Daylight evidence for a rear dormer in a terrace

A rear dormer on a terraced house almost never causes a measurable daylight loss to a neighbour, and saying so clearly in the application is usually more useful than commissioning a study. The geometry explains it. A dormer sits above eaves level, set back from the rear wall, and it faces a garden rather than a window. The obstruction it creates, measured from a neighbour's ground floor rear window, is a narrow sliver of sky high above the horizon. The dormer daylight neighbour question that officers actually worry about is overlooking, not light.

There are terrace layouts where a dormer does cause harm, and knowing which they are saves an argument.

Why the numbers usually fall the right way

The tests in the Building Research Establishment guidance on site layout planning for daylight and sunlight measure the sky visible from a neighbouring window. Vertical sky component looks at the proportion of sky visible at the centre of the window. Annual probable sunlight hours looks at sunlight over the year and in winter. Both depend heavily on obstruction close to the window and at a low angle.

A dormer fails to produce either, for three reasons.

It is high. Light lost from high in the sky contributes far less at a window than light lost near the horizon.

It is set back. Class B permitted development requires a dormer to sit at least 0.2 metres back from the original eaves, and most designs set back more, which increases the distance to the neighbour's window.

It is narrow relative to the roof. A dormer that occupies the middle of the rear roof plane, with a margin to each party wall, subtends a small angle from anywhere in the neighbour's garden.

On a standard terrace with rear gardens of any depth, a proper Daylight and Sunlight Assessment is disproportionate. Councils know this. What they want is a drawing that lets them see the geometry.

Where dormers do cause harm

Four layouts, and they are recognisable from the roof plan.

Back-to-back terraces with very short gardens. Where rear elevations face each other across 10 metres or less, adding habitable accommodation at second floor level creates a genuine overlooking relationship, and in the tightest cases the dormer does begin to affect sunlight to the garden and to upper floor windows opposite.

Dormers on the side slope of a rear outrigger. Many Victorian terraces have a two storey rear back addition with a pitched roof. A dormer in the flank of that roof looks straight into the neighbour's rear windows across the boundary at close range. This is the single most contentious dormer type in London and the northern industrial cities, and it fails on privacy and often on light to the neighbour's rear outrigger window.

Dormers built out to the party wall. Where the dormer cheek sits on or hard against the party wall line, the obstruction angle to the neighbour's own roof windows and to any existing dormer rises sharply, and the visual effect on the terrace is much worse.

Stepped terraces on a slope. Where the neighbour sits lower, a dormer that is invisible on the level can present a large obstruction to the lower property's rear windows.

Testing against the elevation opposite

Where a test is needed, two assessments cover almost every householder dormer.

The 45 degree test, applied in plan and in section from the centre of the nearest habitable room window in the neighbouring property. Most councils adopt a version of it in a householder design guide, and the local wording is what matters, because some measure from the window centre and some from the nearest edge.

A sectional study through the dormer and the neighbouring rear elevation, drawn to scale, showing the obstruction angle from the affected window. One drawing, existing and proposed on the same sheet, usually resolves the question.

Three drafting points that make the difference:

• Draw the section on the correct line. A section taken through the centre of your own roof, rather than through the affected window, proves nothing.
• Show the existing situation, including any existing rear extension or outrigger, so the officer can see the increment rather than the total.
• Label the room use of the affected window if you know it. A bathroom or a landing carries far less weight than a living room, and the assumptions made about room use are discussed in our note on room use assumptions in a daylight assessment.

Where a ground floor rear extension is proposed at the same time, the two effects are cumulative and should be assessed together, as covered in our note on rear extensions and neighbour amenity in a terrace.

Party walls and boundaries

Planning and the party wall regime are separate, and both apply.

A dormer that involves cutting into, raising or inserting beams into the party wall engages the Party Wall etc. Act 1996, which requires notice to the adjoining owner before work starts. Planning permission does not authorise that work and the absence of a party wall award does not affect the planning decision. Serve the notice in good time, because the statutory notice period will otherwise sit on the critical path.

For planning purposes the boundary matters in a different way. A dormer cheek set in from the party wall by a clear margin reads better on the terrace, reduces the obstruction to the neighbour, and removes the argument about flashing details. Set-in margins of roughly the width of a rafter bay are common in council design guidance. Check the local householder guide, because the figure varies.

Design changes that reduce impact

The fourth and fifth rows are worth offering before the officer asks. Obscure glazing conditions work in some situations and not others, and our note on obscure glazing conditions sets out when they hold up.

Permitted development or an application

Class B of Schedule 2 Part 1 of the General Permitted Development Order 2015 allows a roof enlargement within limits. The ones that decide most loft conversions:

1. A volume allowance of 40 cubic metres for a terraced house and 50 cubic metres for other houses, including any earlier roof enlargement.
2. No part higher than the highest part of the existing roof.
3. No extension beyond the plane of the existing roof slope on the principal elevation fronting a highway.
4. A set back of at least 0.2 metres from the original eaves, measured along the roof plane.
5. Side-facing windows obscure glazed and non-opening below 1.7 metres above floor level.
6. Materials similar in appearance to the existing house.

Class B is not available on designated land, which includes conservation areas, National Parks, National Landscapes and World Heritage Sites. In those areas a rear dormer needs an application, and rooflights are treated separately, as covered in our note on rooflights and glazing in conservation areas.

The volume allowance is cumulative and includes work done by previous owners. Check the planning history before relying on it, and remember that a lawful development certificate is the only formal way to confirm that works are lawful.

Evidence worth submitting

For a householder dormer application, the pack that gets a quick recommendation:

• Existing and proposed roof plans, with the set-back dimensions annotated.
• Existing and proposed rear elevations, showing the neighbours on both sides.
• A section through the dormer and the affected neighbouring window, to scale.
• A short amenity note, one page, addressing daylight, sunlight, overlooking and the local design guidance by name.
• Photographs of the rear elevation and of the terrace roofscape, showing any existing dormers.

That last item does more work than people expect. Where a terrace already has three rear dormers, a photograph establishes the pattern and shifts the design argument decisively.

If you are putting a householder application together and want the supporting statements drafted, enter your postcode on PlanScope to see which reports the site is likely to need.

This is general guidance, not legal advice, and a lawful development certificate is the only formal way to confirm that works do not need permission. 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.