What Is Biodiversity Net Gain? A Developer's Complete Guide

Since February 2024, every planning application in England must demonstrate a minimum 10% biodiversity net gain (BNG). This isn't optional — it's the law, introduced through the Environment Act 2021 and enforced by local planning authorities.

If you're a developer, landowner, or anyone involved in the planning process, this guide explains exactly what BNG means for your project, what it costs, and how to comply without losing weeks to ecological surveys.

What Is Biodiversity Net Gain?

Biodiversity Net Gain is a legal requirement that developments must leave biodiversity — the variety of plant and animal life — in a measurably better state than before the development took place.

In practice, this means:

• Your site's baseline biodiversity value is measured in "biodiversity units" using the Statutory Biodiversity Metric
• Your post-development plan must show at least 10% more units than the baseline
• This must be maintained for at least 30 years

Think of it like carbon offsetting, but for nature. If you build on grassland, you need to create or enhance enough habitat elsewhere to more than replace what was lost.

Who Does It Apply To?

Since April 2024, BNG applies to virtually all planning permissions in England, including:

• Major developments (10+ dwellings or 1+ hectare) — since February 2024
• Small developments (1-9 dwellings, under 1 hectare) — since April 2024

Some exemptions exist for householder applications (extensions, loft conversions), permitted development, and urgent Crown developments. But if you're building new homes or commercial space, you almost certainly need BNG.

The Statutory Biodiversity Metric

The metric is the official tool for calculating biodiversity units. Currently at the current statutory version, it considers:

Factor	What It Measures
Habitat type	What's growing there (woodland, grassland, wetland, etc.)
Area or length	Size in hectares (area habitats) or kilometres (hedgerows, watercourses)
Distinctiveness	How ecologically valuable the habitat type is (V.Low to V.High)
Condition	How healthy the habitat is (Poor, Moderate, Good)
Strategic significance	Whether it's identified in local nature recovery strategies

The formula multiplies these factors together. Higher quality habitats generate more biodiversity units — which means losing them costs more to replace.

Three Separate Modules

The metric calculates three types of biodiversity units separately:

1. Area habitat units — for woodlands, grasslands, urban habitats, etc.
2. Hedgerow units — measured in linear metres
3. Watercourse units — rivers, streams, ditches, canals

The 10% gain target must be met in each module independently. You can't offset lost hedgerows with extra grassland.

Small Sites Metric (SSM)

For small developments — sites under 1 hectare with no priority habitats — there's a simplified version called the Small Sites Metric. Key differences:

• Fewer habitat types to choose from
• Distinctiveness is auto-assigned (you don't need to score it)
• No ecologist required — a "competent person" can complete it
• Simpler condition assessment

The SSM is designed so that developers, architects, or planning consultants can handle BNG compliance themselves on small sites, without the cost of hiring an ecologist.

How Much Does BNG Cost?

This is where developers get nervous. The costs vary enormously depending on your site:

Approach	Typical Cost	Best For
Automated screening (BioGain)	14-day free trial, then £39/mo	Early feasibility, due diligence, small sites
Ecologist survey	£2,000 – £10,000+	Complex sites, priority habitats, planning submission
On-site habitat creation	Varies	Landscaping within your development
Off-site credits	£20,000 – £50,000+ per unit	When on-site gains aren't achievable
Statutory credits (last resort)	£42,000+ per unit	When no other option exists

The smart approach: Use an automated screening tool first to understand what's on your site. If it's a small site with no priority habitats, you may be able to handle BNG yourself using the Small Sites Metric. If the site is complex, you'll know before you've committed £5,000 to an ecologist.

What You Need to Submit

For a planning application with BNG, your Local Planning Authority will typically require:

1. Completed biodiversity metric — the statutory biodiversity metric calculation tool (or SSM for small sites)
2. Biodiversity Gain Plan — showing how you'll achieve the 10% gain
3. Habitat Management and Monitoring Plan — how habitats will be managed for 30 years
4. Pre-development habitat map — what's on the site now
5. Post-development habitat map — what you plan to create/retain

The Mitigation Hierarchy

BNG doesn't mean you can bulldoze ancient woodland and plant some wildflowers. There's a strict hierarchy:

1. Avoid — Design around existing habitats where possible
2. Minimise — Reduce the impact on habitats you can't avoid
3. Restore — Rehabilitate damaged habitats on-site
4. Offset — Create new habitats to compensate for unavoidable losses

LPAs will want to see evidence that you've followed this hierarchy before approving off-site solutions.

Trading Rules

You can't replace high-value habitats with low-value ones. The statutory biodiversity metric enforces trading rules:

• Medium distinctiveness losses must be replaced by medium+ habitats in the same broad habitat type
• High/Very High distinctiveness losses require like-for-like or better replacement
• Low/Very Low habitats have more flexible trading options
• Trading rules apply until you reach "no net loss" — after that, you can use any habitat type to hit the 10% target

Common Mistakes

1. Not checking early enough — Finding out your site has ancient woodland at planning stage is an expensive surprise. Screen the site early.
2. Ignoring hedgerows — They're a separate module in the metric. Removing hedgerows requires hedgerow replacement, not just grassland.
3. Assuming off-site credits are cheap — At £20,000-£50,000+ per unit, buying credits can significantly affect project viability.
4. Using the wrong metric version — Make sure you're using Statutory Biodiversity Metric (the current version), not an older version.
5. Forgetting the 30-year commitment — Habitat management plans need funding and monitoring for three decades.

Key Dates

Date	What Happened
November 2021	Environment Act receives Royal Assent
November 2023	Statutory Biodiversity Metric published
February 2024	BNG mandatory for major developments
April 2024	BNG mandatory for small developments
2025	Statutory Biodiversity Metric updated, Small Sites Metric revised

Getting Started

If you're a developer with a site in England, here's the practical path:

1. Screen your site — Use BioGain to check what's there (free, takes 60 seconds)
2. Check SSM eligibility — If under 1 hectare with no priority habitats, you may not need an ecologist
3. Understand the baseline — Know your biodiversity units before designing your scheme
4. Design for gain — Green roofs, SuDS, native planting, hedgerow retention all contribute
5. Get professional help if needed — Complex sites with protected species or priority habitats need an ecologist

The earlier you understand your site's biodiversity, the cheaper and easier compliance will be.