CDM 2015 makes the people who commission and design a building responsible for the foreseeable health risks built into it, and Legionella is one of those risks. The duty to eliminate or reduce it begins at the drawing board, not on the day the first tap runs.

That idea catches a lot of project teams off guard. CDM is usually framed around falls, dust, plant and structural safety, the visible dangers on site. But the regulations are deliberately broad: they cover risks to anyone who builds, uses, maintains or eventually demolishes the structure. A water system that grows Legionella because it was designed with long dead legs, oversized pipework and a calorifier crammed next to a cold-water riser is exactly the kind of foreseeable, designed-in risk the principles of prevention expect you to tackle first by design.

Where water control sits inside CDM

CDM 2015 does not name Legionella, set temperatures, or tell you how to size a tank. It is not in the same family of documents as L8 or HSG274, and it carries no Legionella clause numbers to quote. What it does is impose a hierarchy: eliminate the risk where you reasonably can, reduce what you cannot eliminate, and properly inform whoever inherits the residual risk.

The Legionella technical detail comes from elsewhere. The Approved Code of Practice L8 and the technical guidance in HSG274 are where good water-system design is actually defined [1][2]. CDM is the legal wrapper that says the design team must apply that good practice, document the decisions, and hand the operator a system they can keep safe. Building Regulations sit alongside this for the fabric and services standard, covered separately in Legionella and building regulations: design-stage considerations.

So the real question is not “does CDM mention Legionella”. It is “have we designed out the water risk we can foresee, and recorded what is left?”

What “designing out” actually looks like

Design-out is concrete, and most of it maps straight to HSG274 and L8 guidance on keeping water moving, hot water hot and cold water cold [2][3].

  • Designing out dead legs and blind ends. Size and route pipework so every outlet sees regular flow. Avoid capped spurs left “for future flexibility”, redundant branches, and oversized headers that hold standing water. Dead legs and blind ends: how to find, assess and remove them covers the operational side; the cheapest time to remove one is before it is installed.
  • Pipe and storage sizing to real demand. Oversized pipes and tanks lower turnover and let water sit. Size cold-water storage and distribution to genuine occupancy, not to a generous margin that quietly becomes a stagnation reservoir.
  • Keeping cold cold and hot hot. Route cold-water pipes away from heat sources and uninsulated hot runs; locate and insulate the calorifier so stored hot water reaches outlets at the right temperature. Separation, lagging and plant location are design decisions, not maintenance fixes [3].
  • Materials and fittings. Specify fittings that do not shed nutrients or harbour biofilm, and avoid components the guidance flags as supporting microbial growth.
  • Outlets that get used. Fewer, better-placed outlets beat a tap in every corner that nobody turns on and that becomes a little-used risk on day one.

These are the levers a good Design-stage Legionella risk assessment for new buildings is meant to pull, and they sit behind Future-proof design: plumbing designs that minimise Legionella.

Who holds which duty

CDM spreads the load across three roles, and the Legionella thread runs through all of them.

The client sets the tone. As the organisation commissioning the work, the construction client’s water-safety duty is to make suitable arrangements and to state, up front, that designing out Legionella risk and handing over a clean, commissioned, fully documented water system are project requirements, not extras to be value-engineered away later.

The principal designer plans, manages and coordinates health and safety in the design phase. In practice the principal designer’s Legionella responsibility is to make sure water risk is actually considered by someone competent to consider it, that clashing design decisions get reconciled, and that the residual risk is recorded and passed on.

Designers, usually the M&E or building-services designer, carry the technical work: applying the L8 and HSG274 principles, eliminating the risks they reasonably can, and flagging what they cannot.

The contractor and principal contractor build it as designed and, critically, manage the system through construction and commissioning, when it is most exposed.

What the design team rarely gets told

Here is the part that gets missed even on well-run jobs.

The single biggest Legionella risk in a new building is usually created during construction, not after handover. Systems are pressure-tested and chlorinated, then left water-filled and motionless for weeks or months while the rest of the build finishes. A system disinfected in spring and first used in autumn can be a worse risk on day one than one in regular use. The “foreseeable risk” language in CDM covers exactly this construction-phase stagnation, and it is the contractor’s flushing and commissioning regime, not the future operator’s, that decides the outcome. That is why Commissioning new water systems: preventing Legionella from day one belongs in the contract, not the snagging list.

Two more things nobody warns you about. First, the principal designer is rarely a water-hygiene specialist, so the design-stage Legionella duty falls through the gap precisely because each party assumes another owns it. Name the owner explicitly. Second, value engineering late in the programme quietly reintroduces the very faults you designed out: a deleted outlet leaves a blind end, a “future-proofing” spur becomes a permanent dead leg.

Then there is the deliverable that ties it together: the as-built schematic and operating information that go into the health and safety file. The operational duty holder’s risk assessment is only as good as the drawings they inherit. Hand over a system with no accurate schematic and you have passed on a building nobody can properly assess.

A genuine caveat

This is a general explanation of how design duties and Legionella control intersect, not legal or design advice. The line between what is reasonably practicable to eliminate and what is acceptable residual risk is settled project by project, by competent people, against a site-specific assessment. Use this as the prompt for the conversation to have with your design team, not a replacement for it.

FAQ

Does CDM 2015 specifically require Legionella control?

Not by name. CDM imposes general duties to eliminate and reduce foreseeable health and safety risks and to inform whoever inherits them. Because Legionella is foreseeable in any water system, it is caught, but the technical “how” comes from L8 and HSG274 [1][2].

Who is responsible for designing out Legionella on a project?

It is shared. The client makes it a requirement, designers eliminate and reduce the risk technically, the principal designer coordinates and records it, and the contractor builds and commissions accordingly. At handover, the operational duty holder takes it on.

HSE guidance expects water risk to be considered as the system is designed, and a design-stage assessment is how that gets evidenced. See Design-stage Legionella risk assessment for new buildings.

Your next step

Before the next design review, put one question in writing: who on this project owns designing out Legionella, and where is it recorded? If no one can name the owner, you have found the gap CDM expects you to close, and the cheapest day to close it is today, while the pipework is still lines on a drawing.

Sources

[1] HSE, “Legionnaires’ disease. The control of legionella bacteria in water systems - Approved Code of Practice and guidance (L8)”. https://www.hse.gov.uk/pubns/books/l8.htm [2] HSE, “Legionnaires’ disease: Technical guidance (HSG274)”. https://www.hse.gov.uk/pubns/books/hsg274.htm [3] HSE, “Hot and cold water systems”. https://www.hse.gov.uk/legionnaires/hot-and-cold.htm