Roof extensions and light to the neighbour's rear window

A rear dormer or a hip-to-gable rarely takes daylight from a neighbour's rear window, and when it does, the cause is almost always a change in the flank wall rather than the dormer itself. That is the honest starting point for anyone worried about a loft conversion next door, or about an objection to their own. Roof-level work sits above the sightline from most ground-floor windows and is usually set back from the eaves, so the geometry that matters for a two-storey rear extension often does not bite. Privacy is the issue that actually decides these cases.

Why roof level extensions raise different amenity issues

Daylight loss depends on how much sky a window can see. A two-storey rear extension built along a boundary blocks sky at a low angle, close to the window, which is why it shows up so strongly in a section drawing. A dormer sits five or six metres higher and, under permitted development conditions, is set back from the original eaves. It subtends a smaller angle at the neighbour's ground-floor window and often sits outside the relevant sky zone altogether.

Two exceptions matter.

Where houses are staggered, or built on a slope so that one is materially lower than the other, a roof addition on the higher property can appear directly above and close to the lower property's rear windows. And where a hip roof is converted to a gable, the flank wall grows by the full triangle of the former hip, which is a solid addition on the boundary side at high level.

Assessing the effect on a rear window below

The technical reference is the BRE guidance in Site Layout Planning for Daylight and Sunlight. It sets out a screening test and two measures worth knowing.

The screening test draws a line at 25 degrees above horizontal from the centre of the lowest affected window, in section, perpendicular to the wall. If no part of the new development crosses that line, the guidance says daylight is unlikely to be materially affected and no further calculation is needed. Rear dormers set back from the eaves usually pass this on a standard terrace.

Where the line is crossed, the vertical sky component is calculated at the centre of the window. The guidance treats a value below 27 per cent, combined with a reduction to less than 0.8 times the former value, as the point at which the loss becomes noticeable. The companion measure is the no-sky line, which maps how much of the room's working plane can still see sky. Our explainer on what 27 per cent actually measures goes into the arithmetic.

For sunlight, only windows within 90 degrees of due south are assessed, so a north-facing rear elevation drops out of the sunlight test entirely. That single fact resolves a lot of arguments.

Hip to gable and the new flank wall

This is the roof alteration most likely to produce a real daylight effect, and it is the one most often waved through as harmless.

Picture two semi-detached houses at right angles to each other, a common arrangement on interwar estates. The hip of house A faces the rear garden and rear windows of house B. Converting that hip to a gable adds a triangular wall roughly the width of the house and the height of the roof pitch, set on the boundary side. From house B's rear window the sky lost is at a high angle, but it can still be several degrees of the sky dome, and where the two houses are close it will cross the 25-degree line.

Draw the section. Take it perpendicular to the affected window, through the centre of that window, at true scale, from a measured survey rather than from the estate agent's floor plan. If it crosses, commission a Daylight and Sunlight Assessment for that one window rather than arguing about it in correspondence. Councils usually expect a suitably qualified author for this work, and professional review is strongly advised.

Overlooking from a new rear dormer

Most objections to loft conversions are really about privacy, and the planning answer depends on what already exists.

If the rear elevation already has first-floor bedroom windows looking over the same gardens, a dormer window at second-floor level looks at broadly the same things from a steeper angle. Councils accept that regularly. Many authorities apply a back-to-back separation guideline of around 21 metres between facing habitable room windows in their householder design guides, and terraced gardens in Victorian streets often fall well short of it already, which is why the existing relationship matters more than the guideline.

A dormer changes the position in three situations. A dormer placed in a side roof slope facing a neighbour's garden at close range. A dormer set on a roof that oversails a single-storey neighbour. And any arrangement that converts roof space into an accessible terrace, which is a different proposition altogether and attracts conditions on screening and use, as set out in the note on roof terraces and the conditions councils impose.

Where permitted development removes the question

If the work is permitted development under Class B of Part 1 of Schedule 2 to the GPDO 2015, amenity is not assessed at all. There is no application, no neighbour consultation and no officer judgement about daylight.

Class B allows an enlargement of the roof within cubic content allowances, with conditions that include the following. No part of the enlargement may extend beyond the plane of the existing roof slope on the principal elevation fronting a highway. Other than for a hip-to-gable, the enlargement must be set back at least 200 millimetres from the original eaves, measured along the roof slope. Any side-facing window in a wall or roof slope forming a side elevation must be obscure-glazed and non-opening below 1.7 metres above the floor. Materials must be similar in appearance to the existing house. Balconies, verandas and raised platforms are excluded.

Class B rights do not apply on article 2(3) land, which covers conservation areas, National Parks, National Landscapes, the Broads and World Heritage Sites, and they do not apply to flats or maisonettes at all. An Article 4 direction can remove them elsewhere. A Lawful Development Certificate is the only formal way to confirm that works are lawful, and it is worth having before you build rather than when you sell.

What to submit when permission is needed

Where the scheme needs permission, keep the amenity evidence proportionate.

1. Existing and proposed elevations and roof plans at a consistent scale.
2. A section through the boundary taken perpendicular to the affected window, showing the 25-degree line.
3. A block plan showing the separation distance to any facing window.
4. Photographs of the rear elevations of both properties and of the wider terrace or street pattern.
5. Daylight calculations for the affected window only, where the screening line is crossed.
6. A short amenity section in the planning statement that names the local plan policy and the design guide paragraph and answers each.

Two things officers notice. Sections drawn from assumed dimensions rather than a survey, and drawings that show the neighbour's window in a position that does not match the photographs.

Answering a neighbour objection

Deal with the point actually made. If the objection is about light, produce the section. If it is about privacy, produce the block plan and the existing window positions, and offer obscure glazing to any side-facing pane if that resolves it. If it is about construction noise and scaffolding, say plainly that these are not planning matters and that access will be agreed separately, because pretending otherwise damages your credibility on the points that are.

Neighbours often assume a right to light exists in planning. It does not. Rights of light are a civil property matter under separate law, and a planning permission neither grants nor extinguishes them. There is more on the tone and structure of a reply in the guide to responding to a neighbour's daylight objection.

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.