Guidance on the use of existing biodiversity metrics in the Scottish planning system

Biodiversity Net Gain (BNG) 

Biodiversity Net Gain (BNG) is a statutory requirement in England for certain developments to offset all biodiversity losses and deliver a 10% BNG, which must be calculated and demonstrated using the English metric. 

Note: The respective planning systems in Scotland and England utilise different delivery mechanisms in seeking to address the global biodiversity crisis.  Although there is familiarity with the term BNG in Scotland, BNG terminology specifically relates to England’s statutory framework and is not referred to in Scottish planning policy. 

Biodiversity Units (BUs)

Biodiversity Units (BUs) are the ‘currency’ of the English metric and SSER toolkit, providing the output unit by which biodiversity value is measured. Baseline (pre-development) BUs can be compared against expected post-development BUs to estimate whether losses have been fully offset, and enhancement has been delivered.  BUs are bespoke to the particular metric methodology that is used to calculate them (i.e. they cannot be compared to anything measurable outside of the metric tool that was used to calculate them).  

In both metric tools, there are three different types of BUs that can be calculated: area-based habitat units, watercourse units and hedgerow units.  Due to their differing input data, these three BU unit types are not directly comparable and are calculated separately within the metric tools.  Any losses and enhancement must be fully achieved for each unit type (e.g. losses of watercourse units cannot be offset by delivering habitat units, additional watercourse units would need to be delivered to offset the loss). 

Compensation

Measures taken to recompense for biodiversity loss cause by a project, after avoidance and minimisation have been considered, that can result in either an overall biodiversity net loss or no biodiversity net loss.  Compensation is often used synonymously with ‘offset’, though ‘offset’ implies compensation to the point where there is a ‘no net loss’ conservation outcome. 

Note: With respect to European sites, the term ‘compensatory measures’ has a separate meaning – compensatory measures are independent of the project and are intended to offset remaining negative impacts in cases where it has not been possible to conclude no adverse effect on site integrity.  For further information, see: European Site Casework Guidance: How to consider plans and projects affecting Special Areas of Conservation (SACs) and Special Protection Areas (SPAs). 

Difficulty risk

One of the three ‘risk multipliers’ applied by the English metric / SSER toolkit to post-development habitat creation or restoration.  This is used to represent the uncertainty in the effectiveness of management techniques used to enhance or create habitat. 

Difficulty risk is a negative multiplier (i.e. value = <1) which is automatically assigned by the metric, dependant on the habitat type that you intend to create / restore.  For example, a ‘high’ difficulty risk (value = 0.33) predicts that there is only a 33% chance that the planned habitat creation / restoration will successfully deliver the intended target outcome within the specified timeframe. 

Distinctiveness

One of the multiplier components of the metrics described in this guidance, habitat distinctiveness seeks to capture differences in the conservation values of different habitats. Distinctiveness values are automatically assigned to each habitat type by the metric tool, based on professional ecological judgement by Natural England, that took into account:

• Total extent of habitat remaining in England (rarity)
• Proportion of habitats in SSSIs (where less is protected in SSSIs, it is considered of higher distinctiveness)
• UK BAP Priority Habitat Status
• European Red List Categories

In most metric tools, habitats are assigned to one of five distinctiveness categories (‘Very High’ (8), ‘High’ (6), ‘Medium’ (4), ‘Low’ (2), ‘Very Low’ (0, with the exception of non-native and ornamental hedgerows, which score 1)), which will affect the Biodiversity Unit value that the metric calculates.  The SSEN Transmission toolkit takes a different approach and removes the ‘Very High’ distinctiveness category, with all habitats assigned to one of four distinctiveness categories.

As a result of differing habitat distinctiveness values, it is possible that there will be a difference in area between the impacted site and any habitat created or restored to compensate for habitat losses.  Such differences are ‘permitted’ in both metric tools described in this guidance. For example, if a habitat of low distinctiveness is impacted and is compensated for by the creation of a habitat of high distinctiveness, the area needed to compensate for losses can theoretically be less than the area impacted.  If a habitat of high distinctiveness is lost, a greater area of high distinctiveness habitat creation / restoration may be required to offset the loss (due to metric risk multipliers acknowledging that some planned creation or restoration may fail). Also, see glossary entry on ‘trading rules’.

