A bat survey is required when a planning authority has reason to believe that bats could be present and would be affected by the work. It is not triggered by the size of the project but by the building and the habitat, which is why a single roof replacement on an old building can need one while a much larger scheme on a modern site does not.
What the survey is
First a daytime inspection for evidence of roosting and for features that could support one. If evidence or potential is found, dusk emergence and dawn re entry surveys follow in the active season, and the results go into a report the planning authority reads alongside the application.
Why the authority asks for it
All UK bat species and their roosts are protected in law. A planning authority cannot lawfully grant permission for work that would breach that protection without being satisfied the tests are met, so it asks for the survey before determining the application rather than conditioning it afterwards.
What triggers a request
Older buildings, barns, churches, buildings with gaps at the eaves or under tiles, roof works, demolition, tree felling, and sites near woodland or water. Local validation lists differ, so the authority's own list is the definitive answer for your site.
What happens if bats are found
The scheme is designed around them: timing, exclusion methods and replacement roosting provision, and where the work would still disturb a roost, a licence. Finding bats slows a project, it very rarely stops one.
Questions people ask about what is a bat survey
Do I need a bat survey for planning permission?
Only where the authority considers bats could be present and affected. The preliminary roost assessment is the cheapest way to find out, and its report is what satisfies or removes the requirement.
Do I need a bat survey to replace my roof?
Frequently, on an older building. Roof works are the classic trigger because they disturb exactly the spaces bats use.
Can I do the survey myself?
No. Surveying for bats requires experience and, for anything involving disturbance, a licence. A report from an unqualified person will not satisfy the authority.