Biodiversity Net Gain in the UK Why It’s Not Quite What It Seems


Biodiversity Net Gain (BNG) is widely heralded as a significant step forward for nature in England. Under the Environment Act 2021 and associated guidance, most new developments now legally must deliver at least a 10 per cent increase in biodiversity compared to what was there before. But, as with many well-intentioned policies, the details matter, and some of them reveal worrying loopholes or weaknesses. In its current form, BNG risks falling short of its promise unless its mechanisms are strengthened.

What the Policy Actually Requires

Under the statutory BNG regime, developers must use a standard biodiversity metric to calculate biodiversity units, based on the size, type, quality and location of habitats. Once they have this metric, they must plan how to deliver an uplift of at least 10%. If they can’t achieve that on the development site (“on-site”), they may deliver gains “off-site” (on different land) or, as a last resort, buy statutory biodiversity credits from the government. 

Crucially gains must be maintained for a minimum of 30 years, secured via a legal agreement such as a Section 106 planning obligation or a conservation covenant.

The Loopholes and Risks

While 30 years might sound like a long time, it is not permanent protection, and that creates some potential perverse incentives and weaknesses:

  1. Limited Duration of Commitment
    After the 30-year period expires, landowners are not legally bound to maintain the habitats under BNG. According to legal analysis, once the 30-year covenant ends, landowners could renegotiate goals, re-sell units, or potentially repurpose the land. This means the “net gain” isn’t guaranteed in perpetuity, it may simply be delayed or deferred.

  2. Off-Site Gains Can Be Far from the Development
    The policy allows developers to meet their biodiversity obligations by creating or restoring habitat outside their development boundary. There is no requirement that the offset land be in the same county as the development. In practice, this means a house built in, say, Kent, could be offset by habitat created hundreds of miles away in northern England, wherever land is cheaper.

    This creates a kind of “geographic arbitrage” developers may favour buying land in regions where land value is lower, rather than investing in local ecological restoration.

  3. Risk That Gains Are Not Realised Properly
    In some cases, off-site land purchased for BNG may not be converted quickly into high-quality habitat. If habitat creation or enhancement is delayed, the risk arises that the land reverts to negotiations and the gains are “used” multiple times for different developments. As one legal commentary warns, if the initial habitat goals are achieved ahead of schedule, there is nothing to stop landowners from setting new targets or selling more credits for a new period.

  4. Nature Quality Not Fully Accounted For
    The biodiversity metric scores units based on habitat type, area, condition, and “strategic significance.” But it does not directly value the full ecological complexity, such as roosting birds, rare species, soil invertebrates, or the biomass of a habitat. As a result, offset sites may be evaluated on a relatively superficial basis, focusing on what “grows there” in broad habitat categories rather than the richness of life. This is a serious flaw: high-value ecological features, like mature woodland used by vulnerable species or sensitive water bodies, may be under-valued.

  5. Potential for “Land Banking”
    Because BNG units can be registered on a national biodiversity gain sites register, developers and landowners can form so-called habitat banks. These banks sell units to developers who need offset credit. While this system provides flexibility, there’s concern that habitat bank land could be over-allocated or creatively managed to serve multiple development obligations, especially when the 30-year horizon is the only binding commitment.

  6. Local Accountability Risks
    Some local planning authorities have noted that off-site delivery should ideally be within their own area, yet developers may choose to deliver gains far afield. For example, a Kent council policy states that off-site delivery should focus on sites within its own borough before looking further away. If these local preferences are overridden by commercial deals, local nature recovery strategies may be bypassed.

Why This Matters  A Regional Example

  • South East England, including counties like Kent, Surrey and Sussex, is among the most expensive land in the country.

  • In contrast, Northern England generally has lower land prices.

  • Developers building in the South East may find it financially attractive to purchase BNG units on cheap land up north rather than deliver high-cost on-site habitat enhancement.

This dynamic could result in less nature investment happening locally in the South East, while distant land bears the burden of “offset” for developments far away. Over time, this could disconnect local communities from tangible nature benefits, and even fill more marginal northern land with habitat banks that serve external demands rather than local ecological priorities.

Why the 30-Year Limit Is a Real Problem

A 30-year covenant may sound long, but in ecological terms, many habitats take far longer to mature, and species dynamics evolve slowly. Trees planted now, for example, may only reach full ecological value after several decades; bird populations or soil ecosystems may take even longer to stabilise.

If the covenant lapses after 30 years, there is no guarantee that the habitat will be preserved:

  • Landowners may choose not to renew agreements or draft looser terms.

  • They could reclassify or repurpose land for other uses if more profitable.

  • Future development or other market pressures could threaten the offset sites.

In effect, the 30-year rule makes BNG more of a time-limited accounting trick than a guarantee of long-term environmental stewardship.

What Needs to Change Recommendations for Stronger Protection

To ensure BNG truly benefits nature, not just developers’ balance sheets, policy reform and stronger safeguards are needed. Here are some proposals:

  1. Extend or Remove the 30-Year Limit

    • Shift from a 30-year covenant to perpetual protection, perhaps via conservation covenants or transferring land into trusted conservation bodies (e.g., NGOs, trusts).

    • Alternatively, require rolling management plans that are renewed with meaningful ecological review, rather than simply ending.

  2. Tighten Geographic Rules for Off-Site Gains

    • Require a minimum proportion of biodiversity gains to be delivered locally (e.g., within the same local authority or Natural Recovery Strategy area) before off-setting further afield.

    • Prioritise habitats that contribute to local nature recovery strategies, not just wherever land is cheapest.

  3. Improve the Metric to Better Capture Ecological Value

    • Refine the biodiversity metric to consider species richness, rarity, biomass, and functional ecological processes (e.g., roosting, nesting, soil health), not just broad habitat categories.

    • Introduce qualitative ecological assessments alongside the metric, so offset sites aren’t judged purely on “what grows there” superficially.

  4. Strengthen Monitoring, Enforcement, and Transparency

    • Require robust, independent monitoring throughout the 30-year (or longer) period, with publicly accessible reports.

    • Empower local planning authorities and nature bodies to enforce the terms of BNG covenants rigorously.

    • Ensure the national biodiversity gain register is transparent and up to date, so communities can track which land is being used for gains, by whom, and whether their ecological obligations are being met.

  5. Support Habitat Banks That Serve Conservation, Not Just Commercial Demand

    • Encourage models where conservation organisations, not just profit-driven developers, run habitat banks.

    • Provide incentives (grants, tax relief) for landowners to commit land to long-term ecological restoration, not just short-term credit sales.

  6. Embed BNG in a Broader Nature Strategy

    • Align BNG with Local Nature Recovery Strategies (LNRS) so gains contribute to coherent, strategic ecological goals, rather than ad hoc offsetting.

    • Integrate BNG into national and local conservation priorities, ensuring that green corridors, protected species, and sensitive habitats are not side-lined.

Conclusion

Biodiversity Net Gain is a powerful and important policy tool, but as currently structured, it risks being less durable and less locally meaningful than public perception suggests. The reliance on 30-year agreements, combined with the flexibility to offset far from development sites, and a metric that undervalues ecological complexity, all weaken its environmental integrity.

To truly deliver for nature and future generations, BNG must be reinforced with stronger, longer-term safeguards, geographically tighter rules, and a richer understanding of ecological value. Only then can the policy move from being a clever compliance mechanism to a genuine force for lasting nature recovery across the UK.

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