Bat surveys are unusual among the checks a planning application throws at you, because they're tied to the biology of the animal. You can't survey for roosting bats when the bats aren't there to be seen leaving and returning. That single fact shapes the whole timeline — and it's why "we'll sort the bat survey later" is the phrase that most often costs people a building season.
Two stages, two different timings
A bat survey usually comes in two parts, and they're governed by the calendar very differently.
The Preliminary Roost Assessment — any time of year
The first step, the Preliminary Roost Assessment, is a daytime inspection that grades how suitable a building or tree is for roosting bats. It doesn't depend on bats being active, so it can be carried out in any month. Getting it done in autumn or winter is the smart move: if it shows that follow-up surveys are needed, you've kept the entire active season ahead of you.
Emergence and re-entry surveys — the active season only
Where follow-up is needed, surveyors watch the structure at dusk to count bats emerging, and at dawn to see them return. These only work when bats are active and using their summer roosts — which means a defined window.
The active season: May to September
In line with the Bat Conservation Trust's survey guidelines, dusk emergence and dawn re-entry surveys are carried out across the active season — broadly May to September — with May to August regarded as the optimal period, when bats are most active and maternity roosts are occupied. Surveys outside this window aren't considered valid, because bats are far less active and may not be using the roost at all.
Book early. Survey slots fill through the spring and summer, and the number of visits you need is spread across the season rather than crammed into one week. A project that gets its PRA done over winter almost never gets held up by bats. A project that starts looking for a surveyor in August often does.
Why the season exists
It comes down to what bats are doing through the year. In winter they hibernate, tucked away and barely moving, so there's nothing to observe emerging at dusk. Through the summer — particularly June and July — females gather in maternity roosts to give birth and raise their young, which is when roosts are most active and most detectable. The survey season is built around catching bats when they're genuinely using a site, so the results actually mean something.
How the visits are spaced
The number of survey visits depends on the roost suitability found at the PRA — typically one for low suitability, two for moderate and three for high. Crucially, these can't be run on consecutive nights to get them out of the way. The guidelines call for them to be spread across the season, separated by a couple of weeks, and falling within the optimal months wherever possible. That spacing is part of why the calendar matters so much: three high-suitability surveys need planning across months, not days.
What this means for your planning timeline
If a survey is required and you reach late September without it done, the emergence and re-entry work usually has to wait until the following May. For a planning application, that's not a small slip — it can push a decision back the better part of a year. The way to avoid it is simple: get the assessment done as early as you can, so any follow-up surveys can be slotted into the season rather than chasing its tail.
How we help you stay ahead of it
When you come to us, the first thing we do is tell you honestly where you stand against the calendar — whether the season is on your side, and what sequence of work keeps your programme moving. We carry out the PRA, schedule any follow-up surveys across the season, and produce the report a planning authority needs. If timing is going to be tight, you'll hear it from us early, while there are still options.