Water sampling
Legionella testing: when you need it and how it works
Testing means taking water samples and having a laboratory look for legionella bacteria. HSE says it is not usually required for domestic hot and cold water systems. It is used where temperature control cannot be relied on, in higher-risk buildings, and when something has gone wrong.
Last reviewed October 2026 against HSE guidance.
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Testing is not the same as a risk assessment
| Risk assessment | Temperature monitoring | Water testing | |
|---|---|---|---|
| What it is | An inspection of the system and how it is used | Measuring hot and cold water temperatures at outlets | Laboratory analysis of water samples for legionella |
| Legal position | A legal duty for employers, those in control of premises and landlords | The usual control for hot and cold water systems | Not usually required for domestic systems |
| Tells you | Where the risks are and what to do | Whether the system is under control | Whether bacteria were found in that sample on that day |
HSE's guidance says testing “should not be confused with temperature monitoring, which is a reliable method for confirming the water system is under control”.
When testing is worth doing
HSE says sampling is only needed in very specific circumstances and points to Part 2 of its technical guidance, HSG274. In practice, testing is considered where:
- hot and cold water temperatures cannot be kept within the control limits
- the system is treated with a biocide instead of relying on temperature
- the building has people at higher risk, such as a care home or healthcare setting
- there is a suspected or confirmed case of Legionnaires' disease linked to the building
- you want to confirm that cleaning or disinfection has worked
- a property has been empty for a long time and you want reassurance before it is used again
A negative result does not prove a system is safe. It describes one sample on one day. Good temperatures, regular use and a clean system are what keep the risk low.
How a legionella test works
- Sampling. Water is collected in a sterile bottle from chosen outlets, often the nearest and furthest from the tank or cylinder, and from the tank or cylinder itself.
- Transport. The sample goes to the laboratory quickly, kept cool and away from light. Most laboratories ask for samples early in the week so they are not held over a weekend.
- Culture. The standard method grows any legionella present on a culture plate. This takes about ten days, which is why results are not instant.
- Report. The laboratory reports the count in colony forming units per litre (cfu/l) with a certificate of analysis.
You can have samples taken by a water hygiene company, or buy a postal kit and take them yourself. Look for a laboratory that is UKAS accredited for legionella analysis. Our sister site compares UK legionella test kits side by side.
What testing costs
| Option | Typical published price |
|---|---|
| Postal kit with laboratory analysis | About £40 to £85 per sample, with discounts for several kits |
| Professional sampling visit | Quoted per site. Depends on the number of samples and travel. |
Prices were gathered from UK sellers in October 2026 and many are quoted before VAT.
If legionella is found
Do not panic, and do not ignore it. Low counts are not unusual and the response depends on the level, the type of system and who uses the building. HSG274 Part 2 sets out action levels and what to do at each.
- Review the risk assessment and the control measures straight away.
- Check stored and outlet temperatures against the control limits.
- Take advice from a competent water hygiene specialist on resampling, cleaning or disinfection.
- Reduce exposure to spray from the affected outlets until the system is back under control.
Questions people ask
Is legionella testing a legal requirement?
Not for most domestic systems. HSE says testing or sampling is not usually required for domestic hot and cold water systems. For other systems, whether to sample depends on your risk assessment and on HSE's technical guidance.
Do landlords have to test for legionella?
Not usually. Landlords must assess and control the risk. HSE says testing is only needed in very specific circumstances and that temperature monitoring is a reliable way to confirm control.
How long do legionella test results take?
The standard laboratory culture method takes about ten days from when the sample reaches the laboratory. Rapid tests exist, but a laboratory culture is the method most guidance refers to.
How often should water be tested for legionella?
There is no general interval for hot and cold water systems. The frequency should come from the risk assessment. Systems that cannot be controlled by temperature, and higher-risk buildings, are sampled more often.
Can I test for legionella myself?
Yes. Postal kits include a sterile bottle, instructions and laboratory analysis. Follow the sampling instructions exactly and post early in the week.
Sources
- HSE: Legionella and landlords' responsibilities
- HSE: HSG274 Legionnaires' disease, technical guidance (Part 2 covers hot and cold water)
- HSE: Hot and cold water systems
- HSE: Hot and cold water systems, things to consider
Checked against these sources in October 2026. This page is general information, not legal advice.
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