Assessing tree shading in a daylight and sunlight study
Vegetation is excluded from daylight calculations by default because it is neither permanent nor opaque. Where retained trees change the answer, model both scenarios and state the transmittance you used.
Trees are usually left out of daylight and sunlight models, and that is the default position in BRE 209 rather than a shortcut. Vegetation is not permanent, it can be felled without warning, it lets a variable amount of light through, and its density changes with the season. So the standard assessment models buildings and treats the existing canopy as absent. Where a scheme depends on trees being there, or depends on them being gone, the assessment has to say so and show both cases.
Whether trees are modelled at all
The 2022 edition of BRE 209 takes the view that trees and shrubs need not be included as obstructions in daylight and sunlight calculations, because their permanence and their transmittance cannot be relied upon. That position is sensible and it is also convenient, since a modelled canopy is the least reliable object in any model.
Two situations pull the other way. A large dense evergreen group, a mature holly hedge or a row of conifers behaves much more like a solid obstruction than a single deciduous specimen, and ignoring it can produce a result that plainly contradicts what a resident experiences. And a sun on ground assessment of a new amenity space is misleading if it shows a lawn in full March sun that in reality sits beneath a retained oak.
The honest approach is to state the decision and its reason at the front of the report. One paragraph. Something to the effect that vegetation has been excluded from the window assessment in line with BRE 209, and included in the amenity space assessment as a separate scenario because retained trees materially affect the result.
Deciduous and evergreen treated differently
The distinction is not decorative. It changes what you can defend.
An evergreen canopy is present all year, including at the equinox and midwinter dates that drive the sunlight criteria, and its transmittance is low and fairly stable. Where an evergreen group is retained and protected, modelling it is arguable.
A deciduous canopy is in full leaf from roughly May to October and bare for the rest of the year. The sunlight criteria bite hardest in the winter period, defined in BRE 209 as 21 September to 21 March, which is precisely when a deciduous tree is doing least. So modelling summer foliage on a deciduous tree and then reporting winter sunlight figures mixes two conditions that never coexist.
Where trees are retained under a Tree Preservation Order, or stand within a conservation area, their permanence is better than a tree with no protection at all. Even then, a consent for works can be granted, and a protected tree can be lost to disease. The Arboricultural Impact Assessment prepared for the same application is the document that tells you which trees have protection, what condition they are in, and what their likely lifespan is, and it should be read before the daylight model is built.
Transmittance assumptions and their justification
If you do model vegetation, you have to give the canopy a light transmittance value, and there is no authoritative published set for UK practice. That is an uncomfortable position for a consultant and it should be admitted rather than hidden.
Values used in practice run from a low figure for a dense evergreen or a summer canopy in full leaf, through a middling figure for open-crowned deciduous species in leaf, to a high figure for bare winter branches where most of the aperture is open. Whatever you adopt, three things belong in the report:
- The value used, per species or per canopy group, with the source or the reasoning.
- The season the value stands for, matched to the date being modelled.
- A run with the canopy removed entirely, so the reader can see the sensitivity.
A reviewer will accept a documented assumption with a sensitivity run. A reviewer will not accept a transmittance value with no provenance that happens to be exactly what the scheme needs.
Canopy geometry deserves the same treatment. A tree modelled as a solid block at full mature spread is a different obstruction from the same tree modelled at its surveyed crown dimensions, and the survey dimensions are in the arboricultural report already.
Modelling with and without vegetation
Running both scenarios is more work and it is almost always the right answer, because it removes the argument rather than inviting it.
For window assessments on neighbouring property, report the no-vegetation case as the primary result, since that is the BRE default and the more onerous test. Then report the with-vegetation case as context where there is a substantial retained canopy, with the transmittance stated.
For sun on ground in the proposed scheme's own amenity space, invert that. Report the with-vegetation case as the primary result, because a resident stands under the tree that is actually there, and give the no-vegetation case as the position if the trees were lost. An Overshadowing Study that ignores a retained mature canopy over the only communal garden is answering the wrong question.
