 to the General Permitted Development Order 2015 caps the eaves height at 3m where any part of the extension lies within two metres of a boundary, and caps a single storey rear extension on a terraced house at 3m projection and 4m height. Anything to the side is limited to half the width of the original house and to single storey with a 4m maximum height. On Article 2(3) land, which includes conservation areas, side extensions fall outside Class A altogether.

A larger single storey rear extension of up to 6m on a terrace can be pursued through the prior approval route, under which the council notifies adjoining owners and may refuse on amenity grounds if an objection is received. Loss of light is squarely within that. So the 6m route is not a way round the daylight question.

Two things to keep separate. The Party Wall etc. Act 1996 governs the physical works to and near the party wall and is a civil process with a surveyor, nothing to do with planning. Rights of light are a separate private law matter capable of arising after twenty years of enjoyment under the Prescription Act 1832, and a planning permission is no defence to a rights of light claim. Say that to a client once, in writing.

What to put in the application

For a householder application on a terrace, the drawings do most of the work if they are honest.

1. A block plan showing the neighbour's rear elevation with every ground floor window marked and the room use noted.
2. A section through the extension and the neighbour's affected window with the 45 degree line drawn on.
3. A plan with the 45 degree line drawn from the window centre.
4. Existing and proposed rear elevations at the same scale, including the neighbour's return so the relationship is visible.
5. Photographs from the neighbour's side of the boundary where access allows, or from your own upstairs window where it does not.
6. A short amenity paragraph in the covering statement that names the affected window, states the room use, reports the angle measurements and explains the design response.

Where the figures are close to the line, add the numerical assessment. An Overshadowing Study is the right document where the complaint is about the garden rather than the window, which on a small terrace garden facing north is a real possibility. The wider pattern of rear extensions and overshadowing in dense terraces covers the garden side of this in more detail.

Talking to the neighbour first

Do it before submission, with drawings, and with the section showing the 45 degree line. A neighbour who first learns about the extension from a council consultation letter objects on principle.

Bring one concession to the conversation and hold it back until it is useful: a reduced height at the boundary end, a change from pitched to flat roof, an agreed construction access route, or an undertaking on working hours. Small offers close cases. And write down what was agreed, because the objection that arrives in week three otherwise becomes your word against theirs in front of a case officer who has never met either of you.

Enter your postcode on PlanScope to see which supporting documents a terrace extension is likely to need, then get the amenity paragraph checked before you submit.

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.