Preliminary roost assessment: the first step in a bat survey

A preliminary roost assessment is the inspection that decides whether your project needs bat surveys at all, and if so how many and in which months. An ecologist looks at the building or tree, records the features bats could use, and puts the structure into a suitability category. Everything that follows in the programme flows from that single judgement.

It can be done in November. The surveys it triggers cannot, and that gap is why the assessment is worth booking as early as the design allows.

What the assessment covers

A preliminary roost assessment combines an external inspection with an internal one where access allows. The ecologist works over the elevations and the roof void looking for the features bats exploit: gaps at the verge and ridge, lifted tiles, soffit and fascia voids, cavity walls with open perpends, cracked render, hanging tiles, and the underside of lead flashing.

Inside a roof space they look for droppings, feeding remains, staining on timbers, and the smooth polished surfaces bats leave at access points. For trees the equivalent exercise is a ground-level assessment of potential roost features, usually with close-focusing binoculars and a torch: woodpecker holes, tear-outs, splits along limbs, lifted bark and old cavities.

The output is a report describing the structure, listing the features, giving a suitability category and setting out what further survey is recommended. That report is the document the council's ecologist reads, so it needs the survey date, the weather, the areas that could not be accessed and the reason.

Suitability categories and what they trigger

The Bat Conservation Trust's good practice guidelines set out a scale that most English ecologists use, running from negligible through low and moderate to high suitability. Categories are not a scoring exercise for its own sake. They set survey effort.

Confirmed roost evidence found during the assessment changes the question entirely. At that point the issue is no longer whether bats are present but which species, how the roost is used and what licence the works will need.

The guidelines are guidance, not law, and a local authority ecologist can ask for more where the context justifies it. A moderate-suitability barn adjacent to woodland in a county with rare species records will attract more effort than a moderate-suitability suburban garage.

Why the assessment runs year round

Nothing in a preliminary roost assessment depends on bats being active. The features are structural, and the field signs, droppings and staining, persist through the winter. So the assessment can be done in any month, in most weather, at short notice.

That makes it the sensible first move on any building older than a few decades, any structure with a roof void, and any scheme involving demolition, re-roofing, soffit replacement or the felling of mature trees. A Preliminary Ecological Appraisal often includes the building assessment as one component, which is the efficient way to buy it on a site with several constraints.

The one caveat is access. An assessment that could not reach the roof void, or was done from ground level only on a three-storey building, will be qualified, and a qualified report gets challenged. Arrange access and, where the elevations are high, a means of inspecting them properly.

Emergence and re-entry survey windows

The surveys the assessment triggers are seasonal. Bats are active from roughly May to September, with the optimal window for emergence and re-entry surveys running May to August, and surveys outside that period carry little weight because a negative result proves nothing about a roost that was empty for the season.

A dusk emergence survey starts before sunset and runs into darkness, with surveyors positioned so every elevation is watched. A dawn re-entry survey works the reverse pattern. Where two or three surveys are required they are spread across the season rather than run in consecutive nights, and weather conditions matter: cold, wet or windy nights produce unreliable results and are normally abandoned.

The practical consequence is arithmetic. A high-suitability building assessed in September will not have its three surveys complete until August the following year at the earliest, and often the year after that if the September window is missed.

Licensing and the competent person rule

All UK bat species are European protected species under the Conservation of Habitats and Species Regulations 2017, and are also protected under the Wildlife and Countryside Act 1981. It is an offence to deliberately capture, injure or kill a bat, to damage or destroy a breeding site or resting place, or to disturb bats in a way that affects their ability to survive or breed.

Surveys that involve disturbing bats, and any works affecting a roost, require a licence, and licences are issued to suitably licensed ecologists. This is a category where a licensed professional is legally required rather than merely advised, and any AI-assisted draft is a starting point for that ecologist, never a substitute for their survey or their judgement.

Where a roost will be affected, the local planning authority has to consider the three derogation tests before granting permission: that there is no satisfactory alternative, that the works serve an imperative reason of overriding public interest or the interests of public health and safety, and that the population will remain at favourable conservation status. Getting that reasoning into the Protected Species Survey rather than leaving it to the licence application is what keeps the planning decision clean.

What a finding does to your programme

Three outcomes, three different projects.

Negligible suitability closes the issue. The report goes in with the application and the ecology objection never arrives.

Low or moderate suitability with a negative survey result produces a report recommending precautionary working methods, usually a condition requiring a method statement and often bat boxes as enhancement. That is a manageable outcome and adds weeks rather than seasons.

An illustrative case shows how the middle outcome behaves. A 1930s bungalow with a concrete tile roof, no ridge gaps and a sealed soffit is assessed in February and comes out as low suitability on the strength of two lifted tiles at the verge. One dusk survey in June records no emergence. The report recommends that the roof strip is done by hand under an agreed method statement, with work stopping if a bat is found, and two boxes fixed to the retained gable. That package costs a few weeks and satisfies the council's ecologist. The same building assessed as high suitability would have needed three visits between May and August, which is a different project on a different timescale.

A confirmed roost puts a licence on the critical path. The licence application follows planning permission, needs a mitigation design, and Natural England has its own determination period. Build that into the programme from the day the roost is found rather than the day the contractor asks.

For the survey side of a project involving buildings or mature trees, the Bat Survey page sets out what the work covers and who is expected to carry it out. Enter your postcode on PlanScope to see which ecology documents your site 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.