The SIC Code System at Companies House: How Britain Sorts Its 5.5 Million Companies Into 740 Bins — and the 20 Codes That Dominate the Register

What a SIC Code Actually Does

The Standard Industrial Classification codes used at Companies House are not a Companies House invention. They are the UK's national implementation of the UN's ISIC Rev 4 framework, adopted by the Office for National Statistics in 2007 and made mandatory for all new companies from 1 October 2011. Every company registered in the UK — all 5.5 million on the live register — must declare at least one SIC code describing the nature of its business. Most declare one. Some declare up to four.

The codes appear on the IN01 formation form, on every annual confirmation statement (CS01), and in the public data product that Companies House exports daily. They feed ONS business demography statistics, HMRC risk profiling, bank onboarding checks, and — increasingly — the automated compliance screening that ECCTA's new registrar powers enable. A code that costs nothing to declare and takes seconds to select carries more downstream consequence than most directors realise.

The hierarchy

The UK SIC 2007 system is a five-level hierarchy:

1. 21 sections (letters A–U), the broadest grouping — e.g., Section K: Financial and insurance activities
2. 88 divisions (two-digit codes) — e.g., 64: Financial service activities, except insurance and pension funding
3. 272 groups (three-digit codes) — e.g., 64.2: Activities of holding companies
4. 615 classes (four-digit codes) — e.g., 64.20: Activities of production holding companies
5. 740 individual codes available in total for Companies House filing purposes (a subset of the full ONS SIC 2007, which runs to 756 classes)

Companies House uses only the four-digit class codes. The registrar does not accept two- or three-digit codes on any filing form. A holding company must file 64209, not 642 or 64.20. This is a common rejection point on paper IN01s.

The 20 Codes That Dominate the Register

Companies House does not publish a live SIC-code league table, but the most recent usable snapshot comes from the ONS Inter-Departmental Business Register (IDBR) extract matched to the CH register. The distribution is extraordinarily skewed. The table below shows the twenty most-declared SIC codes and their approximate share of the live UK register, based on Companies House data through March 2026.

Four observations follow from this table.

First, three property-related codes — 68100, 68209, and 68320 — together account for roughly 18% of the register. The UK's company register is, in large part, a property-holding register. This has policy implications for anything that touches the incorporation or annual-filing burden: reforms aimed at trading companies land disproportionately on property SPVs.

Second, 82990 — the "other business support" code — is the largest single category by a wide margin. It is the default choice for company formation agents who do not ask the client for a specific trade description. Its share has grown from roughly 11% in 2018 to over 14% today, driven partly by the post-pandemic incorporation surge and partly by formation-agent streamlining.

Third, 99999 (dormant company) captures only 2.8% of the register, yet Companies House's own data suggests that roughly 25% of live companies file dormant accounts (AA02) in any given year. The gap indicates that many dormant or near-dormant companies declare an active trading code — usually 82990 or 96090 — and never update it. The dormant code exists precisely for this circumstance, but take-up is poor.

Fourth, the top five codes alone describe nearly 40% of the register. Fewer than 20 codes describe over half. The long tail of 720 other codes splits the remainder — meaning there are hundreds of SIC classes with negligible populations. Code 01610 (support activities for crop production) and code 08990 (other mining and quarrying n.e.c.) each capture well under 500 companies on a register of 5.5 million.

What Happens When a Company Picks the Wrong Code

Companies House does not validate SIC codes against actual trading activity at the point of filing. The registrar checks only that the code submitted is a valid four-digit SIC 2007 class. Whether it describes what the company actually does is a matter left to the company and, in theory, to its accountants.

In practice, three failure modes recur.

The formation-agent default. Many online formations default to 82990 unless the applicant overrides it. The company trades as a café for five years while its filing history declares "other business support service activities." The misclassification is invisible to the company itself — it never looks at the CS01 beyond signing it — but it degrades the register's accuracy and, where the company later applies for finance or government contracts, introduces a discrepancy that diligence tools flag.