Note: It is important to recognise that the distinctiveness values assigned by the English metric (and largely by the SSER toolkit) consider the conservation value of habitats within an English context. These values may therefore not provide an appropriate approximation of habitat extents and conservation value in Scotland.

Enhancement

Enhancement refers to measures taken that result in biodiversity being in an overall better state than before intervention (i.e. a net benefit), and secured within a reasonable timescale and with reasonable certainty.  A distinction can be made with ‘offsetting’, in that enhancement is beyond what is required to ‘offset’ (i.e. fully compensate for) impacts. 

Note: It is important to recognise that both the English metric and the SSER toolkit use the term ‘enhancement’ to define one of the two available mechanisms / interventions that can be used to deliver Biodiversity Units. In this context, ‘enhancement’ relates to the process of improving an existing habitat to a better condition (the preferred term to describe this in Scotland would be ‘restoration’).  The other possible intervention is habitat ‘creation’.  Therefore, when using the English metric / SSER toolkit, an ‘enhancement’ type of intervention might be delivering compensation for habitat loss, or it might be delivering a net biodiversity benefit (‘enhancement’ as defined above). 

Mitigation

Measures that avoid or reduce the impact on-site by considering changes to the scale, design, location, process, sequencing, management and / or monitoring of the proposed activity.

Mitigation hierarchy

Before significant biodiversity effects can be delivered, NPF4 first requires that all potential negative effects should be fully mitigated in accordance with the mitigation hierarchy. The hierarchy indicates the order in which the impacts of development should be considered and addressed.

1. Avoid – by removing the impact at the outset, wherever feasible
2. Minimise – by reducing the impacts that are unavoidable
3. Restore – by repairing damaged habitats
4. Offset – by compensating for the residual impact that remains, with preference to on-site over off-site measures. 

Offset

Offsetting is sometimes used synonymously with ‘compensating’, though while compensation can fall short of achieving ‘no net loss’, offsetting implies fully compensating for the residual impact that remains after avoidance and minimisation have been considered.  Offsetting can either be delivered on-site or off-site.

The term ‘offsetting’ is also sometimes used to include an element of enhancement (or net gain), though to aid clarity we would encourage use of the term ‘enhancement’ to describe measures that go beyond ‘no net loss’ and result in a net benefit. 

Note: In some contexts, offsetting is used to infer any compensation measures that are delivered ‘off-site’.  This approach has possibly derived from the use of the term ‘offsetting’ in the carbon offset market, as meaning emissions reductions (including removals) that are used to compensate for emissions that occur elsewhere. For metric purposes, use of the term ‘offsetting’ in this way should be avoided for clarity and transparency purposes.  Preferred terms include ‘off-site offsetting’, ‘off-site delivery’ or ‘off-site measures’.

On-site and off-site

On-site refers to all land within a red line boundary of a development.  Off-site refers to land outside of the red line boundary.

Note: Applications for consent required for overhead lines under section 37 of the Electricity Act 1989 (as amended) do not use red line boundaries and are often present tower locations and other infrastructure with ‘limits of deviation’.

Restoration

The process of assisting the recovery of an existing habitat towards a better condition, as a means of conserving and / or enhancing biodiversity and ecosystem resilience.  See also glossary entry on ‘enhancement’.

Spatial risk

One of the three ‘risk multipliers’ applied by the English metric / SSER toolkit to post-development habitat creation or restoration.  The spatial risk multiplier is applied to off-site interventions (creation / restoration) only.  This represents the relationship between the location of biodiversity loss (on-site) and where the off-site habitat intervention(s) are being delivered.

Spatial risk is a negative multiplier (i.e. value = <1), which must be applied by the user and acts to incentivise local off-site delivery (in situations where full offsetting and enhancement cannot be delivered on-site).  In England, if the off-site delivery location is not within the same Local Planning Authority (LPA) or National Character Area (NCA) (or within the same catchment for watercourses) as the development site, a penalty is applied to the post-development BU score.  Penalty categories differ depending on whether the off-site location is within a neighbouring LPA / NCA or further away. 