Label the scenarios in the drawing titles rather than in a footnote. Plots get extracted from reports and circulated on their own.
Trees that may be removed by others
This is where assessments get into trouble. A tree on third-party land can be felled by its owner at any time unless it is protected, and a daylight result that relies on that tree to screen an impact is built on something you do not control.
So the rule is asymmetric, and deliberately so. Do not rely on a third-party tree as an obstruction that reduces your scheme's apparent impact. Do not rely on a third-party tree to screen overlooking either, which is the same error in a different discipline.
The reverse case, trees on your own site that you propose to remove, is simpler. Model the proposed condition without them, because they will not be there. And where new planting is proposed, do not model it as mature, since a nursery-stock tree takes many years to reach the crown spread shown on the drawing. Our note on tree planting conditions: species, stock size and establishment covers how stock size and establishment periods are secured.
How councils respond to vegetation assumptions
Officers and their retained consultants tend to look for one thing: whether the vegetation assumption moves any result across a threshold. If it does not, the point is academic and the report is accepted. If it does, expect a request for the alternative run.
Three responses recur. A request to remove all vegetation and re-report, which is the safest request an officer can make. A request to model the trees where residents have objected on the basis of losing a treed outlook, which is really an amenity point rather than a daylight one. And a request to reconcile the daylight model's tree positions with the arboricultural plan, because the two documents are frequently drawn from different surveys.
That last one is avoidable. Use the same topographical and tree survey for both, and say in each document that you have.
Presenting both scenarios
A clean presentation is short. A table listing each assessed window or amenity space, with the result in the no-vegetation case and the with-vegetation case side by side, and a difference column. Then the drawings, titled with the scenario.
Add a two-line statement of the tree data source: the survey date, the surveyor, and whether crown dimensions are as-surveyed or projected. Record protection status in the same table, because a protected tree and an unprotected one carry different weight in the argument even when they cast the same shadow.
And say plainly where the answer is uncertain. A retained veteran tree in declining condition is an obstruction today and may not be in fifteen years, and a Daylight and Sunlight Assessment that acknowledges that reads better than one that pretends otherwise.
PlanScope provides AI-assisted draft reports for planning applications. Reports should be reviewed before submission. We do not guarantee planning approval. Daylight modelling and arboricultural assessment are both specialist fields, so councils expect suitably qualified authors and professional review is strongly advised.
Enter your postcode on PlanScope to see which tree and daylight reports your scheme is likely to need.
Frequently asked questions
Does BRE 209 require trees to be modelled?
No. The 2022 edition takes the view that trees and shrubs need not be included as obstructions, because their permanence and light transmittance cannot be relied on. Large dense evergreen groups are the exception worth considering, and retained trees over a scheme's own amenity space should be modelled if the sun on ground result would otherwise be misleading.
What transmittance value should I use for a tree canopy?
There is no authoritative published set for UK practice, so state the value, the species or canopy group it applies to, the season it covers and your reasoning. Always include a run with the canopy removed so the reader can see the sensitivity. A value with no provenance that happens to deliver compliance will not survive review.
Can I rely on a neighbour's tree as an obstruction?
No. A tree on third-party land can be felled by its owner at any time unless protected by a Tree Preservation Order or conservation area status, so a result that depends on it is built on something outside your control. The same applies to relying on a neighbour's tree to screen overlooking, which is the same error in another discipline.
How are deciduous trees handled for winter sunlight figures?
BRE 209 defines the winter period as 21 September to 21 March, which is when a deciduous canopy is largely bare. Modelling full summer foliage and then reporting winter sunlight results combines two conditions that never occur together. Match the transmittance to the date being modelled, or exclude the canopy and say so.
Should proposed new planting be modelled as mature?
No. Nursery stock takes many years to reach the crown spread shown on a planting drawing, so modelling it at maturity overstates screening and shading that will not exist at occupation. Model the proposed condition as built, and handle establishment separately through the planting condition and its stock size and aftercare requirements.
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.