Guide
BNG for small sites (under 10 units)
Biodiversity net gain guidance specifically for small residential developments. Covers exemptions, the Small Sites Metric, simplified assessments, and cost-effective delivery options for SME developers.
Biodiversity net gain (BNG) applies to small sites from April 2024. While you must still achieve 10% net gain, the requirements are simplified for smaller developments with easier-to-use tools and some exemptions.
This guide is for SME developers and small housebuilders who need practical, proportionate BNG compliance for minor residential schemes.
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
- ✓ The site is below the proposed 0.2ha threshold (if introduced)
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:
- On-site first: Maximise what you can achieve within your site
- Off-site second: Purchase units from habitat banks if on-site falls short
- 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
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Check if you're exempt
Review the exemption criteria: self-build, householder application, or de minimis impact (under 25sqm). If exempt, note this in your planning statement and you're done.
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Walk the site and identify habitats
Survey your site noting: areas of grass (lawn vs meadow), trees and shrubs (native vs ornamental), hedgerows, water features, hardstanding, and bare ground. Photograph everything.
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Check for priority habitats and protected species
Use the MAGIC map to check for designated sites, priority habitats, and protected species records near your site. If any are present, you'll need professional ecological input.
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Complete the Small Sites Metric
Download the SSM spreadsheet from GOV.UK. Input your existing habitats (baseline) and proposed habitats (post-development). The spreadsheet calculates whether you achieve 10% net gain.
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Design to achieve net gain
If your initial calculation falls short, adjust your design: retain more existing features, add more native planting, include wildlife features. Iterate until you achieve 10%.
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Write your biodiversity gain plan
Document your approach: baseline assessment, proposed habitats, how you'll achieve net gain, and how habitats will be managed for 30 years. For small sites this can be a few pages.
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Include in your planning application
Submit the completed SSM spreadsheet, biodiversity gain plan, and habitat maps with your planning application. The LPA will assess as part of validation and determination.
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Secure through planning conditions
Expect a planning condition requiring delivery of your biodiversity gain plan. Your landscaping scheme should match what you've committed to in the plan.
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Deliver and maintain
Implement your habitat creation as part of development. Ensure your landscape contractor understands the specification. Maintain for 30 years as committed.
SME developers: Make BNG work for you
BNG is a new cost, but it doesn't have to break small site viability. Key points for SME developers:
- Design-in from the start: BNG is cheapest when integrated into initial design, not retrofitted
- Landscape architects are key: A good landscape design achieves BNG through specification, not additional spend
- Avoid statutory credits: At £48,000-650,000 per unit, credits can kill small site viability. Always exhaust on-site and off-site options first.
- Watch for upcoming exemptions: The proposed 0.2ha exemption would exempt most small infill sites - monitor policy announcements
- Build expertise: Understanding BNG gives you competitive advantage. Sites that others can't make viable become opportunities.