Small Sites Metric (SSM) Assessments
The Small Sites Metric (SSM) is a simplified version of the Statutory Biodiversity Metric (SBM). It was developed by DEFRA and Natural England to support Biodiversity Net Gain (BNG) delivery for small-scale developments. Where the statutory tool can be highly detailed, the SSM streamlines the process for projects with limited ecological complexity.
ACP Consultants provide clear, planning-ready SSM assessments for housebuilders, architects, and developers across England. By combining UKHab metric mapping with the SSM calculator, we deliver accurate biodiversity unit calculations that meet the legal 10% net gain requirement.
You can also drop us an email at hello@acp-consultants.com and we’ll get back to you within 24 hours to help with your inquiry!
We follow a simple, transparent process to ensure your project runs smoothly from the very first contact to the final report. Our approach is designed to provide you with clarity at every step, so you’re fully informed and confident in moving forward.
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Step 1: Request a Quote
Fill out our quick quote form or call us, and our team will provide a free, no-obligation quote, outlining the services tailored to your needs.
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Step 2: Confirm Your Booking
Once you approve the quote, simply return the booking form. We’ll schedule your survey and ensure all the details are taken care of.
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Step 3: Receive Your Report
After your survey is completed and payment is received, we’ll promptly issue your survey report, ensuring you get the results as quickly as possible.
What is the Small Sites Metric?
The Small Sites Metric (SSM) is a government-issued biodiversity calculation tool designed specifically for BNG small sites. It sits within the statutory biodiversity metric family and provides a streamlined way for minor developments to demonstrate the required 10% Biodiversity Net Gain (BNG).
Unlike the full Statutory Biodiversity Metric (SBM), which involves detailed habitat condition assessments, the SSM uses simplified inputs and broad habitat categories. This makes it quicker to apply, easier for non-specialists to understand, and proportionate to the scale of small developments.
Key features of the SSM:
- Focuses on habitat area and distinctiveness, rather than detailed ecological metrics.
- Groups habitats into broad categories, avoiding the need for complex classifications.
- Designed to reduce survey costs and preparation time while still ensuring compliance.
- Provides transparent evidence that local planning authorities (LPAs) can validate easily.
Because the SSM is still part of the statutory biodiversity framework, outputs are fully recognised by LPAs. This means a completed SSM calculator/workbook, accompanied by UKHab mapping and a summary report, is accepted as planning evidence.
For developers of BNG small sites, this tool offers a balance between regulatory compliance and efficiency, ensuring smaller projects can meet biodiversity responsibilities without unnecessary cost or delay.
When can you use the Small Sites Metric (SSM)?
The Small Sites Metric (SSM) is only appropriate in certain circumstances. It was designed to simplify BNG small site assessments, but it cannot be applied universally. Understanding the eligibility rules is important to avoid delays or invalid submissions.
When SSM is suitable
The SSM can normally be used where the proposed development is under 1 hectare in size and habitats are straightforward. It is particularly useful for small housing schemes, minor commercial plots, or infill developments that do not involve significant ecological risks. A further requirement is that the site must not contain priority habitats and must not trigger a full Environmental Impact Assessment (EIA). For these reasons, the SSM is often the preferred option for smaller sites where impacts are limited and can be clearly defined.
When SSM is not permitted
There are also strict circumstances where the SSM cannot be used. If a site contains irreplaceable habitats such as ancient woodland, veteran trees, or designated sites, the simplified tool is not sufficient. Similarly, projects with complex habitat mosaics that require detailed condition assessments must default to the Statutory Biodiversity Metric (SBM). If a development relies on off-site biodiversity units or statutory credits to achieve the required 10% gain, the SSM is also not acceptable. Finally, some local planning authorities (LPAs) will request that the full SBM is used even for small sites.
At ACP Consultants, we always carry out an eligibility check before starting a project. This ensures your assessment is compliant, avoids wasted costs, and prevents your planning application from being delayed or rejected.
Planning & compliance essentials
Using the Small Sites Metric (SSM) does not remove the need to comply with planning rules. Local planning authorities expect BNG small sites to demonstrate the same legal standard as larger projects, but through a proportionate method. Developers should keep the following points in mind when preparing their applications.
