Statutory Biodiversity
Metric Summary
This tool is the UK Government’s official system for assessing and quantifying biodiversity value in development projects. This is the tool we follow within Ireland. It plays a critical role in calculating what’s needed to meet Biodiversity Net Gain (BNG) requirements.
Statutory Biodiversity Metric Summary Overview
The Statutory Biodiversity Metric is the UK Government’s official tool for calculating and demonstrating biodiversity value in land development projects. It ensures that any loss of natural habitat is properly accounted for and compensated, guiding developers and ecologists in achieving measurable improvements through Biodiversity Net Gain (BNG). This summary outlines the tool’s core purpose, rules, and guiding principles.
Please first read the drop down below
The Statutory Biodiversity Metric is designed to:
Quantify biodiversity value across land parcels impacted by development.
Calculate the biodiversity units required to meet the Biodiversity Net Gain (BNG) target.
Apply to:
Area-based habitats (measured in hectares)
Hedgerows (measured in kilometres)
Watercourses (measured in kilometres)
Key considerations:
It is intended for use in planning and environmental assessments where BNG is a legal or policy requirement.
It does not apply to irreplaceable habitats, such as ancient woodland or veteran trees, which must be preserved under separate guidance.
The tool enforces several key rules to ensure biodiversity outcomes are meaningful and ecologically sound:
Minimum Net Gain: A minimum 10% increase in biodiversity units is required for all developments.
Trading Rule: Habitats lost must be replaced with others of equal or higher distinctiveness.
For example, a high-distinctiveness habitat (e.g. species-rich grassland) must be replaced by another high or very high habitat type.
Strict Application: These rules apply rigidly until the point of no net loss. Any biodiversity gains beyond this threshold may allow more flexibility in application.
Qualified Use: The metric must be used and interpreted by trained professionals, typically ecologists.
Not a Substitute: It is not a replacement for ecological judgment or habitat surveys, but should support them.
Relativity: Results are relative and context-dependent – not absolute values.
Data Integration: The metric should be used alongside other ecological data to inform decisions.
Exceeding Minimums: Developers are encouraged to go beyond the minimum 10% gain where possible.
Realistic Delivery: All proposed enhancements or habitat changes must be deliverable within realistic timeframes.
Local Restoration Focus: Priority should be given to restoration opportunities within the local landscape wherever feasible.
BNG Overview
Habitat Quality Inputs
To assess habitat value using the Biodiversity Metric, three key inputs are considered: distinctiveness, condition, and strategic significance. Each plays a role in determining the overall biodiversity value of a habitat.
1. Distinctiveness
Defines how rare or ecologically valuable a habitat type is.
Categories: Very High, High, Medium, Low, Very Low
Determined by a professional ecologist
Additional supporting evidence is required for habitats rated High or Very High
2. Condition
Reflects the current ecological state and management of a habitat.
Categories: Good, Fairly Good, Moderate, Fairly Poor, Poor
Based on management history and current ecological quality
Assessed using standard condition documents with photos and seasonal accuracy
Mixed-condition areas may be split and assessed separately
3. Strategic Significance
Indicates how important the habitat is in the context of local conservation priorities.
Levels: High, Medium, Low
Defined by local nature recovery strategies or wider landscape plans
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BNG Overview
Watercourse-Specific Measures
The Biodiversity Metric includes specific assessments for impacts on watercourses and their surrounding zones. These are evaluated separately due to their unique ecological roles.
Riparian Encroachment
Assesses impacts within the riparian zone — typically 10 metres from river or stream banks, and 5 metres from ditches.
Includes obstructions such as buildings, infrastructure, or agricultural activity
Impacts are graded as: None, Minor, Moderate, or Major
Watercourse Encroachment
Evaluates changes that affect the natural flow or ecological function of a watercourse.
Covers physical alterations such as culverting, channel straightening, or flow diversion
Graded as: None, Minor, Major (or N/A in the case of culverts)
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BNG Overview
Project Actions for Non-Watercourse Habitats
The Biodiversity Metric recognises three types of project actions that can be applied to non-watercourse habitats, each with varying levels of ecological value and impact.
Habitat Retention
Maintains the existing habitat or condition with no change to its baseline state.
This approach preserves current ecological value without enhancement or degradation
Habitat Enhancement
Improves the quality of existing habitat or transitions it to a higher-quality type.
Actions may include better management, restoration of ecological features, or species-specific interventions
Habitat Creation
Establishes new habitat areas or transforms existing land into a different broad habitat type.
For example, converting grassland into woodland
Often used to compensate for losses elsewhere or to meet Biodiversity Net Gain targets
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BNG Overview
Project Actions for Watercourse Habitats
Project actions related to watercourses focus on either preserving existing conditions or making physical or ecological improvements to enhance habitat quality and function.
Retention
Maintains the existing watercourse and riparian zone without alteration.
No physical or ecological changes are made
Enhancement
Improves the natural processes of a watercourse.
May involve actions such as removing culverts, restoring natural meanders, or improving connectivity and flow
Creation
Introduces new artificial features or significantly alters existing ones to modify the watercourse.
Examples include installing new culverts or conducting channel realignment
Note
Estuarine and sub-tidal habitats are not included in this metric.
Ponds and lakes are only recorded if they exceed a 2-hectare size threshold.
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BNG Overview
Risk Multipliers
Risk multipliers are applied to biodiversity unit calculations to reflect the uncertainty, difficulty, and proximity of habitat delivery. These adjustments ensure that projected biodiversity gains are realistic and ecologically meaningful.
1. Difficulty of Creation/Enhancement
Reflects how feasible it is to create or enhance a given habitat type.
Assessed automatically based on habitat type
Accounts for technical challenges, resources needed, and likelihood of success
2. Temporal Risk
Considers the time lag between habitat loss and the full ecological maturity of newly created or enhanced habitats.
Longer delays reduce the certainty of achieving biodiversity gains
Particularly relevant for habitats that take years to establish
3. Spatial Risk
Adjusts for the location of biodiversity improvements relative to the development site.
Within: Same Local Planning Authority (LPA) or catchment
Neighbouring: Adjacent LPA or catchment
Outside: Any other location
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BNG Overview
Individual Tree Recording (“Tree Helper”)
The Tree Helper function is used to assess individual trees based on their size, status, and location. This ensures that both losses and gains involving trees are properly accounted for within the Biodiversity Metric.
Tree Size Classification (DBH – Diameter at Breast Height)
Each tree is categorised by its trunk diameter:
Small: 7.5–30 cm
Medium: >30–60 cm
Large: >60–90 cm
Very Large: >90 cm
Recording Requirements
Trees in private gardens must be recorded if they fall into the medium size category or above
Ancient and veteran trees must always be recorded, regardless of size or location
Trees that are removed during development are recorded as losses in the baseline
Newly planted trees after development must also be recorded — unless they are part of a hedgerow or located within a private garden
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BNG Overview
Results Output
The Biodiversity Metric tool generates four key result summaries. These outputs help determine whether a development achieves the required biodiversity gains and how those gains are distributed spatially and temporally.
1. On-site Results
Shows the baseline biodiversity value, the post-intervention value, and the net unit change within the development site.
2. Off-site Results
Same as the on-site calculation but applied to off-site habitat interventions, excluding spatial risk adjustments.
3. Combined Units
Presents the total biodiversity unit change after applying spatial risk multipliers across both on-site and off-site actions.
4. Overall Net Outcome
The final summary combining all inputs, adjustments, and risk factors to give a clear picture of the total net gain or loss achieved by the project.
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