---
title: "Legionella and building regulations: design-stage considerations"
source_url: https://legionella.io/articles/legionella-and-building-regulations-design-stage-considerations/
canonical_url: https://legionella.io/articles/legionella-and-building-regulations-design-stage-considerations/
pillar: "UK Legionella Law & Compliance"
summary: "Design-stage choices decide how safe and cheap a water system is to run. See the drawing-board forks that create dead legs, stagnation and warm cold water."
primary_keyword: "Legionella building regulations"
date_published: 2025-11-01
date_reviewed: 2026-06-26
author: "Legionella.io editorial team (REMOTE TECH LTD)"
reviewed_against: "HSE L8 and HSG274 guidance"
region: "United Kingdom"
license: "(c) REMOTE TECH LTD. Quote freely with attribution and a link to source_url."
---

# Legionella and building regulations: design-stage considerations

The cheapest Legionella control measure is the one you never have to carry out, because the building was drawn so the risk was never there. A redundant branch capped at the main on the drawing never needs flushing for thirty years. A cold tank sized to its real demand never warms up in a hot plant room. By the time anyone is responsible for the running building, those decisions are set in the slab, and undoing them costs many times what getting them right on paper would have.

That is why the design stage matters to people who never normally open a drawing. If you will one day be the duty holder, the responsible person, or the estates lead signing off the water hygiene file, the most valuable thing you can do for that file happens before the first length of pipe is fitted.

## The decision hiding in the drawings

Every line on a services drawing is a quiet trade-off between what is cheap or convenient to install now and what is cheap or safe to run later. Designers are judged on the first number; duty holders live with the second. The two rarely sit in the same room.

UK Legionella duties do not switch on at handover. They apply for the operational life of the system, and HSE frames the core duty as identifying the risk, managing it, and keeping the evidence that you have [1]. HSG274 goes a step further and treats the design and layout of hot and cold water systems as part of control, not a separate engineering matter [2]. So a layout that bakes in stagnation is not just a design preference. It hands the future duty holder a problem that no amount of monitoring will fully solve.

## What separates a good water design from a bad one

Three questions sort the genuinely good choices from the merely cheap ones:

- How much residual risk does the choice create? Does it tend to leave water warm, still, or both?
- How easy will it be to prove control? Can the system be monitored, cleaned, drained and sampled without acrobatics?
- How much ongoing effort does it commit you to? Every week, for the life of the building.

Held against the recurring decisions below, the convenient option usually stops looking convenient.

## The design forks that lock in your risk

These are the choices where the drawing-stage call echoes for decades. The right-hand column is the part finance never sees at tender, and the part estates pays for every week thereafter.

| Design fork | The lower-risk call | What the convenient choice costs you later |
| --- | --- | --- |
| Cold water storage sizing | Size storage to genuine demand, or go mains-fed where pressure and demand allow | An oversized tank turns over slowly, the water warms and stagnates, and you inherit a permanent tank-cleaning and monitoring burden |
| Cold pipe routing | Run and insulate cold pipes away from heat sources, out of warm risers and ceiling voids shared with hot services | Cold runs warmed by adjacent hot pipes or plant drift into the growth range; flushing cannot fix a pipe that is warm by design |
| Hot water circulation | Design the return so hot water arrives near temperature at the furthest point, with branches kept short | Long uncirculated branches cool to lukewarm between uses, and the far outlets become the ones that fail your temperature checks |
| Dead legs and "future" spurs | Cap redundant branches close to the main; resist capped spurs left for possible expansion | Every blind end is a reservoir that must be flushed forever, and the spur saved "for phase two" becomes a decade-long stagnation point |
| Access for cleaning and sampling | Build in reachable tank covers, drain points, sampling points and TMV access | Sealed-in plant means cleaning and verification quietly get skipped, and your evidence of control goes missing with it |

None of these are exotic. Most failures I have seen trace back to one of these five being settled for first cost alone.

## When the convenient option is genuinely the right one

This is not an argument for picking the cautious option every time. A point-of-use water heater at a single remote handwash basin can beat dragging a long hot circulation loop across a building to one tap, because it removes metres of warm standing pipe rather than adding them. Mains-fed cold with no storage removes a stored-water risk entirely where the supply can carry the peak demand, though plenty of buildings genuinely need storage for resilience.

