Understanding the CIRIA C790 Code of Practice

When it comes to managing flood risk at property level, the CIRIA C790 Code of Practice (CoP) underpins almost everything we do. It’s voluntary, but it has set the benchmark for how Property Flood Resilience (PFR) should be delivered.

The trouble is, most homeowners haven’t heard of it, and when they do encounter it, it can feel confusing. Why so many stages? Why all the surveys? Why the costs up front?

This blog is my attempt to explain what the Code means in practise, with some real-life examples of where it works and why following the process matters.

Why We Needed a Code of Practice

Before the CoP, too many measures were installed in haste. I’ve seen flood walls collapse under water load, barriers outflanked by drainage beneath them, or worse still, works that pushed flooding onto neighbouring homes. In some cases, I’ve even acted as an expert witness when regulatory action was taken because permits or consents weren’t sought and the flood risk was worsened for neighbouring properties.

So, whilst I understand there can be frustration around “front-loaded” cost with surveys and design, the Code of Practice exists for a reason: to avoid poor outcomes and make sure investment in resilience measures will bring the best