Biodiversity Net Gain asks a simple question with a complicated answer: is the land better for wildlife after this development than it was before, and by how much?
How the measurement works
Habitats are surveyed and classified, then scored using a statutory metric. The score reflects habitat type, its condition, its distinctiveness, and where it sits. The pre-development score is the baseline; the proposed post-development habitats are modelled against it, and the difference is the gain.
The statutory requirement, the exemptions and the required percentage have all moved since net gain became mandatory, so this note deliberately describes the mechanism rather than quoting a figure that will date. For the position that applies to a particular site, ask.
Why the baseline is the whole game
Almost every difficulty with net gain traces back to the baseline survey.
Score the existing habitat too generously and the required uplift becomes impossible to deliver on site — which means buying off-site units, at cost. Score it too meanly and the assessment gets challenged. The accuracy of the condition assessment is the single thing most worth paying attention to.
Designing for it rather than around it
Net gain works best when it enters the process at the layout stage. Retained hedgerows, a wider buffer to a watercourse, a grassland margin that was going to be amenity mown anyway — these are cheap when they are drawn in early and expensive when they are retrofitted.
Treated as a compliance exercise at the end, it becomes a cost. Treated as a design input, it is often close to free.
What to ask for
A baseline habitat survey in the growing season, a completed metric, and a strategy that sets out how the uplift is actually delivered and maintained. Anything less than that tends to come back.
General guidance only. Ecological requirements depend on the site, the proposal and the local planning authority — for advice on a specific project, get in touch.
