Barn conversion bat survey: the sequence and the seasons

A barn conversion bat survey is not one survey. It is a sequence that starts with a daytime inspection you can book in any month, and it may end eighteen months later with a licence from Natural England. The season, not the council, sets your programme. Miss the summer window and the scheme waits until the following May.

Traditional farm buildings are the single most likely structure on a holding to hold a bat roost, and all bat species and their roosts are protected whether or not the building is in use, whether or not the roof leaks, and whether the conversion route is a full planning application or prior approval under Class Q.

Why traditional farm buildings are high risk for roosts

Stone and brick barns with slate or clay tile roofs give bats what a modern portal frame does not: crevices in the walls, gaps under ridge tiles, unsealed sarking, a stable temperature and undisturbed dark space at the apex. Timber-framed threshing barns with open ridges and loose weatherboarding are just as attractive.

An ecologist looking at a building is reading for signs rather than counting animals. Droppings on beams and window sills, urine staining on stonework, feeding remains such as moth wings, oil staining at access points, scratch marks, and the smell of an occupied roost.

Age helps but is not decisive. A 1960s asbestos-roofed building with a cavity wall can hold a roost. So can a lean-to.

The preliminary roost assessment and when to do it

The first stage is a preliminary roost assessment, a daytime inspection of the building inside and out with a ladder, endoscope and torch. It can be carried out at any time of year, which makes it the one piece of ecology you should commission the moment the conversion becomes a serious idea.

The assessment does two things. It records any evidence of current or past roosting, and it grades the building for roost suitability against the Bat Conservation Trust Good Practice Guidelines, now in their fourth edition. The grades run from negligible through low and moderate to high suitability, and the grade sets the survey effort that follows.

Negligible means no further survey. Low, moderate and high all mean nocturnal survey work in season, with the number of visits rising with the grade. A preliminary ecological appraisal usually wraps this up with the wider habitat baseline for the site, which saves a second site visit.

Book it in winter. An inspection in January gives you the whole of the following survey season to work with.

Emergence and re entry survey windows

Nocturnal surveys establish whether bats are using the building now, how many, which species and where they get in and out. Surveyors sit outside the building at dusk to watch bats leave, or before dawn to watch them return, using bat detectors and infrared cameras.

The survey season for buildings runs from May to September, with the core maternity period from May to August. Guidance expects visits to be spread across that window rather than clustered in a fortnight, and at least one visit inside the maternity period where the building has moderate or high suitability.

That is the hard constraint on the programme. A building graded high in October cannot complete its survey work until the following August at the earliest. Nothing in the planning system accelerates it, and a council that accepts a condition instead of a survey is taking a risk that an objector can challenge.

Barn owls and their separate protection

Barn owls are listed on Schedule 1 of the Wildlife and Countryside Act 1981, which makes it an offence to disturb a bird at or near an active nest, as well as to damage or destroy the nest while in use. Farm buildings with open gables and internal ledges are classic nest sites.

Evidence is easier to read than for bats. Pellets, whitewash on beams and walls below a ledge, and the flattened debris of a nest site. An inspection in the same visit as the roost assessment picks it up.

Where barn owls are present, the usual planning outcome is a replacement nesting provision, either an external box on a pole away from the road or an integrated owl loft in the converted building, plus timing restrictions on works during the nesting period. Roads are the main cause of barn owl mortality, so siting a box near a fast road is worse than useless.

Mitigation design and roost provision

Mitigation is designed around the roost you found, not around a catalogue. The principles the licensing regime looks for are like-for-like replacement, continuity of access and a design that will still work in twenty years.

Common measures in a farm building conversion:

• Retained roost voids in an uninhabited part of the structure, such as a cart shed, gable void or a section of roof left unconverted.
• Bat access tiles or gaps at the ridge and eaves that connect to a void, sized for the species present.
• Breathable roofing membrane avoided where bats will crawl on it, because bitumen felt or a non-woven membrane is specified instead to prevent entanglement.
• Timing of roof stripping outside the maternity period, under ecological supervision.
• External lighting kept off the flight line, with the roost access point left dark.

Lighting is where most schemes come unstuck after approval. A security light aimed at the gable that holds the new access tile undoes the mitigation, and the condition will usually require a lighting scheme to be approved before occupation.

The licensing route and its timing

If the works will damage or destroy a roost, or disturb bats, the developer needs a European protected species mitigation licence from Natural England before those works start. Planning permission does not confer that licence and does not remove the offence.

The licence application needs a method statement, a reasoned justification against the three derogation tests, and drawings of the mitigation. It is prepared and submitted by a licensed ecologist. A low impact class licence route exists for cases where a registered consultant assesses the impact as low, and it is materially faster than the full application, but it is only available where the roost type and numbers fall within its scope.

Sequence it properly: survey, then planning decision, then licence, then works. Applying for a licence before permission is granted wastes the fee if the scheme changes.

A protected species survey must be carried out by a suitably licensed ecologist under the Wildlife and Countryside Act 1981 and the Conservation of Habitats and Species Regulations 2017. PlanScope can draft the supporting planning narrative around that ecologist's findings, and a bat survey draft is a starting point for the licensed professional rather than a substitute for their survey work or their signature.

How survey seasons drive the programme and what councils expect

The local planning authority has a duty under the Habitats Regulations to have regard to the derogation tests before granting permission where a European protected species is affected. In practice that means the survey has to be in front of the officer at determination, not deferred to a condition.

Class Q prior approval does not change this. Protected species is not one of the matters the authority considers under Class Q, but the offence still applies, the authority's Habitats Regulations duty still applies, and a prior approval granted without survey information does not protect anyone from prosecution.

Build the sequence backwards from the date you want to start on site. A daytime assessment in February, nocturnal surveys across May to August, a report in September, an application validated in October, a decision around Christmas and a licence in the spring is a realistic eighteen-month run for a building with a confirmed maternity roost. Where ownership or tenancy affects who signs the forms, who applies: tenancies, ownership certificates and notices covers the certificates.

This is general guidance, not legal advice, so check the position with your local planning authority or a qualified ecologist before relying on it. Enter your postcode on PlanScope to see which reports a farm building conversion is likely to need.

PlanScope provides AI-assisted draft reports for planning applications. Reports should be reviewed before submission. We do not guarantee planning approval.