BNG for small sites (under 10 units)

Does BNG apply to your small site?

Quick exemption check

Your development is exempt from BNG if:

• ✓ It's a single dwelling for your own occupation (self-build)
• ✓ It's a householder application (extending your existing home)
• ✓ The total habitat impact is less than 25 square metres and less than 5 metres of linear feature, and no onsite priority habitat is affected
• From end July 2026, sites of 0.2 hectares or less are exempt (a confirmed exemption, unless priority habitat is affected); until then, small sites of any size remain in scope

Not exempt: Sites that are entirely hardstanding still require BNG - your baseline is zero, so any habitat creation achieves gain.

The Small Sites Metric

For small sites (under 0.5 hectares or fewer than 10 dwellings), you can use the Small Sites Metric (SSM) instead of the full statutory metric. This is simpler and faster to complete.

Key differences from the full metric

• Fewer habitat types: Broad habitat categories instead of detailed UKHab classification
• Simplified condition assessment: Good, moderate, or poor instead of detailed criteria
• Quicker surveys: Competent surveyors (not necessarily full ecologists) can complete the assessment
• Same 10% target: You still need to achieve 10% net gain, but calculation is simpler

When you MUST use the full metric

The Small Sites Metric cannot be used if your site has:

• Priority habitats (ancient woodland, species-rich grassland, etc.)
• Protected species (great crested newts, bats, etc.)
• Irreplaceable habitats that require bespoke assessment

In these cases, you'll need the full statutory metric and professional ecological input.

Cost-effective BNG for small sites

BNG can feel disproportionately expensive for small schemes. Here's how to minimise costs while achieving compliance:

Maximise on-site delivery

On-site habitat creation is almost always cheaper than buying off-site units or credits. Even small gardens and shared spaces can deliver biodiversity units:

• Native hedgerow: 5m of new native hedge can offset significant losses elsewhere
• Wildlife-friendly planting: Specify native species in soft landscaping
• Bird and bat boxes: Built-in features count towards net gain
• Green roofs: Even small areas of sedum roof generate units
• Wildflower turf: Use instead of standard lawn where appropriate

Retain existing features

Avoiding loss is cheaper than compensating. Design your scheme to retain:

• Mature trees (especially native species)
• Existing hedgerows and boundary vegetation
• Any areas of semi-natural vegetation
• Ponds and water features

Every feature retained reduces your net gain requirement.

Use the hierarchy

Only move to off-site delivery if on-site is genuinely impossible:

1. On-site first: Maximise what you can achieve within your site
2. Off-site second: Purchase units from habitat banks if on-site falls short
3. Statutory credits last: Only as genuine last resort (very expensive)

Typical small site scenarios

Infill garden site (0.1 hectares)

Typical baseline: Amenity grassland (lawn), ornamental planting, maybe a few trees

Typical strategy: Retain trees, replace lawn with wildflower meadow or native planting, add native hedging along boundaries, include bat/bird boxes

Likely outcome: Achievable on-site with good landscape design

Brownfield conversion (0.2 hectares)

Typical baseline: Hardstanding, some colonising vegetation, possibly scrub

Watch out: Open mosaic habitats on brownfield sites can have surprisingly high biodiversity value

Typical strategy: Create habitat where there was none - green roofs, permeable paving with planting, wildlife areas in corners

Likely outcome: Usually achievable on-site as baseline is often low

Agricultural field (0.4 hectares)

Typical baseline: Improved grassland or arable - low biodiversity value

Typical strategy: Retain field boundaries, create wildflower areas in communal spaces, native planting throughout

Likely outcome: Often achievable on-site; agricultural baselines are usually modest

What you need to submit

For exempt development

If your development is genuinely exempt (self-build, de minimis impact), you should still explain this in your planning statement. No biodiversity gain plan is required.

For small sites using the SSM

• Completed Small Sites Metric spreadsheet showing baseline and proposed values
• Brief biodiversity gain plan explaining your strategy
• Simple habitat map showing existing and proposed habitats
• Landscape plan indicating habitat creation measures
• 30-year management outline (can be brief for small sites)

Professional input

For simple small sites, a competent landscape architect or surveyor may be able to complete the SSM without a full ecological consultant. However, if your site has:

• Trees subject to TPO
• Potential protected species
• Priority habitats
• Complex vegetation requiring UKHab classification

You'll need professional ecological input.

Step-by-step: BNG for your small site