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3rd October 2025

8 minute read

Filling out the Small Site Metric (SSM) for Planning Applications Step by Step

In this post, I want to share how we approached the challenge using the Small Sites Metric (SSM) and what we learned along the way.

This guide takes Willow Mill fisheries as a case study and walks you through each section of the SSM.

If you are working on a small development and wondering how Biodiversity Net Gain (BNG) applies, this article should give you some practical pointers.

Maverick Gent

Director

Recent Case Study

At Willow Mill Fisheries in Hyndburn, we recently secured planning permission for drainage remediation and landscaping improvements (Application Ref: 11/25/0280). It was not a straightforward journey.

The site had enforcement issues, required retrospective applications, and even included a sensitive area under notice following concerns from neighbours.

So what is the Small Sites Metric (SSM)?

If you are new to BNG, the Small Sites Metric is a tool created by DEFRA and Natural England to help smaller projects demonstrate compliance with biodiversity rules without unnecessary complexity.

Think of it as a streamlined version of the full Biodiversity Metric 4.0, designed specifically for:

- Developments of fewer than 10 homes
- Sites under 1 hectare in size
- Projects with less than 0.5 hectares of habitat impact

In practice, the SSM helps you:

1. Record what habitats exist on your site now (the baseline)
2. Measure the likely impact of your project
3. Plan enhancements so you can deliver the mandatory 10% biodiversity net gain

The SSM is simpler to use than the full metric, but it is not a shortcut. It is still a robust tool that ensures ecological outcomes are meaningful while remaining manageable for SMEs, self-builders, and architects.

Does your project even require SSM?

Not every project requires the SSM, so it is worth checking before you get stuck into spreadsheets.

The 25m² Rule (De Minimis Exemption)

If your development impacts less than 25m² of habitat (that is smaller than a 5m x 5m patch) and less than 5m of linear habitat such as a hedgerow, then the SSM is not required.

However, there are caveats:

- the development must not impact on any onsite priority habitat; and

- If there is an impact on other onsite habitat, that impact must be on less than 25 square metres (e.g. less than a 5m by 5m square) of onsite habitat with a biodiversity value greater than zero and on less than 5 metres of onsite linear habitat (such as a hedgerow)

The rules and requirements are set out in the Biodiversity Gain Requirements (Exemptions)
Regulations 2024 and explained in the government’s official guidance

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Top Tips Before You Start Using the SSM

If you’re about to dive into the Small Sites Metric (SSM), here are a few things worth checking off your list first. Think of this as a friendly “heads-up” to save you time (and a few headaches) later on.

1. Check for Priority Habitats

The SSM can only be used if your site does not contain priority habitats. If there is a priority habitat on site, you must use the full Biodiversity Metric instead.

That said, being close to a priority habitat does not automatically rule out the SSM. You can still use it if the habitat is off-site and your project will not directly or indirectly affect it. For example, if a neighbouring woodland or hedgerow will not be lost, shaded, polluted, or altered by drainage changes, the SSM may still apply.

How to check: Use Natural England’s mapping tools or your local environmental records centre. If you are unsure, it’s best to ask an ecologist to confirm whether your site qualifies.

2. Understand Your Baseline Habitats

The SSM works by comparing what’s on your site now with what will be there after development. This means you need a good understanding of your baseline habitats before you start.

What are they? Features such as grassland, hedgerows, trees, ponds, or any other ecological areas on your site. How to check: Begin with a walkover survey of your site and back it up with aerial images or existing site plans.

What drawings do you need? A site plan that shows your existing habitats is the starting point. An example is provided further in the article.

3. Use the Right Software

We’ve found the SSM spreadsheet only works properly in Excel with macros enabled. If you try to run it in Google Sheets, the formulas can break and the tool won’t function correctly.

How to check: Make sure you’re using Microsoft Excel. If you don’t have it installed, Microsoft offers a free online version that works perfectly well with the SSM. Also, always double-check that you’ve downloaded the latest version of the SSM from the government's website, as updates are released from time to time.

4. Read the User Guide

The SSM has its own user guide, which explains the definitions, core elements, and how to interpret results. It also sets out how landscaping features are treated in the metric. It might be tempting to dive straight into the spreadsheet, but the guide really is a must-read if you want to get things right the first time.

How to check: Download the most recent Small Sites Metric User Guide from Natural England’s website and keep it alongside your project files. Use it as a reference while you work through the metric.

5. Make Sure You’re Competent

The SSM should only be completed by someone with the right expertise. DEFRA defines a competent person as someone who can:

- Accurately identify habitats on site (pre-development)
- Understand management requirements for habitats to be created or enhanced (post-development)

How to check: Be honest about your skills. If you are not confident in identifying and classifying habitats, bring in a professional. This will give your submission credibility and avoid delays.

How to Use the Small Sites Metric: A Practical Guide

So you have completed the top tips, what is next? Before opening the spreadsheet, make sure you have your proposed design and landscaping plan ready. Jumping into the metric without them is a bit like trying to bake without a recipe: you will only end up guessing, which leads to errors and delays.

