The Preliminary Roost Assessment, and what happens next.

If your planning application has flagged bats, the word you'll hear first is usually "PRA" — a Preliminary Roost Assessment. It's not the full survey people imagine, with ecologists stationed at dusk. It's the assessment that decides whether any of that is needed at all.

What a PRA actually involves

A licensed bat ecologist inspects the building or tree in daylight, inside and out where access allows. They're doing two things: assessing the structure's potential to support roosting bats, and searching for evidence that bats are already using it. On a building that means checking the roof void, ridge, soffits, fascias, gaps in brickwork, hanging tiles and similar features, and looking for droppings, staining, feeding remains or the bats themselves. On a tree it means assessing cavities, splits, woodpecker holes, lifting bark and dense ivy from the ground, sometimes with close-focusing binoculars or an endoscope.

The roost suitability category

The assessment grades the building or tree against the bat roost suitability categories set out in the Bat Conservation Trust's Bat Surveys for Professional Ecologists: Good Practice Guidelines — the standard reference for bat survey work in the UK. The category reflects how suitable the features are for roosting bats, and it sets the number of follow-up survey visits that the guidelines recommend:

• Negligible — no habitat features capable of being used by roosting bats. No further bat survey needed.
• Low — one or more features that could be used by individual bats opportunistically, but unlikely to support a roost of conservation significance. Guidance recommends one survey visit — a dusk emergence or a dawn re-entry survey.
• Moderate — one or more features that could be used by bats given their size, shelter, conditions or surrounding habitat, but not obviously suitable for larger numbers. Guidance recommends two survey visits, on separate evenings or mornings.
• High — one or more features highly likely to support roosting bats, or confirmed evidence of a roost. Guidance recommends three survey visits.

The visits can't simply be run back to back. The guidelines call for the effort to be spread across the active season — broadly May to September — with the surveys ideally falling in the optimal months of May to August, and successive visits separated rather than bunched together. The exact pattern depends on whether it's a building or a tree, the species likely to be present, and what the early visits turn up. These figures are the standard for buildings and structures; trees are assessed on the same categories with comparable effort.

What happens after the PRA

If the result is negligible, you typically have what you need for the application and can move on. If it's low, moderate or high, the next stage is emergence and re-entry surveys — ecologists watching the structure at dusk to count bats leaving, and at dawn to see them return, across the number of visits the grade dictates. Those visits confirm whether a roost is present, what species, and how it's used, which in turn informs any mitigation and licensing.

If a roost is confirmed

A confirmed roost doesn't mean your project stops. It means the work needs to be done lawfully — usually through a mitigation strategy and, where the roost will be affected, a European Protected Species licence from Natural England. Handled properly and early, this is a manageable process rather than a barrier. The job of a good field team is to find the route through.

How we handle it

We tell you the likely outcome and the likely number of surveys up front — before you instruct us — so the cost and timescale are clear from the start. For private clients we produce the full written report a planning authority needs. For consultancies and architects we provide the survey data, notes and photographs in whatever format slots into your own reporting. Either way, the aim is the same: evidence that's accepted first time.