Bat roost potential recorded during a tree survey

A bat roost potential tree survey note is one of the most consequential lines an arboriculturist writes. The tree survey itself is an arboricultural document prepared to BS 5837:2012, which is about trees in relation to design, demolition and construction rather than about wildlife. But a surveyor standing under a mature oak with binoculars will see the features that matter, and a note recording moderate roost potential on three trees will change your programme by a season.

Better to have that note than to find out after the chainsaw has started.

Why the arboriculturist records roost features at all

All bat species in the United Kingdom are European protected species. Under the Conservation of Habitats and Species Regulations 2017 it is an offence to deliberately capture, injure or kill a bat, and to damage or destroy a breeding site or resting place, whether or not a bat is present at the time.

A tree with a roost in it is a resting place. Felling it, or pruning out the limb that holds the feature, is capable of committing the offence even where nobody knew the roost was there, because the protection attaches to the structure rather than to the occupancy on the day.

So the arboriculturist notes what they see and flags it for an ecologist. That is not the same as carrying out a bat assessment. A Tree Survey records condition, category, root protection areas and constraints on layout. Where roost features are present, a separate ecological survey is needed, and a bat survey must be carried out by a suitably licensed ecologist. An AI-assisted draft of any such report is a starting point for that professional rather than a substitute for the survey itself.

Features that indicate potential

The features are structural, and once you know them they are visible from the ground on most trees.

• Woodpecker holes, which are the classic indicator on mature broadleaves.
• Knot holes and cavities left where a limb has been shed or cut.
• Vertical splits, frost cracks and lightning scars.
• Lifting or loose bark, particularly on dead standing limbs.
• Hollows at compression forks and in hazard beams.
• Cavities behind dense ivy, which also conceals everything else.
• Old wounds, cankers and decay columns with an entry point.

Size is not a reliable filter. Bats use gaps of a few millimetres, and a crevice roost can be almost invisible from below. Staining, scratch marks around an entrance, droppings on bark beneath a feature and an audible response on a detector are all supporting signs, but their absence proves nothing from ground level.

Age and species influence likelihood. Veteran and over-mature trees carry more features than young ones, and ash, oak, beech and poplar commonly develop the cavities that suit crevice-dwelling species. A recently planted amenity tree rarely does.

Categorising potential and what follows

Assessment against the published good practice guidelines produces a suitability category for each tree, running from negligible through low and moderate to high. The categories are about the features present and the habitat context, not about whether bats were seen.

The number of dusk emergence or dawn re-entry surveys required rises with the category. Take the figures from the current edition of the guidelines rather than from an older report, because the survey effort recommendations have been revised.

Where several trees are affected, the survey effort is per tree, not per site. A scheme removing eight trees with moderate potential is a substantially larger survey exercise than one removing a single tree. The practical implications are set out in checking trees for bat roost features before felling.

Ground level assessment against climbing inspection

A ground level assessment uses binoculars, a torch and experience. It is quick, it costs little to commission, and it will miss features on the far side of a crown or above the first fork.

An aerial or climbing inspection puts an ecologist with the right training into the tree, with an endoscope, to look inside the features that were identified from below. It can confirm that a cavity is shallow and unsuitable, which removes the tree from the programme entirely, or it can confirm evidence of use, which escalates it.

Climbing is the better investment on any tree that is central to the scheme. It is also constrained: the tree must be structurally safe to climb, the inspection must not disturb a roost, and the ecologist carrying it out needs the appropriate licence to disturb where a roost is found. On a tree in poor condition, a mobile work platform may be the only safe option and that changes the access requirements on site.

One honest caveat. A negative climbing inspection lowers the risk but does not remove it, because bats move between roosts through the season and a feature empty in March may be occupied in June.

Survey seasons for emergence work

Emergence and re-entry surveys are seasonal. The active season runs from around May to September, with the core period for maternity roost detection in the earlier part of that window.

Outside the season, an ecologist can carry out an assessment of the features and a climbing inspection, but cannot complete emergence surveys. That is the constraint that delays schemes. A tree survey done in November that identifies moderate potential leaves you waiting until the following May for the survey work that supports a decision.

Two planning consequences follow. First, commission the arboricultural and ecological work together at the start rather than in sequence, so the roost question is raised in the first winter rather than the second. A Preliminary Ecological Appraisal run alongside the tree survey does this at modest cost. Second, resist the temptation to submit without the survey on the basis that a condition can secure it. Where the survey could determine whether the scheme is acceptable at all, national practice guidance is clear that it should be carried out before the application is determined, not afterwards.

Licensing where a roost is confirmed

A confirmed roost that will be lost or disturbed requires a licence from the statutory nature conservation body before the works proceed. The licence is applied for after planning permission is granted, because the licensing test includes whether there is a satisfactory alternative and whether the works serve an overriding public interest, and that assessment relies on the permission.

The application needs a method statement: the survey results, the roost type and species, the proposed timing, the mitigation and compensation, the monitoring, and the named ecologist who will supervise. Compensation for a tree roost usually means bat boxes or purpose-built features, sited and oriented to the ecologist's specification, installed before the roost is lost.

Programme it properly. Licence determination takes weeks, the works then have to sit inside the seasonal window the licence allows, and the mitigation has to be in place beforehand. A Bat Survey report that stops at findings, without a mitigation strategy, is not enough to get a licence application moving.

Sequencing tree works around the findings

Where the assessment allows works to proceed without a licence, the method is what protects you.

Soft felling, also called sectional dismantling under supervision, involves taking limbs down in sections, lowering them gently rather than dropping them, leaving sections on the ground overnight before processing, and stopping immediately if a bat is found. It is appropriate where suitability is low and an ecologist has signed off the approach. It is not a substitute for a licence on a tree with a confirmed roost.

Timing helps as well. Works outside the maternity period and outside hibernation reduce risk, though no period is risk free, since bats use tree roosts year round.

Set all of this out in the submitted documents rather than leaving it to a condition drafted by someone who has not seen the trees. Name the trees by their survey reference, state the category, state the method and state who will supervise. Where the retained trees also need protection during construction, the measures belong in the same document set as the tree protection plan.

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. To see which tree and ecology documents your site is likely to need, enter your postcode on PlanScope.