Rear extensions and overshadowing in dense terraces

Rear extension overshadowing arguments in a terrace turn on three numbers: how deep the extension goes, how high its flank wall stands, and how far the neighbour's rear window sits from the shared boundary. Terraced plots are narrow, the houses sit close, and rear windows often serve the only habitable room at the back. So a 3 m extension that would be invisible on a detached house in a large plot can remove a measurable share of a neighbour's daylight, and councils assess it accordingly.

Orientation decides how much of this matters. An extension on the northern side of a neighbour casts very little shadow onto them. The same extension to their south shades the garden through the middle of the day. Establish which you are dealing with before drawing anything.

How terrace geometry concentrates the impact

Three features of a Victorian or Edwardian terrace make overshadowing worse than the dimensions suggest.

The plots are narrow, often 4 m to 6 m wide, so an extension built to the boundary sits directly against the neighbour's outdoor space with no buffer. The houses have original rear outriggers, so the new work is frequently a second projection stacked beside an existing one, doubling the effective depth of built form along the boundary. And the rear windows of the main room are typically close to the party wall rather than centred, which puts them nearest to whatever is built next door.

Add the fact that terraced gardens run to a narrow strip, and the shadow of a 3 m high wall covers a large proportion of the neighbour's usable ground in the morning or afternoon depending on which side you are on.

None of this stops an extension. It does mean that the standard depth allowances designed for suburban housing rarely transfer cleanly.

Depth, height and the neighbour's window

Most councils apply a 45 degree guide drawn in plan from the centre of the nearest habitable room window on the neighbouring property to the nearest edge of the proposed extension, and many apply a second test in section from the same window at 25 degrees or 45 degrees depending on the local design guide.

The tests are guidance rather than law and the exact geometry differs, so read the council's householder design guide rather than assuming the version used in the next borough applies.

What moves the result, in order of effectiveness:

1. Reducing the depth nearest the boundary while keeping full depth on the far side, so the extension steps in plan.
2. Lowering the flank wall and eaves height, which usually matters more than depth for a window close to the boundary.
3. Setting the flank wall 0.5 m to 1 m off the boundary, which opens the 45 degree line and gives room for maintenance.
4. Sloping the roof away from the boundary rather than using a flat roof at full height.

An extension that passes the geometry but has a 3.2 m parapet along the boundary will still attract objection. One that steps down to 2.4 m at the boundary and rises away from it usually will not.

Side return infill and flank walls

Side return infill fills the gap beside the original outrigger, which means building a wall along the full length of the boundary for the depth of the return.

The effect on the neighbour depends on what is already there. Where they have their own outrigger, the two flank walls face each other across the boundary and the new wall may change very little. Where their side return is open, the new wall arrives against their kitchen window or their side passage and the effect is real.

The workable design response is a roof that falls away from the boundary with rooflights set back from it, a parapet no higher than the existing outrigger eaves, and no first floor element over the return. Where the neighbour's window is in the flank of their own outrigger, look at whether that window serves the only source of light to a habitable room, because that is the case officers treat most seriously.

Garden overshadowing in small plots

BRE guidance tests amenity areas by asking whether at least half of the space receives at least two hours of sunlight on 21 March, and treats a reduction below 0.8 times the former sunlit area as a noticeable loss.

In a terrace with a 20 m garden the test is usually comfortable. In a plot where the garden is 8 m long and 5 m wide it is not, and an extension on the southern boundary can fail it without being unusually large.

Where a garden result is marginal, an Overshadowing Study showing sun on ground at 21 March with comparison at 21 June and 21 December is more persuasive than an assertion that the effect is limited. Distinguish, too, between shadow that moves across a garden during the day and shadow that sits permanently on the same strip, a distinction covered in transient overshadowing and permanent loss of sunlight.

Rooflights and glazed links

Rooflights solve the applicant's problem and do nothing for the neighbour's, and it is worth being clear about that when writing the supporting statement.

A deep rear extension darkens the original back room of the applicant's own house, and rooflights, a glazed link between the old and new structure, or a partially glazed roof are the standard responses. They matter because officers do consider the daylight of the resulting dwelling as well as the neighbour's, and a scheme that creates a permanently dark middle room is poor design even if nobody objects.

What they do not do is reduce the shadow cast next door. An application that offers rooflights as mitigation for a neighbour's loss of light has misunderstood the objection.

Negotiating a reduced depth

The most reliable way to get a deep extension approved beside a close neighbour is to make the geometry work at the boundary and take the depth further from it.

Practical approaches that officers accept: matching the depth of an extension the neighbour has already built, since it is hard to object to a mirror of your own; stepping the rear wall so the deepest point sits away from the shared boundary; and splitting a full width extension into a deeper section and a shallower one.

Talk to the neighbour before submission with drawings in hand. Objections raised after an application is registered harden into positions, while concerns raised over a fence are often about something specific and fixable. The evidence that actually carries weight in a neighbour's objection is technical, as set out in daylight evidence in a neighbour objection.

Evidence that supports the application

For a householder scheme the evidence does not need to be elaborate, but it does need to exist.

Include a plan showing the 45 degree line from the centre of each affected neighbouring window, with the dimension from the window to the boundary marked. Include a section through the boundary showing the flank wall height against the neighbour's window head. Include a north point on every drawing, which gets left off more often than you would expect and is the first thing a case officer looks for.

Where the scheme is marginal or the neighbour has already objected, a short Daylight and Sunlight Assessment covering the affected windows and the garden is proportionate. Councils expect this work from a suitably qualified assessor, and professional review is strongly advised.

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. To see which documents your extension is likely to need, enter your postcode on PlanScope.