Short answer: before a subcontractor starts, check the documents that match the work: Gas Safe registration and the engineer's ID card categories for gas work; personal F-gas qualifications and a company certificate for refrigeration, air conditioning and heat pumps; employers' liability insurance if they employ anyone; a CSCS card where the site asks for one; and HMRC verification if the work is inside CIS. Record every expiry date.
Why the checks fall on you
Several laws make the firm that brings in a subcontractor responsible for checking them:
- Gas work: regulation 3(2) of the Gas Safety (Installation and Use) Regulations 1998 puts a duty on "every other employer and self-employed person who has control to any extent of such work" to make sure whoever does it is competent. HSE tells employers: "It's your responsibility to check a person is registered and is qualified."
- Construction work: under regulation 8(3) of the Construction (Design and Management) Regulations 2015, a person appointing a contractor "must take reasonable steps to satisfy themselves" that the contractor has the skills, knowledge and experience for the role. CDM's definition of construction work includes "the installation, commissioning, maintenance, repair or removal of mechanical, electrical, gas, compressed air, hydraulic, telecommunications, computer or similar services which are normally fixed within or to a structure" (reg 2(1)).
- Building work in England: since 1 October 2023, regulation 11E of the Building Regulations 2010 says anyone using another person to carry out building work must take all reasonable steps to satisfy themselves, before the work starts, that the person is competent for it. "Building work" here has the meaning in regulation 3, such as replacing a boiler; routine servicing on its own is not on that list.
- Payments: if the work is a construction operation under CIS, you must verify a new subcontractor with HMRC before you pay them.
The documents, one by one
| Document | When you need it | How to check it | Expiry |
|---|---|---|---|
| Gas Safe registration (the business) and ID card (each engineer) | Any work on gas fittings in homes and in workplaces such as shops, restaurants, schools and hospitals (GSIUR reg 3(3)) | Gas Safe Register website or 0800 408 5500; read the categories on the card | The card shows whether the holder's qualifications are up to date |
| Personal F-gas qualification (category 1 to 4) | Each person who works on stationary refrigeration, air conditioning or heat pump systems containing F gas | Ask for the certificate; GOV.UK lists City and Guilds, BESA and LCL Awards as the accredited bodies (or an equivalent qualification an EU member state recognises) | Record any date shown |
| F-gas company certificate | A company, including a sole trader, servicing stationary equipment containing F gas that others operate | Ask for the certificate from Refcom, F-Gas Register (Quidos) or Bureau Veritas | Renewed every 3 years |
| Employers' liability insurance certificate | A subcontractor who employs staff | Check the cover is at least £5 million and the insurer is authorised (Financial Conduct Authority register) | The policy end date |
| Public liability insurance | When your contract with the client sets a level | Check the certificate meets the level in your contract | The policy end date |
| CSCS card (or another CSCS-logo card) | When the site or principal contractor requires one | CSCS Smart Check app or website | The date on the card |
| HMRC CIS verification | Before you pay a new subcontractor for work inside CIS | HMRC CIS online service or CIS software, with the subcontractor's UTR and other details | Verify again if they have not been on your CIS return in the current or last 2 tax years |
Gas Safe: the business and the engineer
Regulation 3(3) of GSIUR says the employer, or the self-employed person, must be "a member of a class of persons approved for the time being by the Health and Safety Executive". In practice that means the subcontracting business is on the Gas Safe Register, and the engineer who turns up holds a Gas Safe ID card for that business. Gas Safe Register's guidance says the engineer must be registered for the type of work, the type of gas and the type of appliance. A card that covers domestic boilers does not on its own cover a commercial catering range. Check the categories against the job, not just the name.
In factories, mines, quarries, agricultural premises, construction site huts and sewage works, HSE says gas work must be done by a "competent person", and that "it's your responsibility to check they're competent". Parts of those premises used as homes or sleeping accommodation still need a Gas Safe registered engineer.
F gas: the person and the company
GOV.UK is plain on the person: "You must have your own qualifications to work on equipment containing F gas, even if you work for someone else." The four categories for stationary systems are:
- Category 1: all activities.
- Category 2: install, maintain, service and recover refrigerant from systems containing less than 3 kg of F gas, or less than 6 kg if hermetically sealed.
- Category 3: recover refrigerant from systems containing less than 3 kg, or less than 6 kg if hermetically sealed.
- Category 4: check for leaks without breaking into the refrigeration circuit.
