Short answer: probably not. If your development is on a site under 1 hectare with no priority habitats or protected species, the Small Sites Metric (SSM) is designed so a "competent person" — not necessarily an ecologist — can complete your BNG assessment.
This could save you £2,000–£5,000 in ecologist fees. Here's everything you need to know.
What Is the Small Sites Metric?
The SSM is a simplified version of the Statutory Biodiversity Metric. It was introduced alongside mandatory BNG for small developments in April 2024.
Where the full metric has hundreds of habitat types and requires detailed ecological assessment, the SSM uses:
- Fewer habitat categories — around 25 area habitat types vs 100+ in the full metric
- Auto-assigned distinctiveness — the tool tells you the score, no interpretation needed
- Simplified condition assessment — Good, Moderate, or Poor
- Three modules — Area habitats, hedgerows, and watercourses (same as full metric)
Can You Use the SSM?
You can only use the SSM if your project meets all of these criteria:
✅ SSM Eligibility Checklist
- Residential: 1–9 dwellings on a site ≤1 hectare (or unknown number of dwellings on site <0.5 ha)
- Commercial: Under 1,000 m² floor space OR site under 1 hectare
- Only SSM-listed habitats are present on site
- No priority habitats on site (e.g., ancient woodland, species-rich grassland)
- No statutory protected sites on site (SSSIs, SACs, SPAs)
- No European protected species on site (great crested newts, bats, etc.)
If any of these conditions aren't met, you must use the full Statutory Biodiversity Metric, which typically requires an ecologist.
Tip: If protected sites or priority habitats are within 500m of your boundary, DEFRA recommends considering the full metric even if your site itself is clear.
What Does "Competent Person" Mean?
The SSM must be completed by someone with appropriate competence, aligned to BS 8683:2021. This doesn't mean they need to be an ecologist — it means they need:
- Understanding of the BNG process and the SSM tool
- Ability to identify the simplified habitat types in the field
- Understanding of habitat condition (Good/Moderate/Poor)
- Knowledge of when to escalate to a full ecological assessment
In practice, this could be a planning consultant, landscape architect, site manager, or even a developer who has done their homework.
How the SSM Calculates Biodiversity Units
The formula is the same concept as the full metric:
📐 Biodiversity Units = Size × Distinctiveness × Condition × Strategic Significance
For post-development habitats, two additional multipliers apply:
- Temporal multiplier — how long the habitat takes to reach target condition (faster = more credit)
- Difficulty multiplier — how hard it is to create or enhance that habitat type
Distinctiveness Scores (Auto-Assigned)
| Band | Score | Examples |
|---|---|---|
| Medium | 4 | Woodland, neutral grassland, scrub, ponds, allotments |
| Low | 2 | Modified grassland, amenity grassland, gardens, arable, green roofs |
| Very Low | 0 | Sealed surfaces, buildings, bare ground |
Note: Very High and High distinctiveness habitats (ancient woodland, lowland meadow, etc.) are priority habitats and are excluded from the SSM entirely.
Condition Scores
| Condition | Score |
|---|---|
| Good | 3 |
| Moderate | 2 |
| Poor | 1 |
Worked Example
Imagine a 0.5 hectare site with:
- 0.3 ha modified grassland (Low, Poor condition) = 0.3 × 2 × 1 = 0.6 units
- 0.15 ha amenity grassland (Low, Poor) = 0.15 × 2 × 1 = 0.3 units
- 0.05 ha sealed surface (V.Low) = 0.05 × 0 × 1 = 0 units
Baseline total: 0.9 units
10% target: 0.99 units
You need to deliver at least 0.99 area habitat units post-development. That's only 0.09 more units — achievable with a modest rain garden, native planting scheme, or green roof.
BioGain has a built-in SSM calculator
Auto-detects eligibility, pre-fills your baseline from government data, and calculates your 10% target. No spreadsheet required.
Try the SSM Calculator →Trading Rules
Even in the simplified metric, you can't just plant anything anywhere:
| Lost Habitat Band | Must Be Replaced By |
|---|---|
| Medium | Medium or higher, same broad habitat type |
| Low | Low or higher (more flexible) |
| Very Low | No trading rules apply |
Trading rules only apply up to the "no net loss" point. Once you've replaced what was lost, additional units to hit the 10% target can come from any habitat type.
SSM vs Full Metric: When to Upgrade
| Scenario | Use SSM | Use Full Metric |
|---|---|---|
| 4 houses on 0.3 ha greenfield | ✅ | |
| 8 houses on 0.8 ha with mature woodland | ✅ (priority habitat) | |
| Small office on 0.5 ha brownfield | ✅ | |
| 9 houses on 0.9 ha near an SSSI | ⚠️ Consider full | ✅ (safer) |
| 12 houses on 1.5 ha | ✅ (exceeds 1 ha) |
What You Submit
For an SSM assessment, you'll need to provide:
- Completed SSM calculation — the filled-in metric tool
- Biodiversity Gain Plan — how you'll achieve +10%
- Simplified Habitat Management and Monitoring Plan — maintenance for 30 years
- Pre and post-development habitat maps
- Evidence of competency — who completed it and their qualifications
Common Mistakes with the SSM
- Using SSM when priority habitats are present — If there's species-rich grassland or traditional orchard, you need the full metric
- Forgetting hedgerows — They're a separate module. You can't offset lost hedgerows with grassland
- Not checking for protected species — Even on small sites, bat surveys or GCN checks may still be needed separately from BNG
- Confusing SSM eligibility with BNG exemption — Using the SSM doesn't exempt you from BNG. It's just a simpler way to calculate it
About BioGain
BioGain auto-checks SSM eligibility using real data — SSSIs, priority habitats, and ancient woodland from government sources. If your site is eligible, our built-in SSM calculator lets you assess your baseline and plan for 10% gain without touching the statutory metric spreadsheet. Try it free →