BNG is mandatory
Even for small schemes, the law requires a minimum 10% Biodiversity Net Gain. The SSM is simply a lighter tool for showing compliance, not an exemption from the policy.
Use the official calculator
LPAs will only accept results produced using the official Small Sites Metric calculator and guidance notes. Substituting alternative spreadsheets or estimates is not acceptable, and risks rejection at validation stage.
Evidence is essential
Every SSM submission must include the completed calculator workbook, a clear site plan, and UKHab mapping that shows both pre-development and post-development habitats. Without this supporting evidence, the results cannot be trusted.
Switching to SBM when required
If your project changes in size, or if previously unknown priority habitats are discovered, the simplified tool is no longer valid. In these cases, the assessment must be upgraded to the full Statutory Biodiversity Metric (SBM).
Legal securing of gains
As with larger sites, all habitat improvements identified in an SSM assessment must be secured for 30 years. This is normally achieved through a Section 106 agreement or a conservation covenant, ensuring that the biodiversity gain is delivered and monitored over the long term.
In the past year, we’ve delivered over 750 audit-ready reports supporting 400+ projects for developers, planners, and architects nationwide. We put quality over quantity and build lasting relationships based on trust, ensuring your planning applications are backed by robust evidence and delivered on time.
You can also drop us an email at hello@acp-consultants.com and we’ll get back to you within 24 hours to help with your inquiry!
Our process
Our Small Sites Metric (SSM) process follows a structured path, ensuring that BNG small sites achieve compliance with confidence. Each stage is designed to deliver accurate data, clear reporting, and planning-ready outputs.
Discovery & eligibility check
The first step is always to establish whether your site qualifies for the Small Sites Metric. Eligibility is not optional: if the project fails the SSM “gates,” the results will not be valid for planning.
We begin by reviewing the site boundary, layout, and constraints. This includes checking whether the site is under one hectare, whether priority or irreplaceable habitats are present, and whether an Environmental Impact Assessment (EIA) is triggered. We also consider LPA expectations, as some authorities insist on the full Statutory Biodiversity Metric (SBM) even for smaller projects.
If we identify risks, we advise switching to the full SBM early on. This protects you from delays and ensures your planning submission is defensible. By carrying out this upfront review, ACP helps clients avoid the common pitfall of using SSM where it is not permitted.
UKHab mapping & baseline survey
Even though the SSM uses simplified habitat categories, the baseline data still needs to be reliable. Our ecologists undertake UKHab mapping, recording habitat polygons, target notes, and condition information.
By applying UKHab classification, we create a transparent record that feeds directly into the SSM calculator. This adds rigour to what could otherwise be a very simple tool. It also means that, if the site later needs to move to the full statutory biodiversity metric, the dataset is already compatible.
The baseline survey is about more than compliance: it provides developers with a clear picture of current habitat value and potential opportunities. This helps inform design choices and can reduce future costs.
Input to the SSM calculator
Once baseline data is collected, we input the information into the Small Sites Metric calculator/workbook. This official DEFRA tool converts habitat data into a biodiversity units calculation, showing the balance between pre-development and post-development conditions.
The SSM calculator is simpler than the full statutory metric, but it still requires careful handling. We ensure that habitat categories are selected correctly, unit scores are consistent with the site survey, and assumptions are clearly explained. The output is a transparent calculation that demonstrates whether the 10% biodiversity net gain target is achieved.
Design review & mitigation advice
Meeting the BNG target is not just about running numbers through a calculator. The design of the site plays a critical role. We work closely with architects, landscape designers, and planning consultants to identify enhancements.
Examples include:
- Planting species-rich grassland to increase habitat distinctiveness.
- Installing native hedgerows to improve structure and habitat connectivity.
- Adding ponds, swales, or rain gardens to support aquatic biodiversity.
- Retaining and enhancing existing scrub or treelines to avoid unnecessary losses.
By applying the BNG hierarchy avoid, minimise, restore, offset we make sure your design supports biodiversity while still being practical and cost-effective.
