Bat Survey

Natural England’s standing advice tells councils to ask for a bat survey where records suggest bats may be present and the site has buildings, trees, water or certain lighting proposals (gov.uk). In practice, councils publish their own triggers. Dorset Council’s list is typical: loft conversions, barn conversions, re-roofing most or all of a roof (including re-thatching), and extensions that join an unused loft (Dorset Council, 2025).

Planning permission is not the only reason to check. Work done under permitted development, such as many loft conversions, still has to avoid harming bats, because the law protects roosts whether or not you need planning permission (Planning Portal).

A licensed ecologist inspects the building or tree from outside and inside, looking for droppings, feeding remains, staining and gaps where bats get in. Historic England confirms that this first inspection can be done at any time of year (Historic England).

The building is then graded for roost potential. If the grade is negligible, the report can usually go straight in with the application. If it is low, moderate or high, the council will expect emergence surveys to confirm whether bats are actually using it.

Emergence surveys watch the building at dusk, when bats leave to feed, using night-vision cameras and bat detectors. The Bat Conservation Trust’s guidelines set the season as May to September, with low-potential buildings surveyed from May to August and at least some visits for moderate and high potential falling in that same May to August window (BCT, 4th edition).

Surveys need a mild, dry, still evening, so a wet spell can push dates back. If the assessment is done in autumn or winter and finds potential, the emergence surveys have to wait for May. That is the most common reason a small planning application is delayed by months.

Finding bats rarely stops a project. It changes the design and the timing. The ecologist will set out mitigation, such as keeping access points, fitting bat tiles or boxes, and timing roof work away from sensitive periods, and the work then goes ahead under a licence from Natural England.

A full mitigation licence costs nothing and Natural England aims to decide within 30 working days (gov.uk). For lower-impact cases, ecologists have used the CL21 class licence; it is closing to new site registrations after 14 December 2026, with the Bat Earned Recognition route taking over (gov.uk). Ask your ecologist which applies.

Damaging or destroying a roost is an offence even when no bats are present at the time, under the Conservation of Habitats and Species Regulations 2017.

A preliminary roost assessment for a house or a single barn is typically £300 to £600 plus VAT. Large or complex buildings can be £2,000 or more. Each emergence survey typically costs £350 to £500, because it needs more than one surveyor at dusk and dawn, so a full set for a building with high potential is £800 to £3,000. Licence applications and mitigation design are priced separately. See the survey cost guide.