Temporal risk / time to target condition

One of the three ‘risk multipliers’ applied by the English metric / SSER toolkit to post-development habitat creation or restoration.  The temporal risk multiplier is used to represent the average time between the start of habitat creation / restoration works and the delivery of a specified target outcome. This is known as the ‘time to target condition’ and is measured in years. 

Temporal risk is a negative multiplier (i.e. value = <1) which is automatically assigned by the metric, dependant on the habitat type that you intend to create / restore.  For example, if restoring ‘upland birch woodland’ from ‘moderate’ to ‘good’ condition, the English metric predicts that it will take 15 years to reach the target outcome.

Trading rules

‘Trading rules’ are set out in the English metric user guide (Rule 1).  These trading rules are also used in the SSER toolkit and user guide (although not strictly applied).  The SSEN Transmission toolkit does not include trading rules but is accompanied by guidance recommending that compensation should be targeted at delivering net gains that are at least ecologically equivalent in type and condition to the habitats lost.

The trading rules set minimum habitat creation and restoration requirements to compensate for specific habitat losses, up to the point of no net loss.  They are based on the habitat type and distinctiveness of the lost habitat.

For example, losses of high distinctiveness habitats must be replaced with BUs of the same habitat type.  Losses of medium distinctiveness habitats can be replaced with BUs of either medium distinctiveness habitats within the same broad habitat type (e.g. grassland), or any habitat type from a higher distinctiveness category.

• Scottish Government’s Biodiversity: Draft Planning Guidance (2023) details ‘core principles’ that can be followed when designing development so that nature and nature recovery are an integral part of any proposal.
• NatureScot’s Developing with Nature Guidance
• NatureScot’s Enhancing Biodiversity Guidance
• Biodiversity Net Gain - Good practice principles for development (CIEEM, CIRIA, IEMA, 2016)
• Biodiversity Net Gain - Good practice principles for development - A practical guide (CIEEM, CIRIA, IEMA, 2019)

There are some key differences between the English metric and SSER toolkit, as set out below.  Users are recommended to be aware of these differences, as well as the SRUC research findings.  This will be important both in considering whether to use one of the two main published metrics (in the absence of a Scottish biodiversity metric), and if so, which metric to use, as well as in interpreting its outputs.  Scottish Government guidance advises that, where a developer wishes to use an established metric or tool, the planning submission should demonstrate how Scotland’s habitats and environmental conditions have been taken into account. 

It is also important to note that beyond the differences described, in many scenarios the SSER toolkit will provide the same (or similar) results as the English metric. 

1. Calculation tool layout

Although both the English metric and SSER toolkit are Microsoft Excel-based tools, their user interfaces are quite different.  The English metric separates elements of the biodiversity calculation process into different tabs (e.g. tabs for on-site habitat baseline, on-site habitat creation, on-site habitat enhancement, on-site watercourse baseline, on-site watercourse creation, etc).  With the SSER toolkit (and the SSEN Transmission toolkit), all calculations are handled within one Excel tab. 

The general formulas are similar between both metric tools and, in both tools, habitat calculations, watercourse calculations and hedgerow calculations are also handled as distinct modules, on the basis that they create different types of Biodiversity Unit. 

2. Tool flexibility

Both tools allow a degree of flexibility in how they are applied.  In the English metric, the approach is set out in metric Rule 4, which states that in exceptional ecological circumstances, deviation from the metric methodology may be permitted.  In such situations, the developer must provide evidence to justify any deviations, and agreement sought from the relevant consenting authority.  The English metric user guide (available from the UK Government website) provides advice on when to use Rule 4, and how to input deviations within the metric tool.