Reporting & outputs
After the calculations are complete, we prepare a clear and concise report. This planning ready document includes:
- The completed SSM calculator workbook.
- A summary of habitat changes, including maps and figures.
- A clear statement of compliance with the 10% BNG requirement.
Reports are structured with planning in mind, making it easy for LPAs to validate your application. We also ensure that all supporting documents, from UKHab mapping outputs to version-controlled workbooks, are included. This avoids the frustration of additional information requests.
Submission & long-term support
Finally, we support you in embedding the results into your planning documents. We provide text and tables ready for your Biodiversity Gain Plan (BGP) and guidance for the Habitat Management & Monitoring Plan (HMMP).
These inputs cover management responsibilities, monitoring triggers, and 30-year securing mechanisms, such as Section 106 agreements or conservation covenants. By addressing long-term delivery, we ensure that gains identified by the SSM are not just theoretical, but implemented and maintained.
Our role does not end with the submission. We remain available to respond to planning officer queries, advise on legal securing, and adapt outputs if your scheme changes. This comprehensive approach means ACP delivers more than just calculations, we provide end-to-end assurance that your BNG small site is fully compliant.
What you get (deliverables)
When you commission ACP to complete a Small Sites Metric (SSM) assessment, you receive a full package of outputs that are accepted by local planning authorities. These deliverables demonstrate compliance with the 10% BNG requirement for small sites and provide a transparent audit trail.
SSM workbook (Excel)
We complete the official SSM calculator, showing pre-development and post-development habitats and producing a clear biodiversity units calculation. This workbook is the core evidence required by LPAs.
UKHab maps
Our reports include UKHab metric mapping, showing baseline and proposed habitats in GIS or PDF format. This gives planners confidence that habitat changes are measured consistently.
Summary report (PDF)
We prepare a concise report explaining methods, results, and compliance with BNG small sites policy. This ensures officers can quickly validate your application.
BGP and HMMP text
We provide prepared wording for your Biodiversity Gain Plan (BGP) and Habitat Management & Monitoring Plan (HMMP), saving time and ensuring long-term delivery is addressed.
Quality assurance and version control
All deliverables are version-controlled against the latest statutory biodiversity metric guidance, so you know your outputs meet current national standards.
Deliverable | Inclusions | Benefit |
SSM workbook (Excel) | Completed calculator with pre/post-development data | Transparent, compliant evidence |
UKHab maps | Baseline and proposed habitat mapping | Easy for LPAs to validate |
Summary report (PDF) | Methods, results, compliance statement | Planning-ready submission |
BGP/HMMP text | Prepared wording for templates | Saves time and reduces risk |
QA checks | Version control and guidance compliance | Confidence that outputs match national standards |
Timelines & costs
One of the main advantages of the Small Sites Metric (SSM) is speed and cost-effectiveness. Because the tool uses simplified habitat categories, assessments are usually quicker to complete than full Statutory Biodiversity Metric (SBM) studies.
In most cases, we can deliver an SSM assessment within 1–2 weeks once site information, boundaries, and drawings are available. This makes it ideal for smaller developments that need to move swiftly through planning. Where additional surveys are required, or where eligibility is uncertain, we advise at the outset to keep your project on track.
Indicative costs
The cost of an SSM assessment will always depend on factors such as site size, location, ecological complexity, and the level of reporting required by the local planning authority. As a guide, our SSM packages start from around £399 + VAT, with final fees confirmed once we review your project details.
Futureproofing your project
If your scheme is close to the eligibility threshold, we often recommend preparing both an SSM and SBM-ready dataset. This approach provides flexibility if the LPA rejects the simplified tool or if project parameters change. It saves time and avoids the cost of commissioning a second assessment later.
ACP always provides fixed-fee quotes after an initial review, so you know exactly what to budget for. This transparency gives you confidence in both timescales and costs, while ensuring your BNG small site remains fully compliant.
Why choose ACP
- Experienced team: we have delivered hundreds of BNG assessments, from small infill plots to complex mixed-use sites.
- Clarity and compliance: our outputs are written for planners and case officers, avoiding jargon.
