06.05.2026
Biodiversity Net Gain (BNG) is now a legal requirement for most infrastructure projects in England. It’s not emerging legislation. It’s in force. Yet many project teams still treat BNG as an ecological addendum to a planning application rather than an engineering constraint that belongs in the brief from day one. That misreading creates real cost and schedule risk, and it tends to surface at the worst possible moment.
This guide, part of our net zero and sustainability consultancy at Morson Praxis, explains what BNG actually demands of engineering teams and programme directors working on complex schemes. Not as a legal briefing: as a practical reference for people making decisions about how to plan and deliver infrastructure in England.
What Is Biodiversity Net Gain?
The legal requirement: Environment Act 2021 and the 10% mandatory BNG threshold
Biodiversity Net Gain (BNG) is an approach to development and land management that leaves biodiversity in a measurably better state than before. When a project disturbs, removes or degrades habitat, it must compensate for that loss and go beyond it. The result, measured in standardised biodiversity units, must show a net improvement on the conditions before development.
In England, that’s a statutory requirement. The Environment Act 2021 inserted a mandatory BNG condition into Schedule 7A of the Town and Country Planning Act 1990, establishing a minimum 10% net gain in biodiversity for most new development. The requirement came into force for major developments on 12 February 2024, and for smaller sites on 2 April 2024.
What distinguishes BNG from earlier biodiversity policy is precision. It’s a calculated outcome, measured using the Statutory Biodiversity Metric published by Natural England and confirmed in a Biodiversity Gain Plan (BGP) approved before construction begins. The habitat created or improved to achieve that gain must then be managed in the agreed condition for a minimum of 30 years.
Why BNG is a design constraint, not a post-consent formality
That structure matters. BNG sits before consent. It has a direct bearing on site selection, site layout, ground modelling and programme decisions made before a planning application is submitted. The project teams managing it well treat it as they would a structural or drainage constraint: something that shapes design, not something that responds to it.
BNG for nationally significant infrastructure projects: the May 2026 deadline
Nationally Significant Infrastructure Projects (NSIPs): major road and rail schemes, power stations, new energy infrastructure, are required to comply from May 2026. The lead time for ecological surveys and biodiversity gain planning at the NSIP scale is considerable, and seasonal survey windows can’t be moved to suit a programme date.
There’s an additional difference for NSIPs. A completed Biodiversity Gain Plan and statutory metric calculation must be submitted with the Development Consent Order (DCO) application itself, not as a pre-commencement condition after consent. BNG needs to be embedded in NSIP briefs now. Adding it later creates a programming problem that doesn’t resolve itself easily.
Key Aspects of Biodiversity Net Gain
The 10% figure is well known. What it actually requires in practice is less clear, and that gap tends to appear as cost and programme risk later in a project. This section covers the mechanics of the statutory metric, the nature of the 30-year obligation, the exemptions most relevant in an infrastructure context, and how two significant developments: the Local Nature Recovery Strategy (LNRS) framework and the Nature Restoration Fund, interact with the BNG regime.
How biodiversity units are calculated: the Statutory Biodiversity Metric explained
The Statutory Biodiversity Metric calculates three distinct categories of biodiversity unit: area (habitat) units, hedgerow units and watercourse units. These can’t be traded across categories. A net loss of watercourse habitat can’t be offset by creating new grassland. Each category must achieve its own minimum gain independently. On linear infrastructure schemes that cross varied habitats and land types, that’s a meaningful constraint on both site layout and survey scope.
Within each category, biodiversity units are derived from four core factors.
| Factor | What it measures |
|---|---|
| Habitat size | Area in hectares, or length in kilometres for hedgerows and watercourses |
| Ecological condition | Assessed on a stepped scale from poor through to good and above |
| Distinctiveness | A Natural England score reflecting the relative rarity and importance of the habitat type |
| Strategic significance | A multiplier linked to Local Nature Recovery Strategies, increasing unit value in identified priority locations |
A fifth factor: the spatial risk multiplier, discounts the value of offsite gains sourced at greater distance from the development.
Area, hedgerow and watercourse units: why they must be managed independently
Each unit type must show a positive outcome separately. Producing an overall surplus doesn’t satisfy the requirement if any single category is in deficit. Getting this right demands precision in both the ecological survey and the site layout, which is precisely why it needs engineering input early.
The metric is calibrated to make losses of highly distinct habitats expensive to compensate for. A site containing habitats rated medium to very high: ancient hedgerows or grasslands rich in species, will require substantially more restoration or habitat creation to achieve 10% gain than a site of low baseline value. For project teams, the implication is direct. Establishing a site’s biodiversity value before acquisition isn’t just ecologically sound. It’s financially material.
