Sunlight to a neighbouring conservatory
Loss of light to a conservatory usually carries less weight than to a living room, but not none. How to assess it honestly and design the extension that gets permitted.
If your rear extension will overshadow the neighbour's conservatory, the position is better for you than it first looks. A conservatory is not normally treated as a main habitable room for the purposes of the BRE guidance on site layout planning for daylight and sunlight, so loss of light to it usually carries less weight than loss of light to a living room window. That is not a free pass. Officers still consider amenity, and a conservatory that is in daily use as a family room will attract real attention.
The useful thing is to deal with it openly in the application rather than hoping nobody mentions it. A short, honest assessment is worth more than silence, because silence invites the neighbour's own version of the numbers.
Whether a conservatory is a habitable room for assessment
The BRE guidance applies its tests by room use, with main living rooms carrying the most weight, then kitchens and dining rooms, then bedrooms, and with bathrooms, utility rooms and circulation not assessed at all. A conservatory sits awkwardly across those categories. It is often used as a dining or sitting space, but it is also a lightly built addition with glazing on three sides and a glazed roof, which means it starts with far more daylight than a room in the main house.
Two questions decide how much weight it gets. Is it in use as a principal living space, or is it a seasonal room, a plant room, a utility space or a route to the garden? And was it designed and built as a thermally separated addition, with doors between it and the house, which points away from it being a main room?
So the position to take in the application is a reasoned one: identify what the conservatory appears to be used for, state the basis for that view, and assess it as a secondary space while acknowledging the alternative. Do not simply assert that conservatories do not count.
How a glazed roof changes the analysis
This is the technical heart of it. Daylight to a conventional room comes through a vertical window, so an obstruction opposite that window has a direct effect. A conservatory receives light through its roof as well as its walls, and roof glazing sees a much larger area of sky.
The consequence is that a single storey extension on the shared boundary reduces the light reaching one vertical face of the conservatory while leaving the roof largely unaffected. The usual vertical sky component test, measured at the centre of a window, does not capture that. An assessment that applies it mechanically to a conservatory wall will overstate the harm, and one that ignores the roof entirely is incomplete.
| Test | What it measures | Relevance to a conservatory |
|---|---|---|
| Vertical sky component | Sky visible from the centre of a vertical window | Applies to the affected wall, but understates total daylight |
| Daylight distribution, the no-sky line | How far into the room sky light reaches | Of limited use where the roof is glazed |
| Annual probable sunlight hours | Sunlight received by a window facing within 90 degrees of south | The test that matters most here |
| Sun on ground | Proportion of amenity space receiving sun on 21 March | Often the more relevant question for the garden |
| Council 45 degree or 25 degree rule | Simple geometric screening test | Usually what the officer applies first |
In practice most councils start with their own geometric rule, taken from a householder design guide, and only look at the detailed tests if the simple one is failed. Find out which rule your authority uses before you fix the depth of the extension. Our note on daylight for a single storey rear extension on a bungalow works through the same geometry on a lower building.
Applying the sunlight benchmark
The sunlight test in the BRE guidance applies to main living rooms with windows facing within 90 degrees of due south. A window is considered to be adversely affected where it receives less than 25 per cent of annual probable sunlight hours, including less than 5 per cent during the winter months from 21 September to 21 March, and the figure falls below 0.8 times its former value in either period, and the reduction over the whole year is more than 4 per cent of annual probable sunlight hours.
All of those limbs have to be satisfied. A conservatory that loses sunlight in absolute terms but keeps more than 25 per cent of annual probable sunlight hours is not adversely affected on the guidance.
Two points that come up repeatedly. Where the conservatory faces north or within 90 degrees of north, the sunlight test does not apply at all, which disposes of a surprising number of objections. And where the conservatory has glazing on more than one elevation, the better reading is to assess the elevation actually affected and to say what the others contribute, rather than averaging. The combination question is covered in our note on when a neighbour's room is served by two windows.
Whether the conservatory itself was lawful
Worth checking, and not for the reason people assume. A conservatory built under permitted development is perfectly lawful and remains part of the dwelling, so its lawfulness does not usually determine the weight given to it.
What does matter is where it was put. A neighbour who built a conservatory hard against the shared boundary, or extended to the rear of their own building line, has reduced their own light and has placed glazing in a position that any ordinary development on the adjoining plot would affect. Inspectors have been willing to say that a householder cannot create a sensitive receptor by building close to a boundary and then rely on it to prevent reasonable development next door.
Check the council's public register for the address. If the conservatory was approved, the approved drawings will show its position and dimensions, which is far better evidence than measuring from your side of the fence. If it was permitted development, the dimensions still tell you whether it sat within the limits at the time.