Similar flexibility exists in the SSER toolkit, though the options to deviate from default metric values are more embedded in the toolkit than the English metric (which is intrinsically linked to compliance with statutory obligations).  For example, it is possible to manually change a habitat’s distinctiveness value, and such changes are flagged (by changing cell colour to orange) to indicate where associated justification for the departure is required.  The SSER toolkit user guide (available from the SSER website) provides guidance on changing distinctiveness values, stating that any changes must be clearly justified and can only be adjusted by one level up or down from the default value.  For example, if the distinctiveness value is ‘low’, it could be altered with justification to ‘medium’ but not ‘high’.  A user can also manually adjust the ‘difficulty’ and ‘time to target condition’ risk multiplier values assigned by the SSER toolkit. Where changes to risk multiplier values are made, these flag in the same way as changes to distinctiveness values and highlight that justification is required. 

3. Habitat data input

The habitat types that can be input into the English metric are primarily based on a simplified version of the UK Habitat (UKHab) hierarchical classification system.  Most habitats within the English metric are classified to UKHab Level 4, however there are some circumstances where habitats are classified using UKHab Level 5 (e.g. Annex 1 habitats), UKHab Secondary codes, or European Nature Information System (EUNIS) definitions.  Bodies of freshwater are classified using the Water Framework Directive Lakes Typology.  In some situations, a metric-specific definition is used to define a habitat.

The SSER toolkit allows for habitat data to be input in a similar manner, if ‘UKHab’ is selected as the chosen classification system on the ‘Project Details’ sheet of the tool workbook. Although identified as UKHab, this is actually the same simplified classification approach used in the English metric, as described above, albeit with some changes to bog habitat classifications (discussed further in relation to peatland habitats below).

Within the English metric tool, there is also guidance on translating Phase 1 habitat data into metric habitat types, to allow historic Phase 1 data to be converted for entry into the metric.  There are recognised challenges with this approach to translating Phase 1 data for use in the metric.  Due to uncertainty surrounding the accuracy of any classification translations derived using this guidance, some English local authorities will not accept metric conclusions based on translated Phase 1 survey data. They will instead specify that sites must by directly surveyed using the UKHab / metric classification systems.

In the SSER toolkit, there is an option to directly input Phase 1 habitat data.  Toolkit values (e.g. distinctiveness) given to Phase 1 habitat types have been derived from UKHab / metric habitat types by using the English metric’s translation guidance.  Given the known limitations of this guidance, there is some risk of mistranslation if inputting Phase 1 habitats into the SSER toolkit without application of good ecological expertise.  Further information may also need to be recorded in the field to allow accurate translation. 

4. Approach to calculating biodiversity uplift

When using the English metric, Defra guidance on calculating biodiversity value states that baseline BUs must be calculated for all habitats within a development’s red line boundary, regardless of whether they will be lost, retained (i.e. not directly affected) or enhanced.  It must then be demonstrated that any habitat losses have been offset (to a point of no net loss) and a BNG uplift of at least 10% has been delivered.  The offsetting requirement is based on delivering enough BUs to fully replace those that have been lost.  However, once offsetting has been achieved, the additional 10% uplift requirement is not directly related to what has been lost through development.  Instead, it is a percentage of the total baseline BU value of all habitats contained within the red line boundary (e.g. if baseline BUs within the red line boundary = 1000, the development must offset any losses and deliver an additional 100 BUs to achieve BNG).

The SRUC report recognises that there are concerns about the applicability of the above approach to the red line boundary within the context of onshore wind development.  Onshore wind farms can have a large red line boundary (to accommodate EIA considerations) but a comparatively small infrastructure footprint. Onshore wind farm boundaries are often set at an early stage, based on what can be agreed with landowners, and may contain a lot of high value habitat (e.g. peatland) not affected by the development.  UK Government is currently inviting views on how the red line boundary is considered in England in relation to NSIPs.

Whilst the general approach to calculating the offsetting / compensation requirement is the same in the SSER toolkit, the method for calculating uplift differs from the English metric.  Rather than calculating uplift as a percentage of the total baseline biodiversity value within a development’s red line boundary, SSER introduces the concept of a ‘biodiversity assessment boundary’.  This is defined as areas which are either directly or indirectly (see point 5 below) affected by the development and areas which are being used to compensate for biodiversity impacts.  Any habitats within the red line boundary that are expected to be retained at baseline habitat type and condition, with no direct or indirect impacts, are excluded from the biodiversity assessment boundary.  Uplift is then calculated as a percentage of the total baseline BU value of habitats within the biodiversity assessment boundary.