The 30-year management obligation: implications for infrastructure asset owners
Habitat measures that contribute to BNG must be secured by legal agreement and maintained in the agreed condition for a minimum of 30 years from the date of development completion. The obligation runs with the land. Changes in ownership don’t extinguish it.
For infrastructure asset owners and operators, this is a long-term liability. It belongs in asset management frameworks alongside maintenance and operational obligations, not as a planning condition to be addressed at the end of asset life. The legal instruments: section 106 obligations and conservation covenants, create binding commitments attached to the relevant land. Offsite gains must also be recorded on the national Biodiversity Gain Sites Register, administered by Natural England.
What development is exempt from biodiversity net gain?
BNG doesn’t apply to all development. The exemptions most relevant in an infrastructure context are worth knowing precisely.
- Developments affecting less than 25 square metres of habitat, or less than 5 metres of onsite linear habitat. This de minimis threshold applies only when no priority habitat is impacted.
- Planning applications submitted before 12 February 2024.
- Development on irreplaceable habitats: ancient woodland and ancient or veteran trees fall under bespoke compensation requirements rather than the standard metric.
- Certain categories of permitted development.
Some local planning authorities set thresholds above the statutory 10% minimum. Certain authorities require 20%, and that must be confirmed at the project preparation stage. It can significantly affect the metric calculation and overall delivery strategy.
Local Nature Recovery Strategies and the BNG strategic significance multiplier
Under the Environment Act 2021, 48 Local Nature Recovery Strategies are being prepared across England, covering the whole country without gaps or overlaps. Several were published in 2025, with others following.
An LNRS identifies where habitat creation will deliver the greatest benefit for nature at a landscape scale. Once published, it feeds into the Statutory Biodiversity Metric through the strategic significance multiplier. Habitat creation aligned with LNRS priorities receives a unit uplift.
For offsite BNG procurement, that’s directly relevant. Units sourced from sites that match LNRS-mapped opportunities have greater metric value than equivalent habitats created elsewhere. Before a local LNRS is published, planning authorities use existing biodiversity opportunity mapping and local policy to apply the multiplier instead.
The Nature Restoration Fund: A distinct mechanism from mandatory BNG
The Planning and Infrastructure Act 2025 introduced the Nature Restoration Fund (NRF), a mechanism that operates separately from BNG and serves a different function. It gives developers a route to discharge obligations arising from impacts on protected sites and species: through a payment to Natural England, which takes strategic responsibility for delivering conservation measures via Environmental Delivery Plans (EDPs). The first EDPs are expected in the second half of 2026.
The NRF doesn’t replace BNG. Both obligations are independent, and developers subject to both must address each in full. For large infrastructure schemes that trigger Habitats Regulations Assessment requirements, common in nuclear, energy and major transport projects, the NRF may offer a more efficient path than individually negotiated mitigation. But it has no effect on BNG accounting.
How BNG Works in Practice
Knowing the framework is one thing. Running BNG through a real infrastructure project is another. The process follows four steps: establish a baseline, calculate the gain requirement, plan delivery, and register and secure. Each has engineering and programme implications worth understanding before work is underway.
Step 1: Biodiversity baseline assessment and ecological survey timing
A Phase 1 habitat survey, supplemented by targeted ecological assessments where necessary, establishes the site’s biodiversity value using the Statutory Biodiversity Metric. The baseline must reflect the site’s current condition and can’t be improved by clearing habitat before an application is submitted. The Environment Act 2021 provides that, where habitat has been degraded or removed since January 2020, the earlier, higher-value conditions are taken as the baseline. For projects with long periods between initial site work and planning, that anti-avoidance provision demands careful programme management.
Seasonal survey windows add a further layer of constraint. Phase 1 surveys are typically conducted between late spring and early autumn. Certain species surveys have narrower windows that can’t be moved to fit a programme date. Confirming the scope of the ecological survey early is one of the most effective ways to manage pre-consent schedule exposure.
Digital tools are increasingly used to support early baseline work. Remote sensing data and aerial habitat mapping can provide an initial read on likely biodiversity unit values before a ground survey is commissioned, helping to prioritise survey scope and flag concentrations of high baseline value that may influence site layout decisions. The Statutory Biodiversity Metric is a freely available Excel-based tool published by Defra. A simplified version, the Small Sites Metric (SSM), is available for smaller developments.
