What an ecologist actually looks for, and when
The first stage is a preliminary roost assessment, sometimes called a preliminary ecological appraisal where the whole site is considered. An ecologist inspects the building externally and, where access allows, internally, looking at two things: evidence of use, and suitability. Evidence includes droppings, which for bats are distinctive and can be identified, staining around access points where fur has polished a surface over time, feeding remains, and in the case of birds the nests themselves. Suitability is about features: gaps under ridge tiles, slipped or lifted tiles, open or damaged soffits, gaps at the wall plate, hanging tiles with voids behind, and access points at gable apexes — a bat needs a remarkably small gap. The building is then assigned a level of potential, and that determines what follows. Negligible potential ends the matter. Low, moderate or high potential leads to further survey, and this is where timing bites, because the standard further survey for bats is an emergence and re-entry survey carried out at dusk and dawn during the active season, and the number of visits required increases with the assessed potential. Outside that season the survey cannot be done at all, so a project that reaches this stage in late autumn may wait months. That single fact is the argument for commissioning the preliminary assessment at feasibility rather than at the point of applying: it costs a modest fee, it takes days, and it either removes the constraint entirely or tells you early enough that the seasonal window can be planned around. On a property adjoining a large area of woodland and open ground, the prior probability of at least moderate potential is meaningfully higher than in dense urban fabric, which is why it is worth budgeting for here rather than treating as an outside chance.