The SSER toolkit approach to calculating biodiversity uplift is therefore more closely related to the predicted loss of biodiversity, rather than the value of all habitats within the red line boundary (whether habitats will be affected or not). 

As each of these tools use a different calculation methodology, percentage biodiversity uplift values reported by the English metric (red line boundary) and SSER toolkit (biodiversity assessment boundary) are not directly comparable and will almost certainly be different. 

5. Considering indirect effects

The English statutory metric tool calculations apply to habitat loss / retention, habitat creation and restoration / enhancement. Only direct impacts on habitats within the footprint of a development project are accounted for.  There is no clear guidance on how to consider indirect impacts in the English metric user guide. 

In contrast, the SSER toolkit does require indirect impacts to be accounted for, with the development’s ‘biodiversity assessment boundary’ (see point 4 above) including both direct and indirect impacts. The SSER toolkit guidance advises that where data is available to predict what indirect impacts the development will have (e.g. a change in hydrology), these should be incorporated into the assessment, with justification provided for the stated BUs (both the pre-development baseline and post-development BU value).  If data on predicted indirect habitats is not available, the toolkit guidance advises that calculations should assume that indirectly affected habitats will decrease in condition by one level (e.g. from Good to Moderate or Moderate to Poor).

Incorporating consideration of indirect impacts can be of particular relevance for those developments affecting upland and peatland habitats, where indirect impacts can potentially be significant.  Indirect drying effects of peatland habitats, caused by changes to hydrology around new tracks and infrastructure, can be an important consideration when calculating overall biodiversity loss.  Such drying could lead to changes in the habitat type (e.g. from blanket bog to heath), or it could reduce the ecological condition of the affected habitat. 

6. Trading Rules

Both the English metric and SSER toolkit apply a series of ‘trading rules’, based on the habitat type and distinctiveness of the habitat lost to development.  For example, high distinctiveness habitat losses must be replaced with BUs of the same habitat type.  Losses of low distinctiveness habitats can be replaced with BUs of a low distinctiveness, or higher distinctiveness habitat type.  ‘Trading down’ is not supported.  These ‘rules’ apply to the point of achieving no net loss.

For the English metric, a full summary of the trading rules can be found in the English metric user guide (Rule 1).  The SSER toolkit uses trading rules based on those in the English metric user guide, with the caveat that they do not need to be strictly applied as the rules may not always be appropriate for use in Scotland. 

7. Handling of peatland habitats

As well as the more general differences described, the SSER toolkit has made a series of adaptations to the English metric to more fully account for peatland habitats within tool calculations.  These include:

(a) Classifying blanket bog habitat

In the English metric, all blanket bog and lowland raised bog habitats are defined using the UKHab Level 4 description within the UKHab Primary Habitat hierarchy and assigned to the Very High Distinctiveness category (score = 8).  Bog condition is considered separately through the habitat condition assessment component of the metric.

When inputting data using UKHab, the SSER toolkit uses the Level 5 descriptions to differentiate between (active) blanket bog / raised bog and degraded blanket bog / raised bog.  Under the SSER approach, active or non-degraded bogs are recognised as Very High Distinctiveness (score = 8), whilst degraded or modified examples of these habitats are assigned to the High Distinctiveness category (score = 6).  As a result, a lower value is placed on degraded bog habitats in the SSER toolkit calculations when compared to the English metric, especially if the lower distinctiveness score is considered alongside the likely lower habitat condition scores for degraded bog. 

(b) Including blanket bog losses and offsetting within metric tool calculations

In England, a number of habitats, including blanket bog, are recognised as ‘irreplaceable’.  These are defined as habitats that are “technically very difficult to recreate once destroyed (or recreation would take a significant amount of time)”.  In England, habitats recognised as irreplaceable are protected under the Biodiversity Gain Requirements (Irreplaceable Habitat) Regulations 2024.  All blanket bog is classed as irreplaceable in the English metric, regardless of condition. In contrast, the SSER toolkit guidance only defines blanket bog as irreplaceable when “the condition score cannot be increased e.g. good condition, which is assumed to mean ‘active’ blanket bog / raised bog”. 

