Daylight to a neighbour's permitted development extension

A window in a neighbour's lawfully built permitted development extension forms part of the existing baseline, and it is assessed like any other window. That is the short answer, and it disappoints applicants who assume that because nobody ever tested the extension against an amenity policy, the room behind it carries no weight. It does carry weight. What changes is the arithmetic, because a deep single-storey rear extension usually starts from a low baseline, and a low baseline makes the proportional test in the BRE guidance very easy to fail.

So the useful question for daylight to a neighbour's permitted development extension is not whether the window counts. It is which test applies, what the room actually is, and how much weight an officer will give the result once the history is on the table.

Whether a lawful extension changes the baseline

Daylight and sunlight assessment compares the existing condition with the proposed condition. The existing condition is the lawful built form on the ground, however it came to be there. An extension built under Class A of Part 1 of Schedule 2 to the GPDO 2015, within the limits and conditions of that class, is lawful development, so its windows are existing windows.

Nothing in the BRE guidance distinguishes between a window in an original building and a window in a permitted development addition. The guidance is written around rooms and their windows.

Two qualifications matter in practice. Where the extension exceeds the Class A limits it may be unlawful, and an unlawful structure does not set the baseline. And where the extension relies on the larger home extension prior approval procedure, there is a planning record of it, which is worth obtaining because it will usually show the internal layout and the room use.

Check the lawfulness before you concede the baseline. A 4.5 metre rear extension on a semi-detached house is outside the 3 metre Class A limit and outside the 6 metre prior approval limit for a semi, so it needed permission, and if none was granted the position is different.

Rooms served by a window in a PD extension

The BRE guidance applies to main living rooms, dining rooms, kitchens and bedrooms, and it says expressly that bathrooms, lavatories, storerooms, circulation space and garages need not be analysed. Kitchens are treated as less important than living rooms and bedrooms.

That hierarchy does most of the work in these cases, because of what people put in rear extensions. A utility room, a boot room, a shower room or an internal hall gets no assessment at all. A home office is not on the list of room types the guidance protects, though a local amenity policy may still ask about it. A kitchen sits on the list but at the bottom of it.

The common arrangement is a knocked-through kitchen and dining space occupying the original rear room and the extension together, lit by sliding doors at the back and often a rooflight. That is a single room for assessment purposes, it is partly a kitchen and partly a dining area, and how you characterise it changes the answer.

Visit the site if access allows, and failing that work from the building control records and the submitted drawings, which are usually obtainable. Guessing room use from the window pattern is how assessments get contradicted at committee.

Single storey rear extension geometry

The geometry of a deep single-storey extension produces a predictable result, and it is worth understanding before running the model.

The rear window sits at the end of a long, low space with a roof over the whole depth. Its vertical sky component is already modest, because the sky it sees is limited by its own roof and by the boundary treatment. Its no sky line position is poor at the back of the room, because the original rear wall and the extension roof together cut off sky from the deeper part of the space.

Then the proportional tests bite. The BRE guidance treats a vertical sky component of 27 per cent as the level at which a window is adequately lit, and where the existing value is already below that figure the relevant comparison is whether the proposed value is at least 0.8 times the existing value. A window starting at 12 per cent fails the ratio once it drops below about 9.6 per cent, and on a tight urban plot a modest two-storey addition next door can take 3 percentage points with ease.

The same ratio applies to the no sky line, where a reduction of the lit area to less than 0.8 of its former value is flagged, and to annual probable sunlight hours on windows within 90 degrees of due south.

A single aspect room at the end of a deep extension behaves much like the flats described in daylight to single aspect flats and how to fix them, and the fixes are similar: more glazing area will not rescue a room that cannot see sky.

The practical consequence is that a numerically large percentage loss can represent a very small absolute change in light. Say so explicitly, in both percentage and absolute terms. An officer reading that a window falls from 12.0 per cent to 9.1 per cent, a 24 per cent reduction, will understand the case better than one who only sees the ratio failure.

Where the neighbour created the problem

This is the part applicants want to lean on, and it has to be handled carefully.

The fact that the neighbour built the extension without any amenity assessment does not strip the room of protection. Permitted development rights exist precisely so that the development does not need assessing, and the absence of a prior test is not a finding that the room is unimportant.

What the history does legitimately support is a point about reasonable expectation. A window installed in a single-storey extension set close to a shared boundary, facing the adjoining plot, was placed in a position where development on that plot was always foreseeable. Inspectors have treated the self-created nature of a sensitivity as relevant to the weight given to the loss, and those decisions are indicative of the approach rather than binding precedent.

There is also a civil law point that is often missed. A right to light is acquired by 20 years of uninterrupted enjoyment through a defined aperture under the Prescription Act 1832. A window built five years ago has no prescriptive right, so the neighbour has a planning amenity interest but no private law remedy. That does not help the planning balance directly, and it changes the negotiating position considerably.

Put the argument in those terms. Not that the window should be ignored, but that the degree of harm should be judged against a baseline the neighbour chose to create.

Habitable room status of a converted space

Room status is contested more often than the numbers. Three situations recur.

A garage converted under Class A or by permission, now used as a bedroom, with a window in the rear extension. The guidance exempts garages, so the status turns on the current lawful use, and conversion to habitable accommodation generally brings it within scope.

An outbuilding in the garden used as a study or gym, built under Class E of Part 1. It is not part of the dwelling, and the BRE guidance does not protect windows in incidental outbuildings. Incidental use is the whole point of Class E, so the structure never became habitable accommodation.

A bedroom created in a rear extension with its only window on the flank wall, a metre from the boundary. The window is assessable, and the geometry is hopeless from the outset, so the existing value will be very low and almost any development next door will breach the ratio.

State the room schedule you have assumed, with its evidence, in the Daylight and Sunlight Assessment. Half of the arguments in these cases are about which rooms belong in the table.

How officers weigh the resulting loss

Officers run a two-stage judgement. First, has the numerical guidance been breached and by how much in absolute terms. Second, how much weight does that breach carry given the room, the context and the baseline.

The factors that reduce weight are a kitchen or secondary room rather than a living room or bedroom, a very low existing value where the absolute loss is small, a second window or rooflight serving the same space, a dense urban context where the local amenity policy expects flexibility, and a baseline created by the neighbour's own recent development.

The factors that increase weight are a sole window serving a bedroom or living room, a loss that pushes a previously compliant window well below the guidance figures, and a scheme that could achieve the same floorspace with a different massing.

Most local plan amenity policies refer to the BRE guidance as guidance rather than as a standard, which is how it was written. The way that reference is drafted matters, so read local plan amenity policies and how they use BRE 209 before you frame the case.

Evidence to submit

Keep the pack tight and make every assumption visible. A measured survey of the neighbouring extension, including window positions, head heights and the internal arrangement where it can be established. The planning and building control history for that extension, with the Class A or prior approval position stated. A room schedule with assumed uses and the source for each. Vertical sky component, no sky line and annual probable sunlight hours results in existing and proposed, in both ratio and absolute terms. A short narrative on the baseline and how it arose.

Where the loss is unavoidable, pair the assessment with design mitigation rather than argument alone: setting the upper floor back from the boundary, chamfering the rear corner, or lowering the eaves on the boundary side. A scheme that visibly responds to the neighbour's window reads very differently from one that merely calculates the loss. An Overshadowing Study covering the neighbour's garden often helps as well, since amenity space loss and window loss are usually raised together.

Councils expect a suitably qualified author for daylight and sunlight work, and professional review of any draft 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.

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