Ecological survey calendar › Bats
When to survey for bats in Wales and England
Updated November 2026. Based on the BCT Bat Surveys for Professional Ecologists, 4th edition (2023), and Natural England's current standing advice.
Bat emergence and re-entry surveys can be done from May to September, and for most roosts at least one survey must fall between May and August. A preliminary roost assessment of a building or tree can be done in any month, so it is the right first step whenever a project starts.
Survey windows, sensitive periods and works timing
| Activity | Type | When | Notes |
|---|---|---|---|
| Roost surveys (emergence and re-entry) | Survey | May to Aug; Sep (not ideal) | The number of surveys depends on roost suitability. At least one should fall between May and August. |
| Activity surveys (transects and static detectors) | Survey | Apr to Oct | Visits spread across spring, summer and autumn. The number depends on how suitable the habitat is for bats. |
| Preliminary roost assessment | Survey | All year | Can be done in any month. Droppings and staining are easiest to find in summer. |
| Hibernation surveys | Survey | Dec to Feb | Checks of known or potential hibernation sites while bats are torpid. Natural England's standing advice gives November to mid-March. |
| Maternity season | Sensitive period | May to Aug | Females gather to give birth and raise young. Avoid disturbing roosts. |
| Hibernation season | Sensitive period | Nov to Mar | Bats are torpid and easily harmed if disturbed. |
| Tree felling and roost works | Works window | Sep to Oct | Between the maternity and hibernation seasons. Late March to April is also used, but clashes with bird nesting. Exact timing is set by survey results and any licence. |
Optimal survey Possible, not ideal Sensitive period Works window
Timing depends on the site, the weather in a given year and what the planning authority and licensing body need. Upland and northern sites can run later. See every species group side by side on the ecological survey calendar.
About bats
Bats move between different roosts through the year: maternity roosts in summer, mating sites in autumn and cool, stable hibernation sites in winter. A building, bridge or tree can matter in one season and be empty in another, which is why survey timing matters so much.
Legal protection
All UK bat species are European protected species under the Conservation of Habitats and Species Regulations 2017. They are also protected by the Wildlife and Countryside Act 1981.
Licensing
Wales: Works affecting a roost need a European protected species licence from Natural Resources Wales, normally applied for after planning permission is granted.
England: Works affecting a roost need a Natural England mitigation licence, or registration under the Bat Mitigation Class Licence for qualifying low-impact cases.
Practical tips
- Book emergence surveys early. The May to August window fills fast, and missing it can delay a planning decision by most of a year.
- A preliminary roost assessment can be done in winter, so the right number of summer surveys can be planned in good time.
- External lighting can affect bats as much as demolition. Plan it to ILP and BCT Guidance Note 08/23.
Common questions
When is bat survey season?
Emergence and re-entry surveys run from May to September. For most roosts at least one survey should be between May and August (Collins, 2023).
Can bat surveys be done in winter?
A preliminary roost assessment can be done in any month, and hibernation surveys run from December to February. Emergence and re-entry surveys cannot be done in winter, so if they are needed they wait until May.
How many bat surveys will a building need?
It depends on the roost suitability found at the preliminary roost assessment. For buildings and structures, low suitability usually means one emergence or re-entry survey, moderate two and high three, spread through the season (Collins, 2023). Trees are assessed differently.
When can trees with bat roost potential be felled?
Usually in September and October, between the maternity and hibernation seasons, after checks by a licensed bat ecologist and under a licence where one is needed.
Need a survey or advice on timing? Ecoleg's licensed ecologists programme surveys across Wales and England so the right work happens in the right season. Call 07356 123706.
Request a quoteSee the full calendarSources
- Collins, J. (ed.) (2023) Bat Surveys for Professional Ecologists: Good Practice Guidelines, 4th edn. Bat Conservation Trust.
- Natural England (2025) Protected species and development: advice for local planning authorities, survey timings table (accessed November 2026).
- Bat Conservation Trust guidance on trees, woodland and bats: March to April and September to October are the least vulnerable times (accessed November 2026).
Updated November 2026. General guidance only: confirm timing for your site with a suitably qualified ecologist.