What counts as gas work?
Gas work covers installing, servicing, repairing, or disconnecting anything connected to the gas supply - boilers, gas fires, gas cookers, gas pipework and flues - as opposed to plumbing that only involves water.
The split isn't always obvious from a homeowner's side, because the same visible object - a boiler, say - has both a water side and a gas side. Bleeding a radiator, fixing a dripping tap, replacing a section of water pipe, or fitting a new bathroom suite are water-only jobs with no gas-registration requirement. But anything involving the gas supply itself - connecting or disconnecting a boiler's gas feed, servicing a gas fire, fitting a new gas cooker point, working on gas pipework, or assessing a flue - is legally gas work, full stop, regardless of who's doing it or how minor the job looks. If a job touches the gas side of an appliance at any point, it needs a Gas Safe registered engineer for that part of the work, even if the rest of the job is routine plumbing.
What's the actual legal requirement?
By law, gas work can only be carried out by an engineer who is on the Gas Safe Register - it's a criminal offence for anyone else to do it, and for a business to advertise itself as able to.
The Gas Safe Register is the official list of engineers and businesses legally permitted to work on gas appliances and installations in the UK, overseen by the Health and Safety Executive. It replaced the old CORGI scheme on 1 April 2009 in Great Britain and the Isle of Man (1 April 2010 in Northern Ireland and Guernsey). Registration isn't a single blanket qualification - each engineer is registered for specific categories of gas work (for example, boilers, gas fires, or cookers), so being Gas Safe registered for one category doesn't automatically cover another. This is why checking an engineer's registration for the specific job, not just confirming they're "on the register" in general terms, is worth doing before work starts.
How do I check an engineer is actually registered?
Every Gas Safe registered engineer carries an ID card showing their licence number and the specific categories of gas work they're qualified for - checkable against the official Gas Safe Register.
The ID card is the quickest check on-site: it shows a photo, a licence number, and the specific categories the engineer is registered for. The same details can be checked in advance against the official Gas Safe Register before booking any gas work. For a landlord specifically, keeping a copy of the engineer's ID (or at least their licence number) alongside the gas safety record is good practice, since it ties the safety check to a specific, verifiable, registered individual rather than just a business name.
What if I use someone who isn't registered?
Unregistered gas work is illegal for the person doing it, and it also invalidates the safety oversight the registration system exists to provide - insurers and landlord gas safety law both assume registered work.
Beyond the legal exposure for whoever carried out the work, unregistered gas work creates a real safety gap: the Gas Safe Register exists specifically because incorrectly installed or maintained gas appliances are a genuine carbon monoxide and fire risk, and registered engineers are inspected against that standard. For landlords, a gas safety check carried out by someone not on the register doesn't satisfy the legal safety-check duty at all - see our landlord gas safety guide for what that duty actually requires.