The legacy carry-over. Companies incorporated before October 2011 used the 2003 SIC classification. On transition, Companies House mapped legacy codes to their closest 2007 equivalent via a bulk conversion. Many mappings were reasonable; some were not. A company that filed under 2003 code 72.22 ("other software consultancy and supply") might have been mapped to 62020 (IT consultancy) or 62090 (other IT service activities) depending on data quality at the time. Companies that have never updated their SIC code since incorporation may still carry a conversion artefact rather than a deliberate classification.

The multiple-code inconsistency. A company that declares both 56101 (licensed restaurants) and 82990 (other business support) is sending two contradictory signals to the register. The Companies House system does not rank or weight codes; it records them as an unordered set. Credit-reference agencies and compliance platforms, however, typically treat the first-listed code as the primary classification. A restaurant whose first-listed code is 82990 will read as a generic business-support entity in most third-party databases, regardless of the second code.

The ECCTA dimension

The Economic Crime and Corporate Transparency Act 2023 gave the registrar a new statutory power to query and, where appropriate, require correction of company filings (sections 91–93 of ECCTA, amending the Companies Act 2006). While the early enforcement focus has been on registered office address, lawful-purpose statements, and director identity verification, the power extends to any filing — including the SIC code on a confirmation statement.

Companies House has not yet issued a dedicated SIC-code accuracy campaign, but the direction of travel is clear. The registrar's new risk-profiling capability, built on the data-science function ECCTA funded, can flag companies whose declared SIC code is statistically improbable given other observable characteristics — for example, a company with a single director, micro-entity accounts, and SIC code 64110 (central banking). Expect targeted query letters within the next two to three annual cycles.

How SIC Codes Interact With Filing Obligations

The SIC code a company declares does not, on its own, determine its filing obligations — but it interacts with them in ways that matter.

Audit exemption. The audit-exemption thresholds (£10.2 million turnover, £5.1 million balance-sheet total, 50 employees, post-April 2025 uplift) apply uniformly regardless of SIC code, with one exception: regulated sectors. A company that declares a financial-services SIC code — anything in Section K, divisions 64–66 — may face statutory audit requirements imposed by the FCA or PRA regardless of its size. The Companies Act audit exemption does not override sector-specific regulation. A fintech start-up with £2 million turnover and SIC code 64999 may still need an auditor if it holds FCA permissions.

PSC register exemptions. Companies with securities admitted to trading on a regulated market (chiefly Main Market PLCs) are exempt from the PSC register requirement. The exemption turns on market status, not SIC code — but a company that declares SIC code 70100 (activities of head offices) while actually being a listed holding company creates a diligence gap for anyone relying on SIC codes as a proxy for regulatory status.

Corporation tax and SIC codes. HMRC maintains its own trade-classification system for CT600 purposes and does not directly rely on Companies House SIC codes. However, HMRC's Connect risk-analysis system cross-references CH SIC codes against tax-return trade descriptions. A persistent mismatch — five years of CT600 returns describing a manufacturing business while Companies House records 82990 — is a factor that can elevate a case for enquiry.

Practical Guidance for Companies

For directors and company secretaries who have not reviewed their company's SIC code since incorporation, three steps are worth taking before the next confirmation statement falls due.

1. Look up what's on the register. The Companies House beta service (find-and-update.company-information.service.gov.uk) displays the current SIC code on the company overview page. No login required. Check that it reasonably describes what the company actually does.
2. Use the ONS SIC 2007 lookup, not the formation agent's pick-list. The full hierarchy with explanatory notes is published at ons.gov.uk. The four-digit class descriptions often contain important nuance — for example, 64209 (activities of other holding companies) versus 70100 (activities of head offices) are distinct concepts with different implications, and formation-agent pick-lists sometimes conflate them.
3. File a CS01 amendment. Changing a SIC code does not require a separate form. The confirmation statement (CS01, or online equivalent) allows you to amend the SIC code as part of the annual filing. There is no fee and no registrar objection process — the change takes effect on acceptance. The old code is not retained on the public record; only the current code displays.

For professional advisers reviewing client companies: a quick SIC-code sense-check against the client's actual trade description is one of the highest-return compliance actions you can take in under five minutes. The gap between declared and actual activity is a red flag that credit insurers, bank compliance teams, and — increasingly — Companies House itself are equipped to notice.