An illustrative case on a semi-detached pair
Take a pair of 1930s semi-detached houses. Number 12 has a 3 metre deep conservatory across half its rear elevation, built against the shared boundary, with its glazed roof sloping away from the boundary. Number 14 proposes a single storey rear extension 4 metres deep and 3 metres high to the eaves, on the boundary.
Measured against the council's 45 degree rule in plan from the nearest edge of the conservatory's rear glazing, the proposal breaches it. Measured against the sunlight test, the conservatory faces north-east, so the annual probable sunlight hours test does not apply. The vertical sky component at the affected side glazing falls below the benchmark, but the roof glazing continues to provide most of the daylight to the space.
The scheme that was permitted reduced the depth to 3.2 metres for the first 1.5 metres from the boundary, then stepped out to 4 metres, with the roof sloping away. The daylight loss to the side glazing became marginal and the officer accepted it. This example is illustrative and the outcome on your site will depend on orientation and your council's own guidance.
Negotiating a reduced depth
Most of these disputes settle on geometry rather than argument. The moves that work, in rough order of cost:
- Step the extension in from the boundary for the first metre or two, which removes the worst of the obstruction angle at little loss of floor area.
- Slope or hip the roof away from the boundary rather than running a flat parapet.
- Reduce the eaves height on the boundary side and take the extra headroom in the middle of the plan.
- Move glazing to a roof light rather than a rear wall window, which lets you keep depth while reducing the apparent bulk.
- Reduce the depth for the whole extension, which is the last resort rather than the first.
Talk to the neighbour before submitting, with the stepped option drawn. A neighbour shown two drawings, one of which they prefer, objects differently from one shown a single drawing they were not consulted about.
What to include in the supporting statement
For a householder application, keep it short and specific:
- The orientation of the conservatory's glazing, stated by compass direction, with a note of whether the sunlight test applies at all.
- The conservatory's position relative to the boundary and to the host building line, with the source of the dimensions.
- Your view of its use, with the basis for that view.
- The result of the council's own geometric rule, worked and shown on a section.
- Where that rule is breached, a proportionate assessment of the detailed tests, acknowledging the contribution of the roof glazing.
- The design response, with the stepped or sloped option drawn.
Where the garden rather than the room is the concern, an Overshadowing Study addressing sun on ground is the more useful document. Where a full technical assessment is warranted, a Daylight and Sunlight Assessment is a report type for which councils expect a suitably qualified author, and professional review is strongly advised. PlanScope can draft the supporting narrative and set out the assumptions, with flags where measurements are still needed.
This is general guidance, not legal advice; check the position with your local planning authority or a qualified professional before relying on it. Enter your postcode on PlanScope to see which reports your project is likely to need.
PlanScope provides AI-assisted draft reports for planning applications. Reports should be reviewed before submission. We do not guarantee planning approval.
Frequently asked questions
Does a conservatory count as a habitable room for daylight assessment?
Not normally as a main habitable room. The BRE guidance weights main living rooms most heavily, then kitchens and dining rooms, then bedrooms, and excludes bathrooms and circulation. A conservatory sits across those categories: it is often used as a sitting space but is glazed on three sides and overhead, so it starts with far more daylight than a room in the house.
Why does the glazed roof matter?
Because most of the daylight reaching a conservatory comes through the roof, which sees a much larger area of sky than a vertical window does. A single storey extension on the boundary obstructs one wall while leaving the roof largely unaffected, so applying the vertical sky component test mechanically to the affected wall overstates the harm.
Does the sunlight test always apply?
No. The annual probable sunlight hours test applies only to windows facing within 90 degrees of due south. Where the conservatory glazing faces north or within 90 degrees of north, the test does not apply at all, which disposes of a good number of objections before any modelling is needed.
Does it matter whether the conservatory had permission?
Less than people expect. A conservatory built under permitted development is lawful and remains part of the dwelling. What matters is its position: a neighbour who built hard against the shared boundary has reduced their own light and placed glazing where ordinary development next door would affect it, and inspectors have given weight to that point.
What design change is most likely to make the scheme acceptable?
Stepping the extension in from the boundary for the first metre or two, which removes the sharpest part of the obstruction angle at very little cost in floor area. Sloping or hipping the roof away from the boundary and dropping the eaves height on that side do most of the remaining work. Reducing the overall depth should be the last resort.
Sources and further reading
This article is general guidance about the planning system in England and, where stated, the rest of the UK. It is not legal or professional advice. Check your local planning authority’s validation requirements and take professional advice for your specific site.