Here is what you need in place:

- Baseline habitat plan – shows what currently exists on site.

- Proposed habitat plan – shows what will remain, be created, or enhanced.

- Evidence of at least a 10% biodiversity improvement – your landscaping plan should make it clear where this gain will come from, such as wildflower planting, new hedgerows, or sustainable drainage with ecological benefits.

Example existing and proposed small site metric habitat plans

Our focus is on producing the best quality architecture with a solid plan and no unpleasant surprises.

Filling out the metric Page-By-Page
Step 1 - Page 02: Site Details

Now that you have your baseline and proposed habitat plans ready, it is time to open the SSM spreadsheet. It can look a little overwhelming at first, but do not worry. We will take it one page at a time so you can see exactly what goes where.

Page 02: Site Details

This is the simplest part. Add the project details, the name of the assessor, and the date. Make sure you reference the correct landscape plan so that everything ties back to your submission.

How to check: Use the same project details as your planning application and ensure consistency across documents.

Example of filling out page 02 Site Details of Small Site Metric

Here's how we filled out page 2 for the Willow Mill Fisherie project.

Step 2 - Page 03: Desktop Assessment

This section pulls together some background information. You will need to fill out:

- Development area – the size of your site in square metres.

- Designated sites and priority habitats – information you should already have from your top tips checks. Do not panic if some boxes turn amber. It is quite common for sites to be within 500m of a priority habitat. What matters is whether there is a priority habitat on your site or if your project will directly impact one nearby.

- European Protected Species – if none are identified during your desk study or walkover, simply select "No" in the dropdown.

- Site Walkover – record who carried out the inspection and when.

How to check: Use your site plan or CAD software to confirm the development area. Confirm priority habitats using Natural England’s Magic Map or your local records centre.

Example of filling out page 03 Desktop Assessment of Small Site Metric

Here's how we filled out page 3 for the Willow Mill Fisherie project.

Step 4.1 - Page 05: Area and Baseline Habitats

This is where you log all the area-based habitats on your site. The spreadsheet looks complicated, but it’s really just about filling in sets of tables:

Baseline Habitats

This table records everything that currently exists on site. For each habitat, you’ll need to input:

- Habitat type (e.g. grassland, scrub, woodland, bare ground)
- Strategic Significance (Aligns with local biodiversity priorities)
- Area in m² (from your baseline plan)

This is how to fill it out step by step and top tips using willow mill fisherie as an example:

1. Select the broad habitat type
Use the dropdown menu to choose the correct broad habitat. If you do not see multiple options available, it usually means there is an issue with your file.

2. Identify the habitat type
For Willow Mill, the baseline habitat was recorded as mixed scrub (although in other cases it could be hawthorn, woodland edge, or similar depending on the survey).

Always cross-check against your site walkover notes and the SSM habitat definitions.

3. Understand strategic significance
This relates to whether the habitats you are creating or enhancing align with local biodiversity priorities such as a Local Nature Recovery Strategy (LNRS) or Green Infrastructure Strategy. If they do, the metric gives extra weight. If they do not, they score lower.

In practice, this box is not often ticked because many areas still do not have a published local strategy.

4. Enter the site area
In our Willow Mill example, the total site area was 517 m². We enhanced 477 m² of mixed scrub while leaving 40 m² off the table, which became a hard surface. This hard surface is then accounted for in the “habitats created” section.

5. Leave the baseline results alone
Do not edit the baseline results table directly. The spreadsheet will automatically calculate this for you based on your entries.

6. Add comments for clarity
Use the comment box to explain your decisions. This helps the assessor understand your approach and gives more context to the numbers.

7. Check the trading rules
Once the table is complete, you should see green tick boxes, which means the trading rules have been satisfied. If an error shows up in amber, it means something is missing or inconsistent and will need to be corrected.

Key of small site metric page 5 - Baseline habitats

Here's how we filled out the first part of page 5 for the Willow Mill Fisherie project.

Step 4.2 - Page 05: Habitats to be Created

Here you log any new habitats your project will deliver. Examples:

- A new wildflower meadow
- A pond created as part of SuDS
- Newly planted native woodland or hard surfaces such as roads

How to fill it out step by step (with Willow Mill as an example)

1. Select the broad habitat type
Use the dropdown menu to choose the correct broad habitat. If you do not see multiple options available, it usually means there is an issue with your file.

2. Identify the habitat type
At Willow Mill, we recorded this as “Artificial unvegetated, unsealed surface” because we were creating a new access track. On other projects, you might be recording grassland,
ponds, or woodland depending on what is proposed.

3. Condition Assessment
For Willow Mill Fisherie, because we were creating a track, the condition was recorded as “Other”. Note that this option is only available for certain artificial habitats. For ecological habitats such as grassland or woodland, you must select a defined condition from the dropdown (e.g. Poor, Moderate, Good).

4. Enter the site area
The total Willow Mill site area was 517 m². Of this, 477 m² was enhanced mixed scrub (already logged in the baseline/enhanced tables). The remaining 40 m² was left over and created as a hard surface for the access track. By entering that here, the table now rounds back up to the full 517 m² site total.

