A bat survey happens in stages, and most projects only need the first one. An ecologist checks local bat records, inspects the building in daylight for bats and the signs they leave, and rates how likely it is that bats roost there. Only if that rating is low, moderate or high do surveyors come back at dusk with bat detectors and night vision cameras to watch for bats leaving the building. Everything is then written up in a report your council reads alongside your planning application. This guide walks through each stage as the homeowner or developer experiences it, based on the Bat Conservation Trust's bat survey guidelines (Bat Surveys for Professional Ecologists: Good Practice Guidelines, 4th edition), which UK ecologists and councils work to.
The bat survey process at a glance
So what is a bat survey in practice? GOV.UK puts it simply: the survey shows what type of bats, how many, and how they use the building or area, so the work can be planned to avoid harming them. All UK bats are European protected species: in England and Wales the Conservation of Habitats and Species Regulations 2017 and the Wildlife and Countryside Act 1981 protect them and their roosts even when no bats are present, which is why local planning authorities ask for a survey before granting planning permission. These are the stages in order:
| Stage | What happens | When | Do you need it? |
|---|---|---|---|
| 1. Desk study | The ecologist requests existing bat records for the area from the local environmental records centre | Before the site visit | Usually part of every survey |
| 2. Preliminary roost assessment (PRA) | A daytime inspection of the building inside and out, ending in a roost suitability rating | Any time of year | Yes, this is the starting point |
| 3. Bat emergence surveys | Surveyors watch the building from just before sunset with bat detectors and night vision aids | May to September | Only if the PRA rates the building low, moderate or high |
| 4. Bat survey report | Findings, impact assessment and any mitigation, submitted with your planning application | After the last survey | Yes |
| 5. Mitigation and licensing | If works would affect a roost, the ecologist designs mitigation and applies for a bat licence | After planning permission | Only if bats are found |
Before anyone visits: the desk study
Before the site visit, the ecologist buys a data search from the local environmental records centre to see which species of bat and which roosts are already recorded near your site, and looks at the surrounding habitat, such as woodland, rivers and hedgerows, that bats use to feed and commute. Historic England's guidance on building works and bats expects consultants to seek these historic records, and the CIEEM homeowner guide notes that you, as the client, pay any charge the records centre makes. A blank search does not mean you have no bats: GOV.UK reminds councils that the absence of a record does not mean bats are absent.
Stage 1: the preliminary roost assessment
The preliminary roost assessment, sometimes called a scoping bat survey, is a detailed daytime inspection of the exterior and interior of the building to look for features bats could use to get in and roost, and to search for signs of a bat roost. The BCT guidelines say it can be done at any time of year and should be carried out systematically in daylight hours, so the ecologist will need enough time on site to cover the whole building. It is the only bat survey visit most homeowners ever have. Trees with cracks, holes or loose bark can be assessed in a similar way from the ground.
Outside the building
The ecologist walks round the building looking for gaps and for evidence on the ground and walls. The BCT guidelines list the places to check, including the ground beneath possible access points, windowsills and windowpanes, peeling paintwork or lifted render, hanging tiles, weatherboarding, eaves, soffit boxes, fascias, lead flashing around chimneys, gaps under roofing felt and under tiles or slates, and gaps in brickwork or stonework that could lead into a cavity wall. Some features are not visible from the ground, and wet weather can wash evidence off walls, so a clean exterior does not rule bats out.
Inside the loft and the house
Where it is safe, the inspection moves inside. On entering a loft, the ecologist looks up first to check the ridge beam and other timbers for free-hanging bats, then searches the floor, water tanks, stored boxes and the edges of insulation for droppings. With all torches switched off, light spilling into a dark loft shows the gaps bats could use. In a house that is lived in, the guidelines say it may not be necessary to inspect every room: the focus is on upper floors, roof spaces, boiler rooms and other dark or little-used spaces. Cellars, outbuildings and garages may be checked too.
The equipment ecologists use
| High-powered torches | To light up loft spaces, roof timbers and dark corners |
|---|---|
| Endoscope | A small camera on a flexible cable, fed slowly into crevices while the ecologist watches the screen. Using one needs specific training and a survey licence |
| Mirrors | To see into crevices and behind beams |
| Ladders | To reach eaves, soffits and roof voids safely |
| Binoculars | To examine roof timbers, ridge tiles and high features from the ground |
| Bat detector | Sometimes used to pick up squeaking from a hidden roost |
Signs of bats they look for
Bats and their droppings are the most reliable evidence. The guidelines list other signs, but warn that these are not always caused by bats and should not be relied on alone:
- Live or dead bats, including single bats hanging from beams or tucked into joints
- Bat droppings, which the Bat Conservation Trust notes are made up of dried insect remains
- Feeding remains, such as discarded moth wings
- Fur-oil staining and urine splashes
- Squeaking from a roost, audible to the ear or on a bat detector
- Bat-fly pupal cases and a distinctive odour
Bats sometimes leave no visible sign at all, particularly in hidden cracks and cavities, and the guidelines are clear that an absence of evidence is not evidence of absence. That is why the assessment rates the building's potential rather than simply recording what was seen.