Step 2: Calculating your BNG requirement using the biodiversity metric tool
Once the baseline is established, the metric calculation determines how many additional units are needed in each category to meet the required gain. This figure drives the delivery strategy. Can 10% be achieved onsite? Or is off-site contribution required? Where a site contains habitats of high distinctiveness, the shortfall to cover may be significant. The earlier that’s understood, the more options remain on the table.
Step 3: Planning BNG delivery using the biodiversity gain hierarchy
The Biodiversity Gain Plan sets out how the required gain will be delivered. It must follow the Biodiversity Gain Hierarchy: a statutory sequence that establishes the order in which delivery options should be pursued before moving to the next.
On-site biodiversity gain: the first priority in the BNG hierarchy
The hierarchy begins with avoiding adverse effects on higher-quality onsite habitats. That means rerouting infrastructure, adjusting the site layout or protecting habitat during construction. Where adverse effects can’t be avoided, they should be mitigated. The remaining biodiversity deficit is then addressed through onsite habitat improvement and the creation of new onsite habitat. On-site delivery is the first preference for a reason: it directly enhances the ecological value of the development site.
Offsite biodiversity units and statutory credits: fallback options under BNG law
Where onsite delivery isn’t sufficient, offsite gains registered on the national Biodiversity Gain Sites Register can be allocated to the scheme. If those options are also exhausted, statutory biodiversity credits can be purchased from Natural England on behalf of Defra. Statutory credits are a last resort. They’re deliberately priced less competitively than market alternatives to push developers toward genuine habitat creation rather than a financial substitute.
It’s worth clarifying how the Biodiversity Gain Hierarchy relates to the Mitigation Hierarchy set out in the National Planning Policy Framework. Both run simultaneously in an infrastructure planning context, but they serve different purposes. The NPPF Mitigation Hierarchy governs how a planning authority assesses the acceptability of ecological harm: whether it can be avoided through alternative site selection, then adequately mitigated, and only as a last resort compensated for. The Biodiversity Gain Hierarchy governs how the mandatory 10% net gain is delivered once harm has been addressed. Meeting BNG doesn’t discharge NPPF Mitigation Hierarchy requirements. Both must be satisfied independently.
Step 4: Registering and securing biodiversity net gain for 30 years
Offsite gains must be recorded on the Biodiversity Gain Sites Register. On-site gains are secured through Section 106 obligations or conservation covenants that create a binding 30-year commitment that runs with the land. These legal instruments take time to establish. Building them into the programme at the same stage as other legal due diligence reduces the risk of delay at the consent stage.
Integrating BNG into infrastructure design and civil engineering
This is where the compliance exercise becomes an engineering one. And it’s where the real opportunity is.
Infrastructure creates large areas of land under ongoing management: drainage corridors, buffer zones, embankments and interface areas with retained open land. On linear schemes, the cumulative habitat value of well-designed verges and water margins can be substantial. Grassland rich in species and wetland features can be designed into drainage and earthworks schemes at low incremental cost, provided they’re planned early enough to influence ground modelling and topsoil management strategies. Trying to fit them into a finalised design is a different order of difficulty.
Wildlife crossing features, such as underpasses and habitat connectivity measures, serve both ecological and engineering functions. At the concept design stage, integrating them into structures adds minimal cost. Retrofitting them doesn’t. Designing sustainable drainage systems and engineered wetland features to serve both flood management and biodiversity unit generation is an area where early collaboration between civil and ecological disciplines typically produces better outcomes than late integration.
Can the BNG requirement be fully met onsite?
For many schemes, the answer is no, and that makes early identification of off-site land and unit availability important. The market for biodiversity units is still developing, and the supply of the right habitat types in the right location can’t be assumed at short notice. For NSIP-scale projects, some promoters are already building estate-wide BNG strategies that combine on-site delivery with long-term habitat banking across operational land portfolios: an approach that reflects both the scale of a 30-year obligation and the practical limits of project-boundary delivery.
The sequencing question sits at the heart of all of this. BNG introduces ecological survey constraints, metric interactions with site-selection decisions, and legal mechanisms that take time to establish. Projects that bring BNG assessment into the brief at the same stage as ground investigation and flood risk assessment are in a materially better position at consent and at delivery than those that don’t.
Morson Praxis works with infrastructure clients across nuclear, defence, major infrastructure, and the built environment to integrate BNG and wider sustainability requirements into design and delivery programmes from the outset. Our net-zero and sustainability consultancy brings engineering expertise to requirements like BNG so they’re resolved on the ground rather than on paper.
