Do I Need a Daylight and Sunlight Report for an Extension in London?

What are the 45-degree and 25-degree tests?

They are rules of thumb from the BRE guide Site Layout Planning for Daylight and Sunlight, which London councils use to decide whether light to a neighbour's window needs a closer look.

The 45-degree test suits extensions that sit at right angles to a neighbour's window, which is what most rear extensions do. Camden's 2018 amenity guidance describes it on a floor plan: draw a 45-degree line from the middle of the neighbour's nearest window towards your extension. If any part of the extension crosses the line, daylight may be affected. The same line can be drawn on an elevation. Where the neighbour's window is a floor-to-ceiling patio door, the line starts 1.6 m above the floor rather than at the middle of the glass.

The 25-degree test suits buildings that face a window across a gap. A line is drawn at 25 degrees above the horizontal from the centre of the bottom window opposite. If the whole development stays below it, it is unlikely to have a substantial effect on that window's daylight; if it rises above it, Camden expects a daylight and sunlight report.

Do London councils carry out the tests themselves?

Yes, for ordinary householder schemes. Camden's current guidance (January 2021) says officers "will use the 45 degree and 25 degree assessment" to make an initial judgement, and that the two tests screen whether a report is required at all. It expects a report up front only from major developments and proposals for new homes.

Brent writes the 45-degree line straight into its rules. Its Residential Extensions and Alterations SPD, adopted in January 2025, says a two-storey rear extension must not extend beyond the 45-degree line from the middle of the nearest habitable room window of a neighbour, on top of a depth limit of 3 m for terraced and semi-detached homes and 4 m for detached homes. Brent counts kitchens as habitable rooms for this purpose, but not bathrooms. An officer checking a Brent scheme measures from your drawings; no consultant is involved unless the design breaks the line.

The national policy behind all of this is short. The National Planning Policy Framework published in August 2026 says, in policy P3, that development should not give rise to an unacceptable level of access to daylight and sunlight for neighbouring residents. It does not set a test of its own, which is why councils borrow the BRE ones.

When does a council ask for a full daylight and sunlight report?

Three situations cover almost every London case:

• The design fails a screening test. A two-storey rear extension that crosses the neighbour's 45-degree line, or a mansard or upward extension that rises above the 25-degree line from the house opposite.
• The scheme is bigger than an extension. New dwellings, including a house in a back garden, and major developments are expected to bring a report in Camden. Richmond's validation checklist, adopted in February 2026, asks for one on all major development and on other proposals that could harm daylight or sunlight, or cause harmful overshadowing of the host building or its neighbours.
• The council's own list says so. Kensington and Chelsea's submission requirements ask for an assessment on any application with potential for a material impact on neighbours' daylight or sunlight. Its guidance explains why: buildings in the borough are so close together that daylight, sunlight and sense of enclosure are frequently controversial.

Neighbour objections can also prompt an officer to ask for one mid-application, usually where the drawings show the extension tight against a side window. National planning guidance limits what a council may ask for: the information must be reasonable having regard to the nature and scale of the development. A request for a full technical report on a small, single-storey extension that passes both tests is worth querying.

What does a daylight and sunlight report measure?

A specialist models your extension and the neighbouring windows and runs the BRE calculations. Camden lists the usual ones: vertical sky component (how much sky the window sees), no sky line (how much of the room still sees sky), average daylight factor, and annual probable sunlight hours. The report compares the results before and after, gives the method and a summary, and is judged against the BRE figures, which Camden treats as guidance to apply flexibly rather than fixed policy.

Richmond asks for it to use the latest BRE edition (2022) and to be prepared by a suitably qualified professional. Kensington and Chelsea says anyone can prepare one but advises using a competent daylight professional. Either way, it is not a document a householder or a drawing service should write.

Which London extensions most often come close to the lines?

The 45-degree test bites hardest where houses are tight together and windows face sideways. That describes much of inner and middle London:

• Victorian and Edwardian terraces with rear outriggers. The neighbour's kitchen window often looks sideways across the side return. A side return infill or a wraparound sits right in front of it. Brent's SPD deals with outriggers directly and asks for lightwells or rooflights where an extension would leave an inner room dark.
• Two-storey rear extensions on semi-detached houses. A first-floor bedroom window next door, close to the shared boundary, is the classic 45-degree failure.
• Mansards and roof extensions on narrow streets. Here it is the 25-degree line from the windows opposite that matters, not the neighbour beside you.

A single-storey rear extension of ordinary depth rarely crosses either line, which is why most of them never see a daylight report. Our side return extension guide walks through the layout choices that keep a side infill clear of the neighbour's window.

What do our drawings do, and what does a daylight specialist do?

We do not prepare daylight and sunlight reports. If your council asks for one, a specialist daylight consultant writes it.

What our complete planning drawing service gives, for £795, is the material both the officer and any specialist work from: a measured survey that records the neighbours' windows where they can be seen from your side, then existing and proposed plans, elevations and sections that show the extension's depth and height against the boundary. If a design sits close to a line, you can change it before submission rather than after an officer raises it. Whether we draw the 45-degree and 25-degree lines on your sheets as a check is something we confirm at the quote stage.

The reports we do sell, with their prices, are on our planning reports page. If your extension uses the larger home extension prior approval route instead, neighbours' objections bring the same light questions in; our prior approval guide explains how.

Related: What reports do I need for a house extension planning application in London?

Frequently asked questions

Is a daylight and sunlight report the same as a right to light? No. The report is a planning document about whether the council should grant permission. A right to light is a private legal right between neighbours, separate from planning, and a planning permission does not override it.

Can I draw the 45-degree line myself? You can sketch it to get a feel for your design. The officer will draw it on your submitted drawings, so the drawings need to show the neighbour's windows accurately, which is why they come from a measured survey rather than an estimate.

My single-storey extension crosses the 45-degree line. Will it be refused? Not automatically. Councils use the tests as screening tools, and Camden says it considers the results flexibly given the site's context. Crossing the line means the impact needs a closer look, sometimes through a report, sometimes through a design change such as a lower eaves height or a set-back from the boundary.

Does a loft dormer need a daylight report? Rarely. A rear dormer faces away from the houses either side, so the 45-degree test seldom applies. A mansard on a narrow street is different: check the 25-degree line from the windows opposite.