Dormers and daylight to the neighbour's rear elevation

A rear dormer takes very little daylight from the neighbour next door, and takes a great deal of privacy. That is the counter-intuitive part of dormer neighbour daylight impact, and understanding it saves a lot of wasted argument: the geometry of a roof addition means the obstruction sits high and close to the party wall, so the angle it subtends at a neighbour's ground-floor window is small, while the new window itself looks down into their garden from above the fence line.

So the objection you get will usually be about overlooking, and the objection you have to answer with modelling is about light. They need different evidence.

Why the geometry favours the dormer on light

Daylight assessments work from the angle of obstruction. BRE guidance treats a vertical angle of 25 degrees measured at the centre of the lowest affected window as the threshold below which skylight is unlikely to be materially affected, with further checks on vertical sky component, daylight distribution and sunlight hours where that angle is exceeded.

A rear dormer sits in the roof slope, set back from the eaves and set in from the party wall. Measured from a neighbour's ground-floor rear window, the top of a dormer is further away and higher than the top of a two-storey rear extension would be, and the angle is usually well inside the threshold. A first-floor window in the adjoining house is the one worth checking, particularly on a close terrace with a projecting rear closet wing.

The 45 degree rules used in many local design guides work differently, and this is where confusion creeps in. Those rules test the plan angle from the nearest edge of a neighbour's habitable room window to the nearest corner of the new work, and sometimes a vertical version too. They were written with ground and first-floor extensions in mind and they apply awkwardly to roof additions, so expect a local guide to say what it means for dormers.

Set backs from the party wall and the eaves

Class B of the General Permitted Development Order requires a roof enlargement to be set back at least 200mm from the original eaves, measured along the roof slope, and to sit no higher than the existing ridge. Many authorities' design guidance asks for more than the Order does: a set back from the party wall, commonly in the region of half a metre to a metre, and a dormer face set down from the ridge line.

Those extra set backs are about appearance as much as amenity. A dormer running flush to both party walls and up to the ridge reads as a new storey, and where several houses in a terrace do it, the roofscape changes character. Officers call it a terracing effect.

The amenity benefit is real too. Setting in from the party wall moves the new window away from the boundary and increases the oblique angle to the neighbour's garden, which reduces direct overlooking without reducing the floor area much at all.

Obscure glazing and fixed lights

Class B already requires side-facing windows in a roof enlargement to be obscure glazed and non-opening below 1.7 metres above the floor of the room. That condition exists precisely because a side window in a dormer cheek looks straight into the adjoining property.

For a full application, the same solution is usually conditioned. Three details decide whether it works.

1. The glazing specification, which should be a stated obscurity level rather than "obscure glass", because thin textured glass gives privacy only until someone stands close to it
2. Whether the opening part is above 1.7 metres from the internal floor level, which has to be shown on the section rather than asserted
3. Whether the frame is fixed shut, since an openable low light defeats the point

Rear-facing dormer windows are not usually obscure glazed, because they serve the main outlook of the room. The privacy argument there is about distance and angle, and the answer is the set back and the position of the window within the dormer face.

Terraces and balconies formed within a dormer

An inset balcony cut into the roof slope, or a dormer with a glazed door and a railing, is excluded from Class B in the same way as any other balcony or raised platform. It is also the single most contentious feature you can put in a roof.

From a standing position at roof level, a person can see over fences, hedges and the usual boundary treatments into two or three gardens. There is no screening solution at that height that works in all directions, and officers know it.

A Juliet arrangement, where the door opens against a railing set in the plane of the dormer face with no standing area outside it, is the version that gets support. It gives the room the outlook and the ventilation without creating a usable external space, and the drawings should make clear that there is no walkable platform.

An illustrative example

A terraced house with a rear closet wing, proposing a full-width rear dormer set 500mm in from each party wall, 300mm down from the ridge, with two windows in the rear face and no side lights. The adjoining house has a first-floor rear window 2.5 metres from the party wall. The dimensions are illustrative.

The light check here is on that first-floor window. With the dormer set in half a metre and set down from the ridge, the obstruction angle from the centre of the neighbour's window is modest, and a simple section drawn at scale will show it. The privacy check is the distance from the new rear windows to the rear boundary and to the facing houses behind, and the honest answer in a tight Victorian terrace is often that the separation is already short and the dormer makes it worse at a higher level.

That is the point to engage with rather than to dodge. An application that acknowledges the existing relationship and explains why the additional effect is acceptable does better than one that claims no effect at all.

Designing out the objection

• Set the dormer in from both party walls, and show the dimension on the roof plan
• Keep the dormer face below the ridge so the roof retains a visible slope above it
• Put habitable windows in the rear face and nothing in the cheeks
• Match the window proportions to those below, because a horizontal band of glazing in a roof slope reads as alien on a house with vertical sashes
• Avoid any standing platform, and if a railing is wanted, draw it in the plane of the dormer face

Evidence that supports the application

For most rear dormers a scaled section through the party wall, showing the neighbour's window positions and the obstruction angle, is enough on the light question. Where a neighbouring window is close and the dormer is large, a Daylight and Sunlight Assessment prepared against BRE guidance is the document that settles it, and councils expect a suitably qualified author for that work with professional review strongly advised.

For privacy, the useful evidence is a roof plan with sightlines drawn from each proposed window across the rear gardens, plus photographs taken from the existing loft or first floor at the same height. Where the garden being overlooked is in constant use, an Overshadowing Study may be asked for alongside, although shade from a dormer is rarely the issue.

Local plan amenity policies vary in how they use the BRE numbers, and reading your own authority's wording before modelling anything avoids producing the wrong report, as local plan amenity policies and how they use BRE 209 explains. Neighbours objecting on light grounds face the mirror image of this exercise, covered in objecting on daylight grounds.

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.