Extending next to a neighbour's conservatory

Building an extension next to a conservatory raises a question councils answer inconsistently, so it helps to know where the guidance actually stands. A conservatory is a lightweight glazed addition rather than an original habitable room, and the daylight guidance most authorities rely on directs attention to main living rooms, kitchens and bedrooms. That usually means a conservatory attracts less protection than a lounge window would. Less protection is not no protection, and a two-storey extension hard against the boundary can still be refused on the loss of light to a neighbour's conservatory if nobody thought about it at design stage.

How the guidance treats a conservatory

BRE 209, Site Layout Planning for Daylight and Sunlight, is the reference nearly every English council uses for neighbour daylight. Its recommendations are framed around rooms where occupants have a reasonable expectation of daylight, and the rooms it names are living rooms, kitchens and bedrooms.

Conservatories sit awkwardly in that scheme. They are usually additions to the original house, often built under permitted development, frequently used as circulation or as an occasional room, and they receive far more daylight than a conventional room to begin with because most of the roof is glass. Inspectors have taken a range of views. The pattern in householder appeals is that a conservatory is treated as a habitable room where it is clearly the main living space of the house, and given reduced weight where it is a secondary space added to a dwelling that already has a lounge and a kitchen.

Ask the practical question rather than the categorical one. What is the room actually used for, and does the house have an alternative?

The 45-degree and 25-degree tests

Two rules of thumb dominate householder cases, and they are not the same thing.

The 45-degree test comes from local supplementary planning documents rather than from BRE. Drawn in plan, a line is taken at 45 degrees from the mid-point of the nearest habitable room window on the neighbouring property. If the proposed extension crosses that line, the council treats loss of light as likely. Many authorities apply the same test in section as well, from the centre of the window at cill or mid-height.

The 25-degree test comes from BRE 209 itself. Drawn in section from the centre of the lowest window of the affected room, a line is taken at 25 degrees above the horizontal. If the new development stays below that line, the guidance says daylight is unlikely to be materially reduced and no further calculation is needed.

Check your council's householder design guide, because the version of the 45-degree test it applies, and the point on the window it measures from, vary and the difference changes the answer.

Why a glazed roof changes things

A conventional room takes its daylight through a vertical window, so the sky it can see is limited to what sits above the 25-degree line. A conservatory takes most of its light through the roof, which sees a much larger area of sky.

That cuts both ways. A single-storey extension alongside a conservatory often has little effect, because the roof glazing still sees plenty of sky above it. A two-storey extension, or a first-floor side extension on the boundary, can cut into that roof view badly, and the effect will not show up in a 45-degree test applied only to the vertical glazing.

Where the conservatory is against the boundary and the proposal goes above single storey, sketch the section properly. It takes twenty minutes and it is the difference between a scheme that survives consultation and one that does not.

The numbers a council may ask for

If the case goes past rules of thumb, the calculations in BRE 209 are the ones an assessment will report.

Vertical sky component measures the amount of skylight reaching the centre of a window on its outside face. The guidance suggests a window retaining at least 27 per cent, or at least 0.8 times its former value, is unlikely to be noticeably affected. Annual probable sunlight hours applies where the window faces within 90 degrees of due south, with reference figures of 25 per cent of annual probable sunlight hours and 5 per cent in the winter months, again with the 0.8 test on the reduction.

The no-sky line test looks at how much of the working plane inside the room can still see the sky, and it is the calculation that most often shows a real effect in a deep room.

A Daylight and Sunlight Assessment reports those figures room by room. Where the concern is a garden or patio rather than an interior, an Overshadowing Study using the 21 March sun path is the right tool, since BRE suggests at least half of an amenity area should receive at least two hours of sunlight on that date.

Design changes that resolve it

Most conservatory conflicts are solved on the drawing board.

• Set the flank wall back from the boundary by even 600mm, which opens the angle noticeably at close range.
• Reduce the depth of the two-storey element and keep the extra length single storey.
• Slope the roof away from the boundary with a hip rather than a gable.
• Move a first-floor side extension to the other flank where the plan allows.
• Where the neighbour's conservatory is old and failing, a negotiated relocation sometimes costs less than a redesign, though nothing about that can be secured through the planning application.

Talk to the neighbour before you submit. A conservatory owner who has seen the section drawing and understands that the effect is limited will usually not object, and an objection you avoided is worth more than an assessment you won.

Permitted development and the neighbour consultation scheme

None of this applies in the same way if the extension is permitted development. Schedule 2 Part 1 Class A of the General Permitted Development Order 2015 sets dimensional limits rather than amenity tests, and a compliant extension does not need permission whatever the effect on a neighbour's light.

The one exception is the larger home extension procedure. A single-storey rear extension beyond the standard 3m limit for a terraced or semi-detached house, or 4m for a detached house, up to 6m and 8m respectively, needs prior approval, and the council notifies adjoining owners. If an adjoining owner objects, the authority considers the impact on the amenity of adjoining premises, and light loss to a conservatory can be raised there.

Height limits still bind. A single-storey rear extension cannot exceed 4m in height, or 3m where it is within 2m of a boundary, and eaves height within 2m of a boundary is capped at 3m.

A lawful development certificate is the only way to confirm formally that works are permitted development, and it is worth the fee where a neighbour dispute is likely.

When an assessment is worth commissioning

For a single-storey rear extension within permitted development limits, almost never. For a two-storey extension or a first-floor side extension within a few metres of a neighbour's glazed room, it often is, particularly where the council's design guide applies a 45-degree test the scheme fails.

The least expensive sequence is to draw the 25-degree section first, and only commission calculations if the proposal breaks that line.

This is general guidance rather than legal advice. Check the position with your local planning authority or a qualified professional before relying on it.

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