Light to a neighbour's conservatory: how it is treated

A neighbour's conservatory usually carries less weight in a daylight assessment than a window to a living room or a bedroom, because it is not a main habitable room and because most of its light arrives through the roof rather than the sides. That is the starting point for conservatory daylight questions, and it is why a side extension that would be refused next to a kitchen window is sometimes acceptable next to a lean-to glazed addition.

It is a starting point, not an answer. Two situations turn it around, and both come up often.

Is a conservatory a habitable room

In planning terms a conservatory is normally treated as an addition rather than a main habitable room. The Building Research Establishment guidance on site layout planning for daylight and sunlight is written around the windows of main habitable rooms, which means living rooms, dining rooms, kitchens where the household sits, and bedrooms.

Local design guidance often puts this in writing. Where your council has adopted a householder design guide or a residential extensions document, look for the paragraph on conservatories, because some say expressly that they will be given reduced weight in assessing loss of light. Read the guide for your area rather than assuming the position.

There is a fairness point underneath this that officers do acknowledge. Many conservatories were built as permitted development, with no assessment of their own light and no neighbour consultation. A structure that was never assessed is a weak basis for restricting someone else's extension.

How glazed additions are treated in assessment

The standard first test for loss of daylight is the 25 degree line. From the centre of the lowest window on the affected elevation, draw a line at 25 degrees above the horizontal towards the proposed obstruction. If the obstruction stays below the line, daylight is unlikely to be materially affected.

For a conservatory with a glazed roof, applying that test to a side pane understates how the structure actually gets its light. A lean-to roof pitched towards the sky collects daylight across its whole area, so an obstruction to one side removes a smaller share of the total than it would from a vertical window in a solid wall.

Many councils also apply a 45 degree test in plan and section, drawn from the nearest edge of the neighbour's nearest habitable room window, as a simple screen for overbearing and light loss. Where the conservatory is not treated as a habitable room, that line is often drawn from the window behind it instead, which is a different and usually more generous starting point for the applicant.

Side glazing and the direction light comes from

The orientation of the conservatory decides how much the extension matters.

• A conservatory with its long glazed side facing the proposed extension, and a solid wall on the opposite side, loses a meaningful proportion of its light.
• A conservatory glazed on three sides loses less, because light continues to arrive from the other two.
• A conservatory on the south side of a house, with the extension to the east, loses morning sun but keeps the main part of the day.
• A north facing conservatory has little direct sun to lose, so the question is diffuse daylight rather than sunlight.

Sunlight to the garden is a separate matter again, measured on 21 March as the proportion of the amenity area receiving a given period of direct sun, and set out in an Overshadowing Study where the effect is in dispute.

Where the effect is still material

Two situations change the answer.

The first is the inner room. Where the conservatory was built across the back of the house and the original rear window now opens into it, the room behind borrows all its daylight through the conservatory. An obstruction to the conservatory then reduces light to a genuine habitable room two steps removed, and that chain has to be assessed rather than dismissed. Draw it in section and the issue becomes obvious.

The second is proximity and height. A two storey extension built up to a boundary, with a conservatory one and a half metres away on the other side, produces a wall of brick along the whole glazed flank. Whatever the technical reading of the guidance, an officer looking at that relationship is thinking about enclosure and outlook as well as daylight, and those are material considerations in their own right.

An illustrative example: a 1930s semi with a 3 metre lean-to conservatory along the rear, and the neighbour proposing a two storey side and rear extension within a metre of the boundary. The conservatory keeps its roof light and most of its far side glazing. But the kitchen behind it, which has no other window, loses a clear share of its daylight. The useful response is a section through both properties showing the kitchen window, the conservatory roof and the proposed flank wall, with the 25 degree line drawn from the kitchen window rather than the conservatory.

Sorting it out between neighbours

Most of these cases settle on the drawing board.

1. Set the extension back from the boundary for the length that runs alongside the conservatory, even by half a metre.
2. Reduce the depth of the two storey element and keep the single storey part further back.
3. Change the roof form, since a hipped or sloped roof near the boundary removes a surprising amount of mass from the view out of the conservatory.
4. Offer rooflights or high level glazing in the extension in exchange for the neighbour's support, which sometimes matters more to them than the technical result.

Rights of light are a separate civil matter with their own law and their own remedies, and a conservatory can acquire them. The distinction is explained in the note on rights of light against planning amenity, and it is worth understanding before anyone starts quoting the twenty year rule.

What to submit

Keep it to drawings and a short note. The material that answers the question:

• A plan showing both properties, the conservatory, the proposed extension and the dimension between them.
• A section cut through the boundary, the conservatory and any window to the room behind it, with the 25 degree line drawn from the correct window.
• A note stating the use of the affected rooms, whether the conservatory is a main habitable room, and whether the room behind it has any other window.
• Photographs from inside the conservatory and the room behind, if the neighbour gives access.

Where the section shows the line broken for a genuine habitable room, a Daylight and Sunlight Assessment with vertical sky component figures is the proportionate next step. Where it is not broken, say so with the drawing rather than arguing about it in prose.

If you are the neighbour rather than the applicant, the structure of an objection that officers can act on is set out in the note on answering a neighbour daylight objection. An objection that says the conservatory will be dark carries little weight. One with a measured section carries real weight.

Enter your postcode on PlanScope to see which reports an extension next to a glazed neighbouring addition is likely to need.

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.