What Is Biodiversity Net Gain (BNG)?

Biodiversity Net Gain, commonly known as BNG, is now a central part of the planning system in England. Introduced under the Environment Act 2021, BNG requires most new developments to leave biodiversity in a measurably better state than before development began.

For developers, landowners and planning consultants, BNG is no longer an emerging policy. It is now a material planning requirement that directly affects land acquisition, site viability, programme timelines and planning risk.

Understanding how Biodiversity Net Gain works is essential for anyone involved in residential, commercial or strategic land development.

What Does Biodiversity Net Gain Mean?

In simple terms, Biodiversity Net Gain requires developers to compensate for biodiversity losses caused by development and deliver at least a 10% increase in biodiversity value.

This uplift must be maintained for a minimum of 30 years.

The biodiversity value of a site is measured using the statutory Biodiversity Metric developed by Natural England. The metric assesses habitats based on factors such as:

• habitat type

• habitat condition

• distinctiveness

• strategic significance

• area and connectivity

The result is calculated in biodiversity units.

If development causes a loss of habitat units, developers must replace and enhance biodiversity either on-site or off-site.

When Did BNG Become Mandatory?

Mandatory BNG became law for most major developments in February 2024, with small sites following in April 2024.

Local planning authorities across Southern England are now routinely requesting:

• biodiversity metric calculations

• baseline habitat surveys

• BNG delivery strategies

• habitat management plans

• legal agreements securing long-term delivery

In practice, BNG is now a standard part of planning applications.

How Is Biodiversity Net Gain Delivered?

There are three main ways developers can achieve BNG compliance.

1. On-Site Biodiversity Enhancements

This involves creating or enhancing habitats within the development boundary itself.

Examples may include:

• new woodland planting

• species-rich grassland

• ponds and wetlands

• hedgerow creation

• sustainable drainage habitats

  
  

While on-site delivery can reduce the need for off-site units, many schemes face practical limitations due to land availability, viability pressures or design constraints.

2. Off-Site Biodiversity Units

Where sufficient gains cannot be achieved on-site, developers can secure off-site biodiversity units from a third-party provider.

This has quickly become one of the most common approaches for housing schemes across Hampshire, Dorset, Surrey, West Sussex and the wider South of England.

Off-site mitigation allows developers to:

• maintain developable area

• reduce site design constraints

• secure planning certainty

• access strategically located habitat banks

• simplify long-term management obligations

  
  

The off-site market is growing rapidly as demand for biodiversity units increases.

3. Statutory Biodiversity Credits

As a final option, developers can purchase statutory biodiversity credits from the government.

However, these credits are intentionally priced at a premium and are generally considered a last resort where private market solutions are unavailable.

Why BNG Matters to Developers

Biodiversity Net Gain is not simply an ecology issue. It is now a commercial planning issue.

Poorly managed BNG requirements can create:

• planning delays

• viability challenges

• redesign costs

• legal complications

• programme uncertainty

  
  

Conversely, developers who address BNG early in the planning process are often able to secure more cost-effective and deliverable solutions.

Key considerations include:

Land Strategy

Sites with limited ecological constraints may still require significant off-site mitigation depending on habitat impacts and unit calculations.

Understanding likely BNG liabilities at acquisition stage is becoming increasingly important.

Cost Certainty

The biodiversity unit market varies significantly depending on location, habitat type and local supply.

Early engagement can help developers secure unit availability before planning submission.

Planning Risk

Many local authorities now expect clear evidence that BNG delivery is both achievable and legally secured prior to determination. A lack of mitigation strategy can delay decision-making.

Opportunities for Landowners

BNG is also creating long-term opportunities for landowners.

Agricultural land, low-yield land or strategically located sites may be suitable for habitat creation schemes capable of generating biodiversity units for sale.

This has led to growing interest in:

• habitat banks

• conservation covenants

• long-term environmental income

• natural capital strategies

  
  

In some areas of Southern England, demand for biodiversity units is already exceeding supply.

What Is the Future of the BNG Market?

The Biodiversity Net Gain market is still developing, but several trends are already becoming clear:

• demand for off-site units is increasing

• local unit supply remains constrained in many regions

• planning authorities are tightening scrutiny

• strategic habitat banks are becoming more valuable

• developers are seeking earlier mitigation agreements

  
  

As the market matures, securing reliable mitigation delivery is likely to become an increasingly important part of development strategy.

Final Thoughts

Biodiversity Net Gain is fundamentally changing how development and environmental mitigation interact within the planning system.

For developers, the key challenge is no longer whether BNG applies. It is how to deliver it efficiently, cost-effectively and without delaying planning.

For landowners, BNG presents a growing opportunity to generate long-term value from land through environmental markets.

As regulation, local authority expectations and mitigation demand continue to evolve, early strategy and access to reliable off-site solutions will become increasingly important.

If you are assessing a development site or exploring off-site mitigation opportunities, Bio Diverse can help you navigate the practical and commercial realities of Biodiversity Net Gain across Southern England.