What a preliminary roost assessment can conclude

A preliminary roost assessment can tell you that bats are present. It can tell you that a building has no features capable of supporting a roost. What it cannot do, on its own, is tell you that bats are absent from a building that looks suitable for them. That asymmetry is the whole of the argument you will have with a council ecologist, and it is worth understanding before you commission the work rather than after.

For an architect, the practical consequence is a programme one. An inspection can be carried out on any dry day of the year. The surveys it may trigger can only be done between May and September. An assessment in October that finds moderate suitability puts the planning submission into the following summer unless the design changes.

What the inspection covers

A preliminary roost assessment, sometimes called a bat scoping survey, is a structured inspection of the building and its surroundings by an ecologist working to the Bat Conservation Trust's good practice guidelines for professional ecologists.

Externally, the surveyor works around the building with binoculars and a torch, looking at ridge tiles, hip tiles, lead flashing, barge boards, soffit and fascia gaps, hanging tiles, weatherboarding, gaps in masonry, and the junctions where these meet. The features that matter are crevices an animal the size of a thumb can enter, and surfaces where droppings or staining would show.

Internally, roof voids are inspected where access allows, with an endoscope used on crevices. The evidence looked for is droppings, feeding remains such as moth wings, urine staining on timbers, scratch or grease marks at access points, cobweb-free gaps that indicate regular use, and live or dead bats. The surveyor also records the surrounding habitat: tree lines and hedges that give commuting routes, watercourses, external lighting, and the relationship of the building to darker foraging ground.

None of that requires disturbing a bat, and none of it requires a licence unless the surveyor needs to enter a known roost. The work still has to be done by an ecologist who can recognise the evidence, because the survey that matters most is the one that finds a single dropping on a rafter.

The suitability categories

The guidelines sort buildings and trees into categories of suitability for roosting bats. Negligible means no habitat features capable of supporting a roost. Low, moderate and high describe an increasing number and quality of features, with the assessment taking account of the wider landscape as well as the structure itself.

The categories are judgements, not measurements, and two competent ecologists can reasonably differ by one category on a marginal building. What they should not differ on is the reasoning, which is why a report that assigns a category without describing the features it relies on is a weak report.

Check the number of visits recommended for each category against the current edition of the guidelines, because the recommendations have been revised and councils generally apply the version current at the time of survey.

When emergence surveys become necessary

Where suitability is anything above negligible, the guidelines recommend dusk emergence and dawn re-entry surveys, with more visits as suitability rises. These involve surveyors positioned around the building from before sunset, or before dawn, with bat detectors and infrared or thermal recording equipment, counting animals in and out.

The point of the survey is to establish whether a roost exists, what type it is, and roughly how many bats use it. A maternity roost in a barn and an occasional day roost used by a single male carry different consequences for licensing and for mitigation design. That distinction is what a bat survey is for, and it cannot be reached from an inspection alone.

The season that decides the programme

Emergence and re-entry surveys are only meaningful when bats are active and using summer roosts. The survey season runs from May to September, with the strongest results between May and August. Outside it, the ecologist can inspect but cannot survey.

Plan backwards from that. A scheme that involves re-roofing, a loft conversion, dormers, removal of soffits or demolition of an outbuilding needs its inspection carried out in the autumn or winter before the application year, so that any surveys can be programmed into the following season. Where an application is submitted without them, councils will either refuse validation, refuse permission for want of information, or hold the application until the survey is done, and the last of those is the one that consumes a year of a client's patience.

Reports that conclude more than the evidence allows

Three conclusions appear in weak reports and are worth checking for.

The first is "no evidence of bats was found, therefore bats are absent". Droppings degrade, roof voids are often inaccessible, and crevice-dwelling species leave very little. Absence of evidence in a building with suitable features is not evidence of absence, and inspectors say so on appeal.

The second is a recommendation that further survey be secured by condition. Where there is a reasonable likelihood of a protected species being present and affected, the survey should be done before the decision, not after it. Deferring it to a condition is the single most common reason ecology chapters are challenged.

The third is a mitigation design written without knowing what roost is present. Bat boxes proposed as a general precaution are not mitigation. They are a gesture, and a licensing officer will read them as one.

How councils treat a negative assessment

A clean assessment on a building of negligible suitability is usually accepted without further comment, particularly for a modern dwelling with a sealed roof and no accessible voids. Say so plainly in the report, with photographs of the eaves and ridge, and the point rarely comes back.

Where suitability is low and no evidence was found, practice varies. Some authorities accept a single further visit, some accept the assessment with an informative on the decision notice reminding the applicant of their legal obligations, and some ask for the full recommended effort. The local plan policy and the council ecologist's own approach decide it, so ask at pre-application stage rather than guessing.

Remember that the legal position is separate from the planning position. Bats and their roosts are protected under the Conservation of Habitats and Species Regulations 2017 and the Wildlife and Countryside Act 1981 regardless of what the council says. A protected species survey, and any mitigation licence that follows, must be carried out by a suitably licensed ecologist. That is a legal requirement, and an AI-assisted draft is a starting point for that ecologist rather than a substitute for their judgement.

Fitting it into a design programme

Commission the assessment at the same time as the measured survey, before the drawings are fixed. It costs little at that stage and it changes what you draw. A roost in the ridge can often be designed around with a retained void, a raised ridge detail or a bat access tile, and those are easy decisions in month one and expensive ones in month nine.

Where the site includes trees, hedges or ponds as well as buildings, a Preliminary Ecological Appraisal covers the wider habitat picture in one visit and will pick up whether other species surveys are triggered. And since mandatory biodiversity net gain applies to most applications, the habitat baseline collected for a Biodiversity Net Gain Assessment is usually gathered on the same visit, which saves a mobilisation fee.

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.