The roost suitability rating
The preliminary roost assessment ends with a rating for each building or structure, based on the features found and the quality of the surrounding habitat. It decides the survey effort that follows, including how many surveyors any later surveys will need.
| Rating | What it means | What happens next |
|---|---|---|
| Negligible | No obvious features bats are likely to use, though a small element of uncertainty remains | No further survey; the report goes with your application |
| Low | Roost spots that single bats could use now and then, but not regularly or in numbers | One dusk emergence survey |
| Moderate | Features bats could use, but unlikely to support a maternity or hibernation roost | Two dusk emergence surveys |
| High | Features obviously suitable for larger numbers of bats, including maternity roosts | Three dusk emergence surveys |
| Bats or signs found | A roost is confirmed | Further surveys to characterise the roost, then mitigation |
DNA analysis of bat droppings
Where the ecologist finds droppings, the BCT guidelines say samples should be collected for DNA analysis unless the species has already been identified reliably, for example by seeing bats in the roost or recording their echolocation calls. Some groups, such as the long-eared bats and the Myotis bats, are difficult or sometimes impossible to tell apart by sound or sight, and DNA analysis is more reliable than judging droppings by their shape or colour. Droppings are collected with clean tweezers, dried if fresh and posted to a specialist laboratory. It can also settle whether they came from a bat at all: the University of Warwick, which developed a bat genotyping service, notes that supposed bat droppings have been identified as belonging to other species. If the first visit finds no fresh droppings, the ecologist may lay paper or plastic sheets in the loft to catch some before the next visit.
Stage 2: bat emergence surveys at dusk
If the preliminary roost assessment rates the building low, moderate or high, the next stage is a bat emergence survey (older reports call these emergence and re-entry surveys). The aim is to find out whether bats are roosting there, which species, how many, and where they get in and out. In some cases the CIEEM homeowner guide says the daytime assessment and the first emergence survey can be combined in one visit.
Timing on the night
Surveyors are in position 15 minutes before sunset. The survey then runs for 1.5 to 2 hours after sunset, so on a midsummer evening it can finish well after dark. Because bats are sensitive to light and noise, surveyors avoid using torches while setting up and during the survey, and stay quiet. Lighting, noise or vibration can change when bats emerge, so tell your ecologist about any outside lights or noisy work planned for survey evenings.
Where surveyors stand and what they use
The positions come from the preliminary roost assessment, which should have found the possible access points. The guidelines ask for enough surveyors for every side of the building to be in view, each covering particular access points and keeping in touch by hand-held radio. Each one listens with a bat detector, which turns bats' echolocation calls into sound and records them so the species can be identified later.
Current guidance also expects night vision aids. The BCT 4th edition says surveys should usually be carried out with night vision aids (night vision, infrared or thermal imaging cameras) because many bats emerge after it is too dark to see them, and any survey without them has to be justified in the report. Infrared cameras need a separate infrared light source, the footage is recorded and reviewed afterwards, and a still frame from the darkest point of the survey goes in the report to prove the camera could see. Used well, cameras can reduce the number of surveyors needed.
How many visits, and how far apart
A building rated low gets one dusk survey, moderate gets two and high gets three. Visits should be spaced at least three weeks apart, preferably more, and fall between May and September, with at least one (moderate) or two (high) of them between May and August, when maternity colonies are present. Your ecologist can adjust the number up or down for your site, but has to justify it in the report. More visits may be needed if parts of the building could not be inspected inside.
Bat activity and hibernation surveys
Emergence surveys focus on one building. On larger or greenfield sites, ecologists may also run bat activity surveys between April and October, walking transect routes at night and leaving static bat detectors out to record how bats feed and commute across the land. Where the preliminary roost assessment finds cool, stable places such as cellars or tunnels, hibernation surveys between November and March check for hibernating bats. Most house and loft projects need neither.
Weather and the northern season
Bats fly less in cold, wet or windy weather, so surveys aim for a sunset temperature of 10°C or above without heavy rain or strong wind. Historic England notes that high winds or heavy rain can mean a survey is rescheduled. The BCT guidelines add that the season is likely to be shorter further north and that September surveys are less suitable in northerly areas, which matters on exposed sites in County Durham and the wider North East.
What about dawn re-entry surveys?
