Small sites metric eligibility: what the Desktop Assessment sheet tests, the strings it writes, where the area and length limits are checked again, and what the workbook does not check

The small sites metric is the version of the statutory biodiversity metric for developments that are not major in planning terms. Its user guide says who may use it and who must use the main calculation tool instead, and GOV.UK is the place to read that rule. This page is about what the workbook does with the question: the nine entries on its 3. Desktop Assessment sheet, the formula behind each string they produce, the three errors that are written without a ▲, the second area test on the habitat sheets, the places where the guide's own troubleshooting table does not match the formulas, the parts of the rule no cell tests, and what the free check on this site lists. Every statement is read from the small sites metric 1.2.3 workbook as published on GOV.UK, with the cell named, or from the July 2025 user guide, with the page named. Nothing here decides whether a site qualifies — that is the applicant's and the authority's judgement, and the workbook's own notes say so.

The rule, as the guide and the Order state it

The user guide (p. 10, Restrictions on use of the SSM) says the small sites metric may be used only for developments that are not major under the Town and Country Planning (Development Management Procedure) (England) Order 2015, and then lists the planning thresholds: residential development of 1 to 9 dwellings on a site of 1 hectare or less, or under 0.5 hectares where the number of dwellings is unknown; commercial development with under 1,000 square metres of floor space created or a site under 1 hectare; not minerals and not waste development. It adds four ecological conditions: only habitats the small sites metric offers are present (a site with any other habitat, riparian zones included, must use the main tool); no priority habitat on site (some hedgerows and arable field margins excepted); no statutory protected site or habitat; no European protected species. A protected site or priority habitat within 500 m is a reason to consider the main tool, not a bar. The Order itself (article 2) defines major development as 10 or more dwellings, or a site of 0.5 hectares or more where the number is not known; floor space of 1,000 square metres or more; or a site of 1 hectare or more. The workbook's thresholds follow the Order's wording, not the guide's paraphrase — the difference matters at exactly 1 hectare, below.

The nine entries on the Desktop Assessment sheet

Sheet 3. Desktop Assessment asks its questions in column B, numbered 14 to 25 (the numbering carries on from 2. Site Details, whose questions are 1 to 13), takes the answers in column C, and writes its verdicts in column D, with one warning in column E. Five answers are drop-down lists; the lists below are read from the sheet's data-validation XML, in the order Excel shows them.

The sheet's Site Name line (C2) is copied from Site Details and reads "Enter site name on 2. Site Details" until that sheet has one. Nothing on this sheet is numbered as a result; the strings below appear beside the question they belong to.

What each formula tests, and the string it writes

The strings are quoted exactly as the formulas write them, double spaces and symbols included, because the symbol decides whether a string search — the free check's included — can see the cell. Three of the eleven carry no ▲ and no ⚠.

Site area (D6, E6, D7)

• D6 is guidance, not a flag: once C6 is filled it reads "Site area must be less than 10,000 m2", or "Site area must be less than 5,000 m2" when the development is Residential and the dwellings are Unknown number. Blank while C6 is blank.
• E6 writes Site area limited by unknown number of dwellings ⚠ for that same pair — Residential with Unknown number — and nothing otherwise. A Mixed development with an unknown number of dwellings gets no warning here, D6 tells it 10,000 m² and D7 tests it at 10,000 m² — while the habitat sheet applies the 5,000 m² limit to it (below), so for such a site between 5,000 and 10,000 m² the two sheets disagree.
• D7 writes ERROR- Site too large for metric  - USE MAIN METRIC ▲ when D6 says 10,000 and C7 is 10,000 or more, or when D6 says 5,000 and C7 is 5,000 or more. The test is ≥, so a site of exactly 1 hectare is refused — which is what the Order's "1 hectare or more" means, and not what the guide's "1 hectare or less" would suggest. It reads the typed site area, not the habitat areas entered later.

Commercial footprint (D8) — three strings, no symbol

D8 tests the footprint in three steps and writes, without a ▲: "ERROR- Not required" when C8 holds a value but B8 says N/A (a footprint typed for a residential or other development); "ERROR- Footprint too large for metric  - USE MAIN METRIC" when C8 is 1,000 or more; and "ERROR - Footprint larger than development area" when C8 exceeds the site area in C7. These are the Order's 1,000 square metres and a sanity check, and they show in red text in the workbook. Because no symbol is written, the workbook's own sheet summaries, the headline results and any search for ▲ and ⚠ pass over them.

