Most rented homes are, on paper, a low Legionella risk. A combi boiler, mains-fed cold water, no loft tank, a couple of showers used every day — there is little for the bacteria to colonise. The catch is that “low risk” is a conclusion you are meant to reach and record, not one you get to assume. The moment a landlord assumes it, two opposite mistakes show up: paying a cold-caller for a meaningless annual “certificate”, or doing nothing at all because it is “just a flat”.

Both are avoidable, and both are common. The duty on a landlord is modest and mostly common sense, but it is specific, and the ways it goes wrong in residential lettings are predictable. Here are the ones worth designing out.

What a landlord actually has to do

HSE is clear that landlords have a duty to assess and manage the Legionella risk in the water systems of properties they let [1]. For most domestic-scale systems that means a proportionate written risk assessment, keeping water either properly hot or properly cold and not left standing, and reviewing the assessment when something changes [1]. That is the whole picture. Every mistake below is a way of missing it in one direction or the other.

The mistakes that catch landlords out

Buying a “Legionella certificate” nobody asked for

What it looks like: an annual invoice for a “Legionella test” or “compliance certificate”, filed next to the gas safety record as though the two are the same kind of thing.

Why it happens: the gas and electrical certificate habit, plus a steady stream of marketing aimed at landlords and agents who would rather pay than read the guidance.

The fix: there is no statutory Legionella certificate for a standard domestic let, and for most simple systems routine water sampling is not expected at all [1][5]. What the law wants is an assessment that is correct and current. Pay for competent help if your property is genuinely unusual — never for a certificate that proves nothing.

Treating a low-risk flat as a no-duty flat

What it looks like: “One bed, combi boiler, nothing to do here.”

Why it happens: low risk quietly reads as no risk.

The fix: the assessment is still the deliverable, even when the honest answer is “low”. A short record showing you checked — no stored water, no obvious dead legs, regular use, no especially vulnerable occupant — is what demonstrates you discharged the duty. Reach “low risk” without writing down how you got there and you keep the risk while throwing away the evidence. (Worth knowing too how long that record should survive before you bin it.)

Forgetting the gap between tenancies

What it looks like: a property sits empty for weeks between lets, nobody runs the taps, and the new tenant’s first shower throws out a fine spray of water that has been standing warm in the pipes.

Why it happens: voids are admin dead time, and water is invisible.

The fix: flushing standing and low-use outlets is the cheapest control you have. Run every outlet, especially showers, before a new tenant moves in, and during long voids where you can get access. It is the same low-use problem hotels manage at scale — only here it is one property at a time, which makes it easier, not optional.

Applying the simple-house model to an HMO or block

What it looks like: a property with a cold water storage tank, a hot cylinder, communal risers or shared bathrooms, assessed as if it were a combi flat.

Why it happens: one mental model gets copied across an entire portfolio.

The fix: stored and shared water is a different risk class. Tanks, calorifiers, long pipe runs and communal showers are exactly the features HSE highlights as more likely to create foreseeable risk [3]. HMOs and blocks of flats deserve a fuller assessment and, more often than not, a competent person rather than a downloaded template.

Assuming “the agent handles compliance” means the duty has moved

What it looks like: a managing agent says they look after compliance; the landlord keeps no copies and could not say who carried out the assessment, or when.

Why it happens: delegation feels like transfer.

The fix: the duty can be shared, but it has to be pinned down. Agree in writing who assesses, who flushes between tenancies and who reviews — then keep copies yourself [2]. If anything ever goes wrong, “the agent dealt with it” is not an answer; the paperwork is. This is where vague handovers turn into the documentation gaps that sink you.

Running a template that does not match the building

What it looks like: a generic assessment with the address typed in, describing a cold water storage tank in a property that has not had one for years.

Why it happens: it is fast and cheap.

The fix: an assessment that describes the wrong system is worse than none, because it looks like control while documenting fiction. Walk the property. Note the real fittings, the outlets that rarely get used — the spare en-suite, the outside tap, the second bathroom — and whether the tenant is elderly or has a weakened immune system, since those occupants raise the stakes considerably if anything goes wrong [6].

If you fix only one thing

Make the assessment match the actual property, then act on the one residential weak point it almost always exposes: water left standing in low-use outlets and empty homes. A real assessment plus a habit of flushing before re-let will do more for genuine safety than any certificate, and it costs close to nothing.

Where this guidance stops

Treat the above as orientation, not a verdict on your building. A combi flat and a converted house split into six bedsits with a rooftop tank are not the same problem, and controls that are perfectly adequate for the first would be negligent for the second. Temperature targets, whether sampling is ever appropriate, and how often to review all depend on the specific system and the people living in it — settle them through a competent, site-specific assessment rather than a general page like this one.

FAQ

Do I legally need a Legionella certificate to let a property?

No. UK guidance for landlords is built around a proportionate risk assessment and sensible control of the water system, not a certificate or licence. For most simple domestic systems there is no requirement to bring in a contractor for an annual test [1].

Who is responsible — me or the letting agent?

The duty sits with whoever controls the premises, and it can be shared by agreement. If your agent manages the property they may take on part of it, but you should agree in writing exactly who does the assessment, the flushing and the reviews, and keep your own copies [1][2].

Do I have to get the water tested for Legionella?

Usually not. For typical domestic systems, sampling is not part of routine control and is reserved for specific situations your assessment identifies — for example after a suspected case, or in a more complex stored-water system [5]. Controlling temperature and keeping the water moving matters far more than a one-off sample [4].

Do this next

Pick one property and put its current risk assessment next to the actual building. Check three things: does it describe the system that is really there, does it name who flushes the outlets between tenancies, and is there a date for the next review? If any of those is missing, you have found this week’s job — fix the document before you touch anything else.

Sources

[1] HSE, “Legionella and landlords’ responsibilities”. https://www.hse.gov.uk/legionnaires/legionella-landlords-responsibilities.htm [2] HSE, “Legionnaires’ disease - what you must do”. https://www.hse.gov.uk/legionnaires/what-you-must-do/index.htm [3] HSE, “Systems most likely to create legionella risk”. https://www.hse.gov.uk/legionnaires/risk-systems.htm [4] HSE, “Hot and cold water systems”. https://www.hse.gov.uk/legionnaires/hot-and-cold.htm [5] HSE, “Testing and monitoring your water system for legionella”. https://www.hse.gov.uk/legionnaires/testing-monitoring-water-system.htm [6] NHS, “Legionnaires’ disease”. https://www.nhs.uk/conditions/legionnaires-disease/