Where a development will result in the deterioration or loss of an irreplaceable habitat (including blanket bog), the English BNG requirement is disapplied and offsetting of these impacts cannot be calculated using the English metric tool.  Instead, the developer must agree bespoke compensation with the relevant decision-making authority.

By comparison, the SSER toolkit does allow blanket bog / irreplaceable habitat losses and offsetting to be calculated within the tool. Although the sensitivity surrounding development on irreplaceable habitats is recognised by SSER, the SSER toolkit sets a higher threshold for irreplaceability and therefore allows more offsetting / enhancement to be handled within the toolkit before bespoke compensation discussions are required. Guidance accompanying the SSER toolkit indicates that if irreplaceable habitats are negatively affected by development, bespoke compensation will still be required. By allowing the losses and offsetting to be fully calculated, the toolkit can be used to identify a minimum offsetting requirement, i.e. the toolkit guidance advises that the calculated compensation level is a starting point for negotiation of suitable compensation with the decision-making authority (and any other relevant stakeholders).   

(c) Estimating the success of peatland restoration

For any proposed creation or restoration of a habitat, the English metric applies a ‘difficulty risk multiplier’.  This is a negative multiplier (i.e. value = <1) which is automatically assigned by the metric, dependant on the habitat type that you intend to create / restore.  An average difficulty value is assigned to each habitat type, reflecting how difficult it is to create / restore and accounting for any uncertainty that the desired outcome will be achieved.  The level of difficulty risk will differ between habitat types, due to ecological factors and the availability of techniques to create / restore habitats within a realistic timeframe (higher risk = lower multiplier value). 

For blanket bog restoration, the English metric sets a high difficulty risk (score = 0.33).  In effect, the metric tool is predicting that there is only a 33% chance of the restoration work successfully delivering the intended target outcome within the specified timeframe.

In contrast, the SSER toolkit assigns a medium difficulty risk (score = 0.67) to blanket bog restoration, estimating that restoration effort is twice as likely to be successful as the English metric suggests.  Therefore, when using the SSER toolkit a relatively smaller area of blanket bog would need to be restored to provide the same number of BUs as the English metric. 

NatureScot recognise that differences remain between both the English metric, and the SSER toolkit, when compared with NatureScot’s current advice on peatland, carbon-rich soils and priority peatland habitats.  It is important that developers and decision makers apply reasoning and judgement based on the circumstances of the individual case.

8. Other differences

In addition to the above, there are other recognised differences between the English metric and the SSER toolkit.  For example, the SSER toolkit takes a different approach to assessing condition of river habitats.  Whereas the English metric uses the Modular River Survey River Condition Assessment (RCA), the SSER toolkit approach to assessing rivers is still to be fully determined, with tool guidance currently recommending that the condition score applied is related to the distinctiveness of the habitat.  For assessments of all other habitats, the SSER toolkit recommends use of the English metric’s habitat condition assessment sheets.  It also retains (from Natural England v2.0) a simple multiplier for habitat connectivity, which assigns scores largely based on English habitat distinctiveness values. There is no consideration of habitat connectivity in later versions of the English metric, including the current statutory v1.0.3.   

The English metric has gone through a number of iterations during its development.  To facilitate consistency of reporting for the statutory BNG requirement in England (adopted spring 2024), the metric has now entered a period of development stability with no major updates expected in the foreseeable future (though a minor update to address some bug fixes is expected later in 2025).  By comparison, the SSER toolkit is still being updated to improve both the tool’s performance and its application to sites in Scotland. 

Upcoming changes to the SSER toolkit include a name change to the “SSE toolkit”, reflecting that both SSE Renewables and SSEN Transmission are intending to move towards use of a single metric tool to demonstrate delivery of NPF4 requirements (currently SSER and SSEN Transmission use different toolkits).  Any single toolkit adopted across all businesses within SSE will be agreed across the SSE group.  The SSE toolkit will also potentially include a new approach to calculating post-development woodland creation, through the use of a ‘Woodland Uplift’ multiplier which will be included within the ‘strategic significance’ component of the toolkit.