Dwellings (D9)

• ERROR- Not required ▲ when C9 holds an answer but B9 says N/A — dwellings given for a Commercial or Other development.
• ERROR- Number of dwellings too large for metric  - USE MAIN METRIC ▲ when the development is Residential or Mixed and C9 is Over 9 dwellings. The count is never typed; the list's three choices are the whole test.
• ERROR - You must specify number of dwellings ▲ when the question is shown (B9 carries its text, so C6 is Residential or Mixed) and C9 is blank.

Designated sites, priority habitats, protected species (D11, D12, D16)

D11 and D12 run the same formula on their own answer: On site writes ERROR- Site too complex for metric - USE MAIN METRIC ▲; Within 500m of site boundary writes Consider using main metric tool ⚠; No and a blank write nothing. D16 writes the same too complex error when C16 is Yes. So the one string covers three different causes — a designated site on site, a priority habitat on site, a European protected species present — and the cell it sits in (D11, D12 or D16) is the only thing that says which. The 500 m case is a ⚠, which matches the guide's "consider whether you need to … use the SBMCT"; the guide also says that, if the small sites metric is still used, "a warning flag will appear where this is the case" — this is that flag.

The walkover (D19, D20) and the photographs (sheet 4)

• D19 writes "A competent person should be able to confidently identify the habitats onsite" for Walkover completed by competent person, and "ERROR- Site walkover required" — no symbol — for No walkover undertaken. (The formula spells the test value "No Walkover Undertaken"; Excel compares text without regard to case, so the list's entry matches.) A qualified ecologist's walkover writes nothing.
• D20 writes Walkover date required ▲ while C20 is blank; with a date it writes "Site walkover data valid until" and the date six months on (EDATE(C20,6), shown as dd/mm/yy). The guide (p. 45) says the validity date is "an aid to age the information" and that an authority decides whether a fresh walkover is needed; the workbook never compares it with the completion date on Site Details (D9) or the authority's review date (D23).
• On 4. Supporting Information, cell E2 writes "Error - Site photographs required to support application" — no symbol — when the walkover answer is competent person or No walkover undertaken and none of the twenty habitat photo rows (D8:E46, alternate rows) holds anything. A qualified ecologist's walkover switches the requirement off. This is the one place another sheet reads the Desktop Assessment's answers for anything but area.

A blank development type

With C6 blank, D6 and D7 are blank, B8 and B9 are blank so D8 and D9 write nothing, and the sheet shows no error at all. The first string a blank development type produces is on the habitat sheet, below — and it is the general site exceeds areas error, whose words do not name the cause.

The second test: the habitat sheets

Sheet 5. Area Habitats repeats the planning test against the areas actually entered. I32 sums the baseline areas in I11:I30 — individual trees (row 31) are outside the range, and the formula excludes by name the two green-wall habitats and the three artificial hard-structure habitats, which the label in H32 summarises as "excl trees, green walls and intertidal hard structures". I33 then reads the Desktop Assessment and writes "Areas Acceptable ✓" in exactly seven cases, and otherwise Error - Site exceeds areas appropriate for the small sites metric ▲:

Three things follow from the formula's shape. A blank development type with any baseline area matches no branch and writes the error, so this is where a forgotten C6 first shows. (A blank footprint does not: Excel reads an empty cell as 0 in C8<1000, so a commercial site with the footprint left blank passes this test, and only sheet 3's question — "or zero if none" — asks for the figure.) Over 9 dwellings matches no branch, so that choice is flagged twice, once on each sheet. And the commercial test needs both the area and the footprint under their limits — the guide's sentence on p. 10 joins them with "or", the Order makes either one alone a major development, and the workbook follows the Order.

Two more cells belong to the same question. I35 writes Error - Areas Entered Does Not Match Stated Site Area ▲ when I32 differs from the site area typed in Desktop Assessment C7 (it is silent when I32 is 0), so the typed area and the entered habitats are held to each other. And on 6. Hedges & Lines of Trees and 7. Watercourses, I33 writes Length Exceeds Length Appropriate for Small Sites Metric ▲ when the baseline length total in I31 is 5,000 m or more — a limit that appears nowhere in the guide's restrictions section, and that reads nothing from the Desktop Assessment.

