A COMAH site is a facility holding dangerous substances above the thresholds set out in the Control of Major Accident Hazards Regulations 2015, the UK's legal regime for preventing major accidents involving hazardous chemicals. Manufacturing at this scale brings additional regulatory obligations that are worth understanding if your product involves large-volume storage of a regulated substance.
Key points
- COMAH sites are classified as lower tier or upper tier, based on the quantity of dangerous substances held against thresholds set out in Schedule 1 of the regulations.
- Example threshold quantities include 10 tonnes (lower tier) and 25 tonnes (upper tier) for chlorine gas, and 2,500 tonnes (lower tier) and 5,000 tonnes (upper tier) for ammonium nitrate.
- Lower-tier sites must have a Major Accident Prevention Policy (MAPP); upper-tier sites face more demanding requirements, including a detailed Safety Report.
- COMAH is enforced by a joint competent authority - HSE (or ONR) alongside the relevant environmental regulator for each GB nation.
What This Capability Involves
A manufacturer operating a COMAH-regulated site has the safety management systems, major accident prevention policy, and (for upper-tier sites) safety report required under the regulations, reflecting the scale of dangerous substances held on site.
When You Need This Capability
You need a COMAH-regulated manufacturer if your product or its raw materials are stored or used in quantities at or above the relevant Schedule 1 threshold - this is more likely for large-volume industrial chemical production than smaller-batch or niche products.
Regulatory Considerations
COMAH thresholds are set per substance in Schedule 1 of the regulations, so whether a site is lower tier, upper tier, or below either threshold depends on the specific dangerous substances and quantities involved. Confirm with a prospective manufacturer their COMAH tier status and how it relates to the substances used in your specific process, since this affects the safety management and reporting obligations they operate under.
What Operating at This Level Means
COMAH sites operate under duties well beyond ordinary industrial safety requirements: a major accident prevention policy, a documented safety management system, emergency planning coordinated with local authorities, and for upper-tier sites a detailed safety report assessed by the regulator. Compliance is continuous rather than periodic, and it shapes how the site runs day to day.
One consequence for buyers is that safety management maturity at these sites tends to be high, since it has to be. Another is that introducing a new substance is not automatic - it may need assessing against the site's existing inventory position and safety case before a first batch can be scheduled, which affects lead time in a way that catches people out.
Whether You Actually Need One
COMAH status depends on the quantities of specified dangerous substances held across the whole establishment, so it is a property of the site rather than of your product alone. Your product may require a COMAH site, or may simply be capable of being made at one - and those are different situations.
If it does require one, the available shortlist is short and there is no route around it, so establish this at the very start of a search. If it does not, treat COMAH status as a positive indicator of safety maturity rather than a requirement, and weigh it against the higher overheads and longer onboarding that usually accompany it. See COMAH manufacturing sites.
Questions to Ask a Manufacturer
- Is your site classified as lower tier or upper tier under COMAH, and for which substances?
- Can you share relevant details of your Major Accident Prevention Policy or Safety Report?
- How does operating a COMAH site affect lead times or scheduling for our product?
- What experience do you have manufacturing at the scale our project requires?
Related Pages
Frequently Asked Questions
Does a small contract manufacturer ever operate a COMAH site?
It's possible if they hold sufficient quantities of a regulated substance, but COMAH status is generally more associated with larger-scale industrial manufacturing and storage - check a manufacturer's specific status rather than assuming based on their size alone.
Does COMAH status affect how quickly a manufacturer can take on new production?
It can - upper-tier sites in particular operate under more extensive safety management and reporting requirements, which may factor into scheduling and onboarding new products. Ask directly about lead times.
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Search the DirectoryLast reviewed: August 2026. Regulation and guidance change — verify anything you rely on against the primary source. Tell us if something here is out of date.