On the company, GOV.UK says: "Your company, including sole traders, must be certified by an approved body to service stationary equipment containing F gas operated by others." A firm servicing only its own equipment does not need a company certificate. A subcontractor servicing your client's air conditioning does. Both the person's qualification and the company certificate are needed: one does not replace the other. GOV.UK says working with F gas without the correct certification or qualifications is against the law and can lead to a civil penalty.
Insurance
GOV.UK says a business must get employers' liability insurance "as soon as you become an employer", cover of "at least £5 million", from an authorised insurer. It adds: "You can be fined £2,500 every day you are not properly insured." A subcontractor that employs staff should be able to show you the certificate. Public liability insurance is not on that GOV.UK list, so the level you ask for comes from your contract with the client.
Check your own policy too. HSE's guide to the Employers' Liability (Compulsory Insurance) Act 1969 says that whether you need cover for someone "depends on the terms of your contract with them", not on what you call them or their tax status, and that someone who is self-employed for tax purposes "may be classed as an employee for other reasons". A self-employed engineer who works only for you, under your control, may need to be covered by your own employers' liability insurance. Ask your insurer or broker.
CSCS cards
CSCS states that "holding a CSCS card is not a legal requirement but most principal contractors and major house builders in the UK require workers on their sites to hold a valid CSCS card". If a site you work on requires cards, your subcontractors' people need them too. CSCS says cards "should be verified regularly using CSCS Smart Check" and that "visual card inspections will not detect cards that are fake, have been revoked or fraudulently obtained".
CIS verification
HMRC's CIS 340 guide lists the repair, servicing and maintenance of heating, ventilation, air-conditioning, drainage, sanitation, water supply and fire protection systems as outside the scheme, while installing complete systems is inside it. Our guide to CIS for building services contractors sets out where the line falls. Where CIS applies, GOV.UK says that "before you can pay a new subcontractor, you'll need to 'verify' them" with HMRC, which tells you the deduction rate. The details you give "must exactly match" those the subcontractor used to register.
Worked example: two subcontractors for one contract
A heating and cooling maintenance firm wins a contract for a secondary school. It brings in two subcontractors:
- A heating company with two engineers to replace a gas-fired boiler in the plant room. The firm checks the company's Gas Safe registration and both engineers' ID cards for the commercial categories this boiler needs, the employers' liability certificate (£10 million cover, ends in 4 months), and CSCS cards because the school's principal contractor requires them on site. The firm checks against the CIS guide whether the boiler replacement is inside CIS and, if it is, verifies the company with HMRC before the first payment.
- A sole trader air conditioning engineer to service six split units. The firm checks the engineer's category 1 F-gas certificate, the company certificate (renewal due in 11 months) and, since the work is servicing only, does not need a CIS verification. The engineer employs nobody, so there is no employers' liability certificate to collect. The engineer runs their own business, brings their own tools and works for other firms, so the firm's insurer agrees they do not need to be on the firm's own employers' liability policy.
The firm records five dates: the insurance end date, the two CSCS card dates, the F-gas company certificate renewal, and the date of each check. The insurance runs out first, so that is the first reminder.
Who does what
- The contracting firm checks documents before the work starts, records expiry dates, and stops sending work when a document lapses.
- The subcontractor sends current documents, tells you when a registration, card or policy changes, and sends only people whose cards cover the job.
- The client or principal contractor sets site rules such as CSCS cards and the insurance levels in the contract.
- The office verifies CIS status with HMRC and keeps the verification number with the subcontractor's record.
What to record
- Subcontractor's legal name, trading name and company number if any
- Gas Safe registration number, each engineer's card number and the categories checked
- F-gas personal certificate (category and body) for each engineer, and the company certificate with its renewal date
- Employers' liability and public liability: insurer, cover and end date
- CSCS card type and expiry for each person going to site, and the date it was checked on Smart Check
- CIS: UTR, verification number and deduction rate, where CIS applies
- Who checked each document, and when
Keeping subcontractor documents in Field Suite
- Subcontractor records: each subcontractor has their UTR, CIS status (gross 0%, net 20% or unverified 30%) and HMRC verification number.
- Documents with expiry dates: insurance, Gas Safe, NICEIC or NAPIT, CSCS and F-gas, each with its expiry date. The office gets a reminder 30 days and 7 days before a document expires, and on the day. The reminders do not stop you offering a job to a subcontractor whose document has run out, and Field Suite does not check registers or card categories: those checks stay with you.