Where the verdict is not echoed

The Desktop Assessment's strings stay on their sheet. 8. Headline Results D5 and D7 read the unit balances and the trading cells; D9 writes Check for input errors/rule breaks present in the metric ⚠ only as the alternative to "Scheme alterations or offsite units required", from those two cells alone. The sheet summaries in J2 that write Input Errors Present On Sheet - Red Cells Or ▲ Highlight Errors exist on sheets 5, 6 and 7 and search their own rows; nothing searches sheet 3 or sheet 4. A small sites metric can therefore read "BNG Targets Met ✓" and "Trading Rules Satisfied ✓" on its results sheet with a too large, too complex or walkover required error standing on the Desktop Assessment. Only the site exceeds areas cell on sheet 5 is inside a summary's reach.

Where the guide's troubleshooting table differs from the formulas

Appendix 1 of the user guide (Table A1, pp. 54–55) lists the sheet's errors with a comment on each. Read against the formulas of 1.2.3, the table differs in six places; a reviewer who searches a metric for the table's wording will not find the workbook's.

• The table's strings end "USE STATUTORY BIODIVERSITY METRIC"; the formulas write "USE MAIN METRIC", with a double space before the dash in three of them (D7, D8, D9). The table writes "ERROR -" throughout; the formulas write "ERROR-" without the space in five of the six.
• The table's "Number of units too large for metric" is the formula's "Number of dwellings too large for metric".
• The table's comment on site too large says the error appears "if your site is above 5000m2 or is above 10,000m2 and classed as residential". The formula applies 10,000 m² to every development type and 5,000 m² only to a residential development with an unknown number of dwellings, and tests at or above, not above.
• The table's comment on site too complex names only "a statutory designated site … within your site boundary". The formula also writes it for a priority habitat on site (D12) and for European protected species present (D16).
• The table lists "Site walkover required" and the two footprint errors as errors; in the workbook those three strings carry no ▲, unlike every other error on the sheet, so the guide's own convention — the ▲ marks a rule break — is not applied to them.
• The table's "Site exceeds areas appropriate for the small site metric" is the formula's "…small sites metric", and its comment ("the total area of habitats entered … is greater than that which is acceptable") omits the formula's other triggers: a blank development type and Over 9 dwellings.

What the workbook does not check

• Minerals and waste. The Order's first two heads of major development are not asked about anywhere. Other in C6 is accepted up to 10,000 m² with no further question.
• Priority habitat, protected site, protected species. Each is a declaration. The cells that would carry the evidence (C13, C14) are free text no formula reads; the guide's "data search of the site and immediate area" (p. 45) is asked for, not tested.
• Only small-sites habitats present. The habitat drop-downs on sheets 5 to 7 offer only the habitats the small sites metric prices, so a habitat outside the list cannot be entered — but nothing asks whether one exists on site and was left out, which is the guide's actual criterion.
• The 500 m case. A ⚠ and nothing more; whether the applicant "contacted an ecologist", as the guide suggests, is not recorded.
• The dates. The walkover's validity date is printed and never compared with the metric's completion or review dates; the completion date on Site Details is not tested against anything.
• The site area itself. C7 is typed; nothing checks it against a plan. I35 only holds the habitat areas to it.

Sources

The small sites metric 1.2.3 (the .xlsm workbook) as published on GOV.UK's Statutory biodiversity metric tools and guides page and read on 5 October 2026: the cells, drop-down lists and formulas named above are its — sheet 3 (C6:C23, D6:D20, E6), sheet 4 (E2), sheet 5 (H32:I35), sheets 6 and 7 (I31, I33), sheet 8 (D5, D7, D9) — and the strings are quoted as the formulas write them. The Small sites metric (statutory biodiversity metric): user guide, July 2025, p. 10 (restrictions on use), p. 45 (Tab 3 – Desktop Assessment) and pp. 54–55 (Appendix 1, Table A1). The Town and Country Planning (Development Management Procedure) (England) Order 2015, article 2, as published on legislation.gov.uk and read the same day, for the definition of major development. A later release may move a cell or change a string; the version each statement rests on is named here so it can be checked. Quadrat is not affiliated with Defra or Natural England. If a cell on this page does not match the workbook in front of you, write to hello@quadrat.uk; replies are signed as Quadrat.