Biodiversity Net Gain: What It Means for Your Planning Application

How the mandatory 10% biodiversity net gain requirement affects planning applications - what it means, who it applies to, and how to comply.

Since February 2024 (for major developments) and April 2024 (for smaller developments), all planning permissions in England must deliver a minimum 10% biodiversity net gain (BNG). This is one of the biggest changes to the planning system in recent years, and it affects almost every planning application that involves new construction.

What Is Biodiversity Net Gain?

Biodiversity net gain means that a development must leave the natural environment in a measurably better state than before the development took place. Specifically, the habitats on the site after development must have at least 10% more biodiversity value than the habitats that were there before.

Biodiversity value is measured using the statutory biodiversity metric (currently version 4.0), which assigns numerical values to different habitat types based on their ecological quality. A wildflower meadow scores higher than improved grassland, which scores higher than hard standing.

Who Does It Apply To?

BNG applies to most planning permissions granted under the Town and Country Planning Act 1990. This includes:

• All major developments (10+ homes or 1,000+ m² commercial)
• All minor developments (including single homes and extensions that require planning permission)
• Outline and reserved matters permissions

Exemptions

The following are currently exempt from mandatory BNG:

• Permitted development - including householder extensions, prior approvals, and other PD rights
• Small sites affecting less than 25 m² of habitat or 5 metres of linear habitat (hedgerow)
• Self-build and custom-build homes (individual plots)
• Biodiversity gain sites themselves
• Urgent crown development

In practice, most householder projects (extensions, outbuildings) are exempt because they either fall under PD or affect less than 25 m² of habitat. BNG primarily affects new-build housing, commercial development, and larger schemes.

How It Works in Practice

Step 1: Baseline assessment

Before submitting a planning application, you need a baseline biodiversity assessment of the site. This measures the existing habitats using the statutory metric. A qualified ecologist surveys the site and maps each habitat type, recording its condition and extent.

Step 2: Calculate the impact

The metric calculation shows how the proposed development will change the biodiversity value - which habitats will be lost (buildings, roads, gardens replacing existing habitats) and which will be created or enhanced (landscaping, green roofs, wildflower areas, ponds).

Step 3: Achieve 10% net gain

The development must deliver at least 10% more biodiversity units than the baseline. This can be achieved through:

1. On-site habitat creation - designing landscaping, green roofs, ponds, hedgerows, and planting within the development site. This is the preferred approach
2. Off-site habitat creation - if the site cannot accommodate enough new habitat, you can create or enhance habitats on another piece of land. This must be secured by a legal agreement for at least 30 years
3. Statutory biodiversity credits - as a last resort, you can purchase credits from the government (via Natural England). These are intentionally expensive (currently around £42,000 per unit) to incentivise on-site delivery

Step 4: Biodiversity gain plan

A biodiversity gain plan must be submitted to (and approved by) the council before development begins. This plan sets out exactly what habitats will be created, where, and how they will be managed for at least 30 years.

Impact on Planning Applications

BNG adds a new layer of information to planning applications:

• Ecology survey - most applications now require a baseline ecology assessment. For major developments, this is typically a full Phase 1 habitat survey
• Metric calculation - the statutory biodiversity metric spreadsheet showing baseline and post-development values
• Landscaping design - landscaping schemes now need to demonstrate biodiversity value, not just visual amenity
• 30-year management plan - newly created habitats must be maintained for at least 30 years, secured by a planning condition or legal agreement

Costs

For a single new dwelling, the BNG cost is typically £2,000-£5,000 for the assessment and a modest amount of on-site planting. For major developments, the cost can be £50,000-£200,000+.

Tips for Applicants

1. Commission the ecology survey early - some surveys can only be done at certain times of year (bat surveys: May-September, great crested newt surveys: March-June)
2. Design for biodiversity from the start - it is much easier (and cheaper) to achieve BNG if the landscape architect works with the ecologist from the initial design stage
3. Don't clear the site before applying - deliberately destroying habitat to reduce the baseline is illegal under the Environment Act 2021. The council can use a pre-development baseline (the habitat value on a reference date) to prevent this
4. Consider green roofs - even small developments can gain biodiversity units from green or brown roofs
5. Check local requirements - some councils require more than 10% (e.g. 20% in some local plans)