Daylight evidence for a roof extension on a terrace

A roof extension on a terraced house rarely takes light from the house next door. The geometry works in your favour: you are building above the level of the neighbour's windows, behind the party wall line, and the windows most at risk are the ones in their own roof rather than the ones in their rear wall. That is why most rear dormers are approved and most objections about light fail.

The exceptions are specific, and worth knowing before you draw anything. A dormer that comes close to the party line and projects well back from the roof plane can reach into the 45 degree line from a neighbouring second floor window. A neighbour with a rooflight serving a converted loft bedroom has a window the extension really can affect. And on a stepped terrace, where one house sits higher than the next, the usual assumptions break down.

Which neighbouring windows are affected by a roof extension

Start by listing every window on the adjoining properties that faces the works, with the room it serves and its floor level. A window serving a bathroom, a hall or a landing carries much less weight in amenity terms than one serving a habitable room.

On a standard two storey terrace with a rear outrigger, the candidates are:

• The neighbour's rear first floor window, which is below your extension and usually unaffected
• A rooflight or dormer in the neighbour's rear roof slope, which can be directly affected if your dormer sits close to the party line
• A second floor rear window in a three storey terrace, which is the case that produces real daylight loss
• A side window in the neighbour's outrigger facing the party boundary, often obscure glazed and serving a bathroom
• A window in a rear extension at ground level, affected by overshadowing rather than by direct obstruction

Rank them. Then measure the geometry only for the ones serving habitable rooms. An assessment that covers every window equally wastes effort and dilutes the argument.

Rear dormers and the light to rear rooms

A rear dormer sits above the neighbour's first floor windows, set back from the eaves, and projects into the roof space rather than beyond the rear wall. Light to the neighbour's rear rooms arrives over the top of their own rear wall at an angle that your dormer does not obstruct.

Two things change that. If the dormer is built out to the party wall with a flank cheek standing above the roof line, it becomes a vertical obstruction right at the boundary, and on a south-facing rear elevation that can cast a real shadow into the neighbour's garden and ground floor. And if the neighbour has already converted their loft with a rooflight in the rear slope close to the party wall, your cheek stands directly beside their window.

The convention that avoids both is to set the dormer in from the party wall by 0.5 to 1 metre on each side, keep the flank cheeks as low as the headroom allows, and set the dormer back from the eaves. Permitted development under Class B requires a set back of 0.2 metres from the eaves in any event, and the volume allowance is 40 cubic metres for a terraced house and 50 cubic metres for other houses, with side-facing windows obscure glazed and non-opening below 1.7 metres above floor level.

Mansard forms and the angle at the party line

A mansard replaces the roof with a steep front slope and a shallow top, gaining a full storey. It is a different proposition in amenity terms because the new wall plane rises from near the eaves, which puts built form where previously there was sky.

Mansards normally need planning permission rather than falling within permitted development, and proposals to extend permitted development rights to them have been announced at various points, so check the current text of Part 1 of the Order before assuming either way. In conservation areas the question is usually whether the terrace has an established mansard pattern. Where it does, matching it is straightforward. Where yours would be the first, the design case is uphill and the daylight case is secondary.

At the party line, a mansard with a party wall upstand reads as a two storey obstruction to the neighbour's rooflight. Where the terrace is being extended as a pair, or where the neighbour has already built one, the geometry resolves itself. Where it is a single intervention, expect the officer to ask for a section through the party wall showing the relationship to any window within 3 metres of it.

Rooflights against dormers for amenity

The privacy point is the one householders underestimate. A dormer window at second floor level looks down the full length of two or three gardens, and the objection that actually succeeds is more often about overlooking than about light. Roof terraces and neighbour privacy in flatted schemes covers the equivalent problem where the roof becomes amenity space.

Preparing simple evidence without a full model

For most householder cases a drawn section does the work. Take a section through the party wall, at right angles to the rear elevation, showing your proposed roof extension and the neighbour's nearest habitable room window. From the centre of that window, draw a line at 45 degrees above the horizontal. If the proposal sits below the line, say so on the drawing and the point is made.

Add a plan showing the 45 degree line taken from the same window edge in plan, because councils' householder design guides apply the rule in both planes. Photographs from the neighbour's side of the boundary, where access allows, settle arguments about what is already there.

That evidence is enough for a simple case. It is not a daylight assessment and it should not be described as one. The BRE guidance on site layout planning for daylight and sunlight sets out the proper tests, including the vertical sky component with its 27 per cent reference value and the annual probable sunlight hours tests, and applying them needs a model.

What the case officer will look for

Officers assessing a roof extension on a terrace work through a short list: whether the form matches the established pattern of the terrace, whether the dormer is subordinate to the roof slope, the set backs from party walls and eaves, the window positions and what they overlook, and whether any neighbouring habitable room window is materially affected.

They will also check the permitted development fallback. If the proposal could be built without permission, that is a material consideration and it limits what the council can reasonably resist. Say so in the covering statement where it applies.

Where a full assessment becomes worth it

Commission a Daylight and Sunlight Assessment where the proposal breaches the 45 degree line to a habitable room window, where a neighbour has objected on daylight grounds and the officer has asked for evidence, where the terrace is stepped or the site falls so that the simple geometry misleads, or where the scheme is large enough that an appeal is a realistic prospect.

For gardens and ground level amenity space rather than windows, an Overshadowing Study is the relevant document, and the equivalent question for a single storey extension is set out in daylight for a single storey rear extension. Where the neighbour's room has more than one window, the analysis changes again, as when a neighbour's room is served by two windows explains.

PlanScope can draft the planning statement and design material for a roof extension from a postcode and a set of answers, flagging where measured survey data is needed rather than filling the gap. Daylight modelling needs a suitably qualified author. Reports should be reviewed before submission and PlanScope does not guarantee planning approval. This is general guidance, not legal advice. Check the position with your local planning authority or a qualified professional before relying on it.