Timing a tree survey: what can be done in winter

A tree survey for a planning application can be carried out in winter. Unlike bat emergence surveys or botanical condition assessments, arboricultural survey work to the British Standard for trees in relation to design, demolition and construction is not locked to a season, and the measurements that shape your layout are all available with the trees out of leaf.

What winter costs you is a reliable judgement about vitality, and confidence on species where identification depends on foliage. Both matter for some trees on some sites, and neither is a reason to hold a project for four months.

What an out of leaf survey records

The survey produces a schedule of trees and a tree constraints plan. The inputs to both are largely structural.

Stem diameter is measured at 1.5 metres above ground level, and the root protection area is calculated from it. That calculation is the single most important output for a site layout, because it defines where you cannot build, and it works identically in January and July.

Height, crown spread and clearance are measured or estimated, and crown spread is measured to the branch tips, which are easier to see without leaves. Structural defects come into view as well: included bark at forks, cavities, old pruning wounds, deadwood, and asymmetry in the branch architecture. Winter is a good time to see how a tree is actually built.

Categorisation follows. Trees are placed in category U, where removal is recommended regardless of development, or in categories A, B and C by quality and remaining contribution. That judgement uses structural condition, life expectancy and amenity value, and it is mostly available out of leaf.

What needs a visit in leaf

Three things do need the growing season.

Vitality is the main one. Leaf size, density, colour and distribution through the crown are how an arboriculturist judges whether a tree is doing well, and a bare crown in February tells you much less. A tree recorded as category B in winter may be a declining specimen in July.

Species identification is the second, and it matters more than people expect. Bark, bud and twig characters identify most trees to species, but hybrids, ornamental cultivars and some willows and whitebeams are difficult without leaves or flowers. Species drives the root protection area calculation only indirectly, though it drives the tolerance of the tree to construction impact quite directly.

Third, decline and disease. Ash dieback is the common case: dieback, lesions and epicormic growth are visible year round, and quantifying crown loss is more reliable in leaf. A winter report on a declining ash population usually recommends a summer re-inspection before felling decisions are finalised, and that is sound advice rather than a way of selling another visit.

Decay detection and specialist techniques

Where a visual inspection raises a question about structural integrity, the follow-up work is not seasonal.

Resistance drilling and sonic tomography both work in winter, and an aerial inspection by a climbing arborist can be carried out whenever conditions are safe. For trees near a proposed building, this is worth doing early, because a tree that turns out to be structurally unsound changes the layout in a helpful direction rather than an awkward one.

Fungal fruiting bodies are the exception that favours autumn. Many decay fungi fruit between late summer and the first frosts, and their presence, species and position on the stem tell an arboriculturist a great deal. A survey in February may miss brackets that were obvious in October. Note the limitation and, on a site where veteran trees are being retained close to new structures, plan an autumn walkover.

How long a survey stays current

There is no statutory validity period. In practice most local planning authorities expect an arboricultural survey to be around twelve months old or less at the point of validation, and the local checklist sometimes says so.

Trees change slowly, and the reasons a survey goes stale are usually events rather than time: a storm, a neighbour's felling, the death of an ash, or works on the site. Where the survey is ageing and nothing has happened, a re-inspection letter confirming that the schedule remains accurate is normally accepted and takes an hour on site.

Build the timing into the programme. A survey commissioned in November for an application submitted the following October will need refreshing, and the refresh is far easier to arrange before the case officer asks for it.

Coordinating with ecology survey seasons

Tree work and ecology overlap on two fronts, and the seasons do not align.

Trees with potential roost features for bats can be identified during a ground level assessment at any time of year, and a winter tree survey is a good opportunity to flag them. What cannot be done in winter is the emergence and re-entry survey work that follows, which runs through the summer months. If the survey identifies features on trees you intend to fell, that constraint lands on the programme immediately, which is why a bat survey assessment early in the winter is worth more than one commissioned in April.

Nesting birds constrain the works rather than the survey. And where vegetation clearance is planned, the tree survey should identify which trees and hedges are affected so that the clearance method statement can be written once.

Planning the commission around the application date

Work backwards from the date you intend to submit.

1. Topographical survey and tree survey together, on the same base drawing, before the layout is designed.
2. Tree constraints plan produced and given to the designer.
3. Layout developed with root protection areas shown, and shading and daylight considered, since retained trees and daylight to habitable rooms produce more post-occupation complaints than most people expect.
4. Arboricultural impact assessment and tree protection plan prepared against the near-final layout.
5. Any specialist inspection or seasonal re-visit completed before submission.

Reversing steps one and three is the common error, and it produces a tree survey used to justify a layout rather than inform it. Case officers and tree officers recognise the difference.

Refreshing an ageing survey

Where an existing survey is out of date, three options exist, in ascending order of cost.

A re-inspection letter, where the arboriculturist walks the site, confirms the schedule and notes any changes. A partial update, where only the trees affected by the proposal are re-surveyed and the rest are carried forward with a stated caveat. Or a full re-survey, which is the right answer where the site has changed materially or where the original survey was thin.

An illustrative example. A self-build plot surveyed in December carried two mature ash on the boundary, categorised B. A summer re-inspection before submission found both had lost more than half their crowns. The trees moved to category U, the layout gained three metres of usable garden, and the replacement planting condition was agreed without argument. The survey did its job twice.

Where a survey supports an application to work on protected trees, the specification of the works matters as much as the survey itself, which is the point covered in specifying crown works councils will accept.

This is general guidance, not legal advice; check the position with your local planning authority or a qualified professional before relying on it. Arboricultural survey and impact assessment should be carried out by a suitably qualified arboriculturist, and an AI-assisted draft is a starting point for that professional rather than a substitute.

To see which tree and ecology documents a plot is likely to need, check the plot on PlanScope with its postcode.

PlanScope provides AI-assisted draft reports for planning applications. Reports should be reviewed before submission. We do not guarantee planning approval.