Common BNG problems, and what to do about them
The biodiversity net gain problems that stall schemes most often, from a metric submitted too early to units sold before planning is granted, and what to do.
Biodiversity net gain has been mandatory for most development in England since 2024, and the same problems come up scheme after scheme. Most of them are not ecological. They are about timing, geography and paperwork. Here are the ones we see most, and what can be done about each.
1. The metric went in before anyone thought about units
The most expensive BNG problem is also the most common. The statutory metric is submitted with the planning application, and once it is with the authority it cannot be changed. Anyone offering to revise it for you is not being straight with you.
What can be done: come to the conversation earlier. How a scheme is laid out decides how much habitat it damages, and that decides what has to be replaced. Thinking about it at appraisal stage, before the metric is fixed, is where the biggest savings are. If the metric is already in, the remaining options are the trading rules and the choice of bank, both covered below.
2. The site loses a hedge, and area units will not cover it
The metric produces three separate scores: area habitats, hedgerows and watercourses. They do not trade against each other. A scheme that removes a hedgerow needs hedgerow units, and no quantity of grassland units will substitute.
Schemes discover this late because hedgerows are often the last thing to go in the layout, when an access is moved or a plot is squeezed. The answer is to check all three scores before the layout is fixed, and to source hedgerow or watercourse units from a bank that generates them. We wrote about this in Hedgerow units, watercourse units, and why area units will not cover them.
3. There are no suitable units near the site
Units from a bank in the same local planning authority or National Character Area count at full value. Units from further away are discounted by the spatial risk multiplier, so you need more of them. In some areas no bank holds the habitat type a scheme needs.
What can be done: look at the whole cost rather than the rate per unit. A nearer bank at a higher headline price can cost less once the multiplier is applied. Where the requirement is large and nothing suitable exists, a habitat bank can be created for the scheme. It takes longer, so it is a conversation to have early, and it is often combined with existing units to discharge the condition now.
4. The trading rules block a like-for-like match
The loss of a distinctive habitat generally cannot be offset with a low-distinctiveness one. When the exact habitat is not available, schemes stall looking for it.
The rules are more flexible than they look. Trading up, compensating a loss with habitat of higher distinctiveness, is permitted, and it can open a route that is cheaper, faster, or the only one that exists. It is worth having someone look at what the requirement could be met with, not only what it appears to demand.
5. Planning is granted, and you still cannot start
The biodiversity gain condition is a pre-commencement condition. It is deemed to apply whether or not it appears on the decision notice, and development cannot lawfully begin until the authority has approved a biodiversity gain plan. Units have to be allocated on the biodiversity gain sites register before that plan can be approved.
What can be done: line the units up while the application is being determined, so that allocation follows the decision rather than starting after it. The gain plan is the buyer’s own document to file, and the evidence the authority needs is the allocation reference from the register. More in Discharging your BNG conditions.
6. The units you were counting on have been sold
Supply is patchy and uneven across the country. Between an appraisal and a decision, the units a scheme was relying on can be sold to someone else. Rare habitat types and large requirements are the most exposed.
A reservation holds specific units for an agreed period against a deposit. The deposit is charged against the reservation and deducted from the final price on completion. It is not refundable, so the period should be set against a realistic view of the determination date, and moved if that date moves.
7. Being pushed towards statutory credits
Statutory biodiversity credits are the government’s last resort. They are deliberately priced well above the private market so that they act as a backstop rather than a route. If a scheme is being steered towards them, it is usually because nobody has looked hard enough at registered off-site units. A second opinion first is nearly always cheaper.
8. Not knowing whether the scheme is in scope at all
The rules changed for applications made on or after 6 August 2026. Sites with a red-line boundary of 0.2 hectares or less are exempt, unless on-site priority habitat is negatively affected. Temporary developments of five years or less are exempt where the land is reinstated. The separate self-build and custom-build exemption has been removed. Applications made before that date run under the previous rules.
Small schemes on both sides of that line get it wrong in both directions: some buy units they do not need, and some assume an exemption that no longer exists. If you are not sure which side you fall on, ask before you spend anything.
9. The price moved between appraisal and purchase
BNG unit prices are not stable. The market is young and values move, sometimes sharply. A figure carried in an appraisal for six months can be out of date by the time the scheme is ready to buy.
What can be done: get a written quote that states the full cost and holds for a defined period. Ours hold for three months, and a reservation extends the certainty while planning is determined.
10. Nutrient neutrality on top of BNG
In catchments where nutrient neutrality applies, a residential scheme has to deal with nitrogen or phosphorus as well as biodiversity, and the two are separate requirements with separate mitigation. Nutrient mitigation has to be secured before permission can be granted, so it bites earlier than BNG does.
The two are best handled together, because the same conversation about land, catchment and timing covers both. Our nutrient mitigation page explains which catchments are affected and how mitigation is met.
The common thread
Nearly every problem on this list gets cheaper the earlier it is looked at, and dearer the longer it waits. If a scheme has a BNG requirement, or a problem that is not on this list, tell us about it and we will say what the options are.