- Efficiency: SSM assessments are turned around quickly to meet application deadlines.
- Futureproofing: we always advise if SBM is more appropriate, protecting you from wasted effort.
- Sustainability commitment: our work supports wider goals such as the Sustainable Development Goals, natural capital approach, and green infrastructure development.
Frequently Asked Questions (FAQs) About the Small Sites Metric (SSM)
The Small Sites Metric (SSM) is a simplified version of the statutory Biodiversity Metric 4.0, designed specifically for smaller development sites that meet defined criteria under the Environment Act 2021. It allows developers to calculate Biodiversity Net Gain (BNG) outcomes more efficiently without the complexity of the full metric, while still meeting statutory requirements. This FAQ section answers common questions about when the SSM can be used, how it differs from the full metric, what data inputs are required, and how planning authorities interpret its results. Whether you’re managing a minor residential project or a small commercial scheme, these FAQs explain how the SSM works in practice, what evidence must accompany it, and how ACP’s ecologists ensure your BNG calculations remain accurate, transparent, and compliant with Natural England guidance.
What is the difference between SSM and SBM?
The Small Sites Metric (SSM) is a simplified calculator for BNG small sites, whereas the Statutory Biodiversity Metric (SBM) is the full statutory tool. SSM is designed to save time and reduce costs, but it cannot be used for projects that affect priority habitats or have complex ecological conditions.
Who can use the Small Sites Metric?
The SSM is intended for developers, architects, and landowners working on projects under one hectare in size with straightforward habitats. ACP always confirms eligibility before starting, to ensure that your site genuinely qualifies and that your submission won’t face rejection.
What happens if my project doesn’t qualify?
If your site includes features that exclude the SSM, such as irreplaceable habitats or EIA requirements, you must use the full Statutory Biodiversity Metric. Our team can seamlessly transfer baseline data into the SBM calculator so there is no wasted effort.
Do I still need a 10% net gain with SSM?
Yes. The BNG requirement is the same regardless of which tool is used. Every SSM assessment must demonstrate at least 10% Biodiversity Net Gain, calculated using the official workbook.
How long does an SSM assessment take?
Most SSM assessments can be completed within 1–2 weeks, provided site boundaries, drawings, and access are available. If eligibility is borderline, we may recommend preparing both SSM and SBM datasets to avoid rework.
Does SSM link to the Biodiversity Gain Plan?
Yes. The outputs from the SSM feed directly into the Biodiversity Gain Plan (BGP) and the Habitat Management & Monitoring Plan (HMMP). This ensures that the 10% gain is both evidenced and secured for 30 years.
Can local planning authorities refuse SSM?
Yes. Some LPAs may insist on the full Statutory Biodiversity Metric even for small sites. This usually happens if habitats are complex, if there are connectivity concerns, or if the planning officer needs more detail. That is why an eligibility check at the start is essential.
Can the Small Sites Metric be used for outline applications?
Yes. The SSM can be applied at outline stage to provide an indicative biodiversity units calculation. However, final numbers and habitat details will need to be confirmed at reserved matters stage once the layout and landscaping are fixed.
What common mistakes occur with SSM?
Typical errors include using SSM on sites with priority or irreplaceable habitats, misclassifying habitat categories, or failing to provide accurate UKHab mapping. Another mistake is relying on outdated versions of the calculator. ACP’s QA process prevents these issues.
How much does an SSM assessment cost?
Costs vary depending on site size, location, and complexity. At ACP, SSM assessments typically start from £399 + VAT, with final fees confirmed after reviewing your site boundary and design. We always provide fixed-fee quotes so you can budget with confidence.
Does SSM allow for off-site units or statutory credits?
No. The Small Sites Metric is only for projects that can demonstrate gains on-site. If your scheme requires off-site biodiversity units or the purchase of statutory credits, you must upgrade to the full SBM.
How is SSM secured for the long term?
Like SBM outputs, any gains identified by SSM must be secured for 30 years. This is normally through a Section 106 agreement or a conservation covenant, ensuring that habitats are maintained and monitored well into the future.
Government Guidance and Statutory References
- Statutory framework & planning condition – Biodiversity Net Gain under Schedule 7A of the Town and Country Planning Act 1990; statutory regime explained in Planning Practice Guidance (GOV.UK).
- Biodiversity Gain Plan – Must be approved before commencement; Local Planning Authority decision normally within 8 weeks. See Biodiversity Net Gain Guidance (GOV.UK).
- Statutory biodiversity metric – How units are calculated, modules, factors, and guidance on early use. Statutory Metric and User Guide (GOV.UK).
- Condition Sheets & Small Sites Metric (SSM) – Official metric tools and guidance sheets. BNG Metric Tools (GOV.UK).
- Exemptions & de minimis thresholds – Householder, small self-build, and very small impacts where no priority habitat is affected. Exemptions Guidance (GOV.UK) and Defra Environment Blog.
- Off-site register & fee – Natural England guidance on registering biodiversity gain sites, with the current £639 registration fee. Register a Biodiversity Gain Site (GOV.UK).
- NSIPs timing – Government proposals indicate that BNG will apply to Nationally Significant Infrastructure Projects (NSIPs) from May 2026, pending final regulations. Defra Consultation (GOV.UK).
- Creating a Habitat Management & Monitoring Plan (HMMP) – GOV.UK guidance on how to prepare HMMPs, including monitoring and reporting requirements. HMMP Guidance (GOV.UK).
- Natural England HMMP Template (JP058) – Official template, checklist, and monitoring report tools for Habitat Management & Monitoring Plans. Natural England Publications.
- Statutory Biodiversity Metric User Guide (July 2025) – Full guidance on how the statutory metric should be applied, including trading rules and worked examples. User Guide PDF.
- Metric Supporting Documents (JP039) – Includes GIS templates, data standards, and case studies for applying the statutory metric. Natural England Publications.
- Statutory Biodiversity metric user guide.
- Small Site Matric Guidance.
Other Supporting References (Quick Links)
- BNG in Practice Report (2025) – Case studies showing how BNG is being delivered in real projects. Institute of Environmental Sciences.
- Biodiversity Net Gain – Principles and Guidance for UK Construction and Developments. Chartered Institute of Ecology and Environmental Management.
- BNG Research Briefing – This post note outlines the mandatory biodiversity net gain policy introduced in England in 2024 and the risks and challenges for delivering its objectives. UK Parliament.
- Biodiversity net gain: where to start. Natural England.
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Next Steps
Need a Small Sites Metric (SSM) assessment for your project? Contact ACP Consultants with your site boundary and layout. We’ll confirm eligibility, complete the SSM calculator, and provide planning-ready outputs so your application runs smoothly.
Explore Related Biodiversity Net Gain Resources
The Small Sites Metric (SSM) offers a simplified way to calculate Biodiversity Net Gain (BNG) for smaller developments while maintaining compliance with the Environment Act 2021. To understand how it fits within the overall BNG framework, begin with the Biodiversity Net Gain Overview and the BNG Legislation & Guidance explaining when SSM can be used.
If your site exceeds SSM thresholds, use the Statutory Biodiversity Metric 4.0 for full assessments. Once calculations are complete, document results through the Biodiversity Gain Plan (BGP) and Habitat Management and Monitoring Plan (HMMP) templates to formalise compliance.
For developers and planners, our BNG Exemptions & Thresholds page clarifies whether SSM applies, while the BNG Templates & Downloads and BNG Monitoring Fee Calculator offer practical tools to support implementation. To see how SSM-based assessments have been delivered successfully, visit our BNG Case Studies & Portfolio.
You can also drop us an email at hello@acp-consultants.com and we’ll get back to you within 24 hours to help with your inquiry!
- Last Updated:
- November 2025
Disclaimer: Our content is prepared by ACP Consultants’ in-house specialists and is based on current guidance, standards, and best practice in environmental consultancy. While we make every effort to keep information accurate and up to date, it is provided for general guidance only and should not be relied upon as a substitute for professional advice on specific projects. Planning authorities retain final decision-making powers, and requirements may vary between local authorities and over time. ACP Consultants accepts no liability for any loss arising from reliance on this content without obtaining tailored advice for your project.