5. Leave the baseline results alone
Do not edit the baseline results table directly. The spreadsheet will automatically calculate this for you based on your entries.

6. Add comments for clarity
Use the comment box to explain your decisions. This helps the assessor understand your approach and gives more context to the numbers.

7. Check the trading rules
Once the table is complete, you should see green tick boxes, which means the trading rules have been satisfied. If an error shows up in amber, it means something is missing or inconsistent and will need to be corrected.

Example of filling out page 05 habitats to be created Small Site Metric

Here's how we filled out page 3 for the Willow Mill Fisherie project.

Step 4.3 - Page 05: Habitats to be Enhanced

This covers habitats that will remain but get better management. For example:

- Scrub that will be restored into species-rich grassland
- Amenity grassland improved into meadow
- Woodland under new active management

How to fill it out step by step (with Willow Mill as an example)

1. Select the broad habitat type
Use the dropdown menu to choose the correct broad habitat. If you do not see multiple options available, it usually means there is an issue with your file.

2. Identify the existing habitat type
For Willow Mill, the baseline habitat was recorded as mixed scrub (although in other cases it could be hawthorn, woodland edge, or similar depending on the survey). Always cross-check against your site walkover notes and the SSM habitat definitions.

3. Enhanced habitat type?
Here you record the target condition you are aiming for through enhancement. For example, improving grassland from “poor” to “moderate” condition, or scrub to better-managed habitat. Enhancements help improve your overall metric score compared to simply leaving the area as retained.

4. Enter the enhanced site area
In the Willow Mill case, we enhanced 477 m² of mixed scrub. Make sure your enhancement figures line up with the baseline totals you entered in Table 1a, so that the areas add up consistently.

5. User Comments
Always use the comment box to explain your approach. At Willow Mill, we noted that the enhancement was part of a sensitive remediation area, explaining how the management would improve the scrub habitat over time. Clear notes help the assessor understand the context and reduce queries.

Example of filling out page 05 habitats to be enhanced of Small Site Metric

Here's how we filled out page 3 for the Willow Mill Fisherie project.

Step 4.4 - Page 05: Tree Area Calculator

If your site has scattered trees or small groups, use this calculator to translate canopy cover into habitat area. It then feeds back into the main tables.

How to fill it out step by step (with Willow Mill as an example)

How to fill it out step by step (with Willow Mill as an example)

1. Select the Size
Use the dropdown menu to choose the size class of trees (small, medium, or large). This is based on their canopy spread, which you can measure from site surveys or aerial imagery.

2. Identify the number of trees
Count the number of individual trees or groups of trees within the site. At Willow Mill, there were no scattered trees within the baseline survey, but on other sites you may need to classify them carefully. Always cross-check your site walkover notes and the SSM definitions.

3. Are you retaining all the trees?
Record whether the existing trees will be retained, lost, or partially removed. Retained trees contribute positively to your baseline and can reduce the need for extensive compensation later on.

4. Are you planting new trees?
If your design includes new tree planting, record the numbers here. These will appear in the “created” section of the table. For example, if you are adding street trees, orchard planting, or amenity trees within landscaping, they should all be included.

5. Area of trees planted post development
Once you input the number and size of new trees, the SSM will automatically calculate the equivalent area of canopy cover. You do not need to edit this manually — the spreadsheet does the maths for you.

Example of filling out page 05 Tree Area Calculator explained

A helper image for the tree area calculator page.

Step 5 - Page 6: Hedges and Lines of Trees

The process is the same as for area habitats but specifically for hedgerows and tree lines. Work through baseline, created, and enhanced categories. How to check: Measure lengths accurately from your site plan and proposed habitat plan. Even short stretches count, so do not leave them out.

Step 6 - Page 7: Watercourses

The process here is the same as for area habitats and hedgerows. Record any baseline watercourses, then add those that will be created or enhanced as part of your scheme.

How to check: Refer to your drainage strategy and landscaping drawings. Sustainable drainage features can often be designed to provide ecological value, which will strengthen your results in the metric.

Important note: If you are proposing any changes to an existing watercourse, you may need separate consent. This is usually from the county council or the Lead Local Flood Authority (LLFA), rather than the local planning authority. Works that affect ordinary watercourses often require permission under the Land Drainage Act 1991, and any alterations to main rivers may need approval from the Environment Agency.

You can still secure planning permission for the works through the SSM process, but you must obtain these additional consents before carrying them out.

Step 7 - Page 8: Headline Results

This is where the spreadsheet pulls everything together and shows whether you have achieved the 10 percent biodiversity net gain.

If the figure is less than 10 percent, do not panic. It simply means you need to adjust the design. Options might include adding wildflower planting, creating an additional hedgerow, or enhancing the management of retained habitats.

How to check: Review the outputs against your landscaping plan. Make sure the enhancements you are proposing are realistic and achievable in the long term.

Example of the results page 08 of the Small Site Metric

This is how the results page looked for Willow Mill Fisherie.