Older guidance paired dusk surveys with dawn re-entry surveys, watching bats return before sunrise. The 4th edition no longer recommends them as standard for presence or absence, because return times vary so much that bats are easily missed and night vision aids have made dusk surveys more accurate. An ecologist may still use one for a specific reason, such as pinning down an entry point on a confirmed roost.
The bat survey report: what it contains
After the last visit, the ecologist writes the bat survey report you submit with your planning application. The BCT guidelines set out what a report for planning should include:
| Summary | A plain-English overview of the purpose, methods, findings, limitations, recommendations and conclusions |
|---|---|
| Desk study | Which records were requested, from whom and when |
| Methods | Survey dates, start and finish times, sunset times, weather, equipment, the number and names of ecologists and their licence numbers |
| Survey positions | A plan showing where each surveyor and camera stood, with a screenshot from each night vision camera |
| Results | Descriptions of the building, access points and evidence found, DNA results, and any bats seen emerging, with times, species and numbers, all marked on a plan with photographs |
| Limitations | Anything that could not be surveyed and why, such as a loft that was unsafe to enter |
| Impact assessment | How your project would affect any roosts or bats |
| Required actions | Further surveys, whether a bat licence is needed, and avoidance, mitigation, compensation and enhancement measures written as firm commitments |
| Validity | How long the survey data will remain valid before a resurvey is needed |
If a licence is needed, Historic England notes the report will include the licence method statement. Councils use the report to decide your application, and GOV.UK tells them to check that the ecologist holds an up-to-date bat survey licence.
How long the bat survey process takes
The preliminary roost assessment is a single daytime visit, with its length depending on the size and complexity of the building. The rest depends on its rating and the time of year. If you need emergence surveys, the calendar sets the pace: they can only run from May to September, so a building rated high in October waits until the next May, as the month-by-month bat survey calendar shows.
| Outcome of the PRA | Minimum survey time on site | Earliest the sequence can finish |
|---|---|---|
| Negligible | One daytime visit | As soon as the report is written |
| Low | PRA plus one dusk survey | After one dusk survey between May and August |
| Moderate | PRA plus two dusk surveys | At least three weeks after the first dusk survey |
| High | PRA plus three dusk surveys | At least six weeks after the first dusk survey |
The simplest way to save time is to book the preliminary roost assessment in winter or early spring, so any emergence surveys can start in May and finish inside one season.
What you need to do during a bat survey
The CIEEM homeowner guide puts the client's role simply: organise and pay for the survey, directly or through an agent, and follow any conditions about how the work is done. In practice:
| Give clear access | Make sure the loft hatch, outbuildings, garages and any cellar can be reached, and tell the ecologist about anywhere unsafe |
|---|---|
| Don't clean up first | Leave any droppings where they are. If a space has been cleaned, tell the ecologist when and why, as cleaning removes evidence |
| Don't disturb bats | It is an offence to deliberately disturb a bat, or damage a roost even when bats are absent. Don't block gaps or start work before the survey |
| Share what you know | Tell the ecologist if you have seen bats or droppings, and when |
| Leave equipment alone | Don't move sheets laid to catch droppings or any cameras and detectors left in place |
| Keep evenings quiet | On survey nights, avoid outside lights and noisy work near the building if asked |
What happens if bats are found
Finding bats rarely stops a project. If emergence surveys confirm a roost, the next step is usually further roost characterisation surveys to establish the species, numbers, access points and type of roost. Your ecologist then designs mitigation and compensation measures, such as timing works around the bats, keeping or replacing roost spaces, keeping flight routes dark and changing working methods, and once planning permission is granted applies on your behalf for a licence from Natural England in England (NatureScot in Scotland, Natural Resources Wales in Wales). Depending on the impact, that can be an individual bat mitigation licence, the bat mitigation class licence or Bat Earned Recognition. GOV.UK says that if a roost has to be destroyed, there should be no net loss of roost sites and roost types should be replaced like for like. The Bat Conservation Trust's own advice to roost owners is that the laws protecting bats should not stop you carrying out works altogether: you just need to take the bats into account first.
Arranging a bat survey in County Durham
If you are planning a loft conversion, a new roof, a barn conversion or a demolition, it pays to start early. Durham County Council's validation checklist asks for a bat risk assessment for buildings with roost features within 200m of woodland or water, and for trees with roost potential, but householder applications for ground-floor-only works that do not disturb the roof are exempt. Beatrice Vasey is a horticulturist, not a bat ecologist, so she does not carry out bat surveys herself. If you are arranging a bat survey in County Durham, we can introduce you to one vetted, licensed bat ecologist who covers your site and will quote you directly. We are paid a referral fee by that business.
