What mandatory BNG actually means for developers, and what most are getting wrong

Biodiversity Net Gain became mandatory for most planning applications in February 2024. For major infrastructure, it followed later that year. By now, most developers, planners and land managers are at least familiar with the 10% net gain requirement and the statutory metric.

But familiarity with the number isn’t the same as being ready for what the obligation actually demands.

The gap between planning and delivery

The BNG process has two distinct phases that most organisations treat as one.

The first is the application phase: calculating baseline habitat units, designing an enhancement scheme, demonstrating how 10% net gain will be achieved, and securing planning consent. This is where most attention and most professional resources go.

The second is the delivery phase: implementing the habitat scheme, monitoring its condition, evidencing delivery, reporting to planning authorities, and maintaining the obligation across a 30-year stewardship period. This is where most problems emerge.

The statutory metric gets you through the door. What happens after the door is where the regulatory exposure lies.

What “30 years” actually means

A 30-year BNG obligation attached to a planning permission is not a planning condition in the conventional sense. It is a legal land management commitment that survives changes in ownership, changes in management, and changes in corporate structure.

A developer who secures planning consent in 2026 is creating an obligation that runs to 2056. The land agent, the habitat manager, and the monitoring contractor may not all exist in their current form by the time that obligation matures.

Most organisations have no system designed to track obligations across that timescale. They have planning files. They have email chains. They may have a spreadsheet. None of this constitutes a reliable evidence trail when a planning authority queries compliance fifteen years in.

The most common failures

1. Treating BNG as a planning condition rather than an asset obligation

Planning conditions are discharged. BNG obligations are managed. The distinction is important because it changes who owns the problem after practical completion. If the answer is “the planning team,” that’s a governance failure.

2. Habitat management plans that don’t survive handover

A habitat management plan prepared by an ecologist for a planning application is usually a good document, but not always. We are working to benchmark that service. The big question is whether the organisation receiving it at handover has the skills, the systems and the contractual arrangements to implement it.

3. Monitoring gaps

Year one monitoring will undoubtedly be done. Year five monitoring may be more variable. Year ten onwards is where we may see significant gaps, particularly where the original delivery team has moved on, and no formal obligation tracking system exists.

4. Off-site unit reliance without verification oversight

Purchasing off-site BNG units is legitimate and often necessary. But it transfers the delivery risk to a third party. Developers who buy units and consider their obligation discharged are exposed if the off-site habitat project fails to deliver. The obligation remains on the planning permission.

What good looks like

Organisations that are managing BNG well tend to have a few things in common.

They treat BNG obligations as they treat other long-term asset liabilities; something tracked on a register, evidenced systematically, and reported on a schedule. They won’t rely on institutional memory or email chains.

They’ll have clear internal ownership of the obligation that doesn’t sit exclusively in the planning team. Facilities, estates or environmental management functions will be better placed to hold long-term monitoring obligations.

They’ll engage with their monitoring contractors through formal, documented arrangements rather than informal relationships that dissolve when personnel change.

And they’ll build an evidence trail from day one, including site photographs, ecological assessments, and management records, in a format that can be retrieved and presented to an authority or an investor years later.

Why is this an issue now?

Obligations that are informally managed through the early years of mandatory BNG will come under increasing scrutiny.

For developers with significant land portfolios, the aggregate obligation across multiple sites is also becoming a material due diligence question. Investors and acquirers are asking about environmental liability trails in a way they weren’t three years ago.

Getting this right is not complicated. But it requires treating BNG as an operational obligation, from the point of consent, not a planning milestone.