- Your own engineers: qualifications in People carry a photo of the card and an expiry date, with reminders to the person and the office. With the People module on, a form can be set so that only someone with an in-date qualification, such as Gas Safe or F-gas, recorded in People and checked by an owner or admin, can sign it off. This applies to your own staff, not to subcontractors.
- Offering the job: the office sends the job to the subcontractor with an agreed price, and the subcontractor accepts, completes it with notes and photos, and adds their invoice number.
- CIS: deductions on labour only, the monthly statement for each subcontractor and the totals for your CIS300 return.
Work out a deduction with the CIS calculator, see subcontractor and CIS management, or book a demo.
Field Suite is job management software. The checks above, and the decision to use a subcontractor, remain the contracting firm's.
Questions
Is a CSCS card a legal requirement?
No. CSCS says so itself. Many principal contractors require one on their sites, which makes it a condition of the contract.
Does the F-gas company certificate cover the engineers?
No. GOV.UK says any individual who works with F gas must have personal qualifications. The company certificate is a separate requirement.
Is a Gas Safe card enough for any gas job?
Only for the categories printed on it. Check the type of work, gas and appliance against the job.
Do I verify a subcontractor who only services equipment?
CIS verification is needed before paying for work inside CIS. Servicing or maintaining a system on its own is usually outside it. Check the job against our CIS guide.
Sources
- Gas Safety (Installation and Use) Regulations 1998, regulation 3. https://www.legislation.gov.uk/uksi/1998/2451/regulation/3
- HSE, Gas safety for employers. https://www.hse.gov.uk/gas/employers.htm
- Gas Safe Register, Commercial catering (issued 3 February 2021). https://www.gassaferegister.co.uk/media/1447/commercial-catering.pdf
- Construction (Design and Management) Regulations 2015, regulation 2. https://www.legislation.gov.uk/uksi/2015/51/regulation/2
- Construction (Design and Management) Regulations 2015, regulation 8. https://www.legislation.gov.uk/uksi/2015/51/regulation/8
- Building Regulations 2010, Part 2A (dutyholders and competence). https://www.legislation.gov.uk/uksi/2010/2214/part/2A
- Building Regulations etc. (Amendment) (England) Regulations 2023, regulation 1 (commencement). https://www.legislation.gov.uk/uksi/2023/911/regulation/1
- GOV.UK, Qualifications to work with F gas (updated 3 March 2026). https://www.gov.uk/guidance/qualifications-required-to-work-on-equipment-containing-f-gas
- GOV.UK, Company certification to work with F gas. https://www.gov.uk/guidance/certification-for-companies-working-on-equipment-containing-f-gas
- GOV.UK, Employers' liability insurance. https://www.gov.uk/employers-liability-insurance
- HSE, Employers' Liability (Compulsory Insurance) Act 1969: a brief guide for employers, HSE40(rev4). https://www.hse.gov.uk/pubns/hse40.pdf
- CSCS, Frequently asked questions. https://www.cscs.uk.com/faqs/
- CSCS, How can I check a CSCS card? https://www.cscs.uk.com/checkcards/
- HMRC, Construction Industry Scheme: a guide for contractors and subcontractors (CIS 340), paragraph A.16 and Appendix C. https://www.gov.uk/government/publications/construction-industry-scheme-cis-340/construction-industry-scheme-a-guide-for-contractors-and-subcontractors-cis-340
- GOV.UK, What you must do as a CIS contractor: verify subcontractors. https://www.gov.uk/what-you-must-do-as-a-cis-contractor/verify-subcontractors
Last checked: 5 November 2026 against the sources above.
Checked in detail: GSIUR 1998 reg 3, CDM 2015 regs 2 and 8, Building Regulations 2010 reg 3 and Part 2A (regs 11E, 11F) and SI 2023/911 reg 1 on legislation.gov.uk, GOV.UK F-gas qualification and company certification pages, employers' liability insurance page and CIS verification page, HSE40, HMRC CIS 340 para A.16 and Appendix C, CSCS FAQs and card checks, HSE gas employers page and Gas Safe Register catering guidance.
Field Suite is job management software for UK building services firms, not a consultancy. Your own risk assessments, contracts and advisers decide what applies to your work. Spotted a mistake? Email info@fieldsuiteapp.com and we correct it.