The design-stage skill is not avoiding risk. It is knowing which risk you are choosing and making sure someone will own it. A risk assessment that starts at the design stage, as the BS 8580-1 code of practice envisages, is where that ownership gets written down rather than discovered after occupation [3].

## The gap nobody draws: handover to full occupancy

Here is the failure mode no approved construction document covers. A water system is installed, filled, chlorinated and pressure-tested at practical completion, and then the building sits empty or part-occupied for weeks or months while fit-out finishes and tenants trickle in. The water that was commissioned clean goes still and warm. A new building is not low-risk by virtue of being new; an unused system is an unused system whatever its age, and stagnation is the condition that lets the bacteria multiply [4].

Design and programme for that gap deliberately. Set a written flushing regime for the void period, name a clear owner of the water system before the duty holder formally takes over, and plan to bring the whole system back to temperature before anyone uses it. The water safety plan approach in BS 8680 is built for exactly this kind of through-life thinking, carrying responsibility across the seam between construction and occupation [5]. The same neglect that affects a finished building applies double to one that has never been properly used, as covered in [Neglected water systems](https://legionella.io/articles/neglected-water-systems-the-danger-of-stagnation/).

## Where building regulations stop and your duty begins

It pays to be blunt about the limits. Building regulations and their approved documents govern how a system must be constructed. They are baseline construction standards, not a Legionella control plan, and meeting them does not mean the system is being controlled. Reviewing a drawing for water hygiene is not a risk assessment either. The operational duty under health and safety law sits with the duty holder once the building is in use, and it is discharged through a competent, site-specific assessment that reflects the system as actually built and used, not the design intent on file [6]. Treat everything here as general guidance to take to your designer and your risk assessor, not a replacement for either.

If a scheme is on your desk now, the single most useful next step is to ask for the hot and cold water services drawings and walk them against the five forks above with your risk assessor, before practical completion, while a change still costs a redraw rather than a rebuild. The same design lens, applied through HSG274, is set out in [HSG274 guidance explained](https://legionella.io/articles/hsg274-guidance-explained-practical-legionella-control/).

## FAQ

### Do building regulations actually require Legionella controls to be designed in?
Building regulations govern how a system is constructed, not how it is operated against Legionella. The enforceable Legionella duty comes from health and safety law and applies once the building is in use, with HSE setting out what the duty holder must do [1]. Good design supports that duty and makes it cheaper to discharge; it does not replace it.

### We inherited a building we did not design. Is any of this still relevant?
Yes, because the design constraints are now yours to manage. Reading the as-built system through a Legionella lens shows where the unavoidable flushing, monitoring and cleaning will fall, and where a modest retrofit, such as capping a dead leg or re-routing and insulating a warm cold run, would cut that burden permanently rather than week by week.

### Who holds the Legionella risk between practical completion and occupation?
Someone must, and it should be agreed in writing before handover. A filled, idle system needs managing from the day it is commissioned, not from the day the first tenant arrives. Settle the owner, the flushing regime and the records in advance so the void period does not become a blind spot in the chain of control.

## Related reading

- [HSG274 guidance explained: practical Legionella control](https://legionella.io/articles/hsg274-guidance-explained-practical-legionella-control/)
- [Neglected water systems: the danger of stagnation](https://legionella.io/articles/neglected-water-systems-the-danger-of-stagnation/)
- [UK water temperature guidance for Legionella control](https://legionella.io/articles/uk-water-temperature-guidance-for-legionella-control/)
- [Managing Legionella risk across multiple sites](https://legionella.io/articles/managing-legionella-risk-across-multiple-sites/)

## Sources

[1] HSE, "Legionnaires' disease - what you must do". https://www.hse.gov.uk/legionnaires/what-you-must-do/index.htm
[2] HSE, "Legionnaires' disease: Technical guidance (HSG274)". https://www.hse.gov.uk/pubns/books/hsg274.htm
[3] BSI, "BS 8580-1:2019 - Risk assessments for Legionella control. Code of practice". https://knowledge.bsigroup.com/products/water-quality-risk-assessments-for-legionella-control-code-of-practice-1
[4] HSE, "Systems most likely to create legionella risk". https://www.hse.gov.uk/legionnaires/risk-systems.htm
[5] BSI, "BS 8680:2020 - Water quality. Water safety plans. Code of practice". https://knowledge.bsigroup.com/products/water-quality-water-safety-plans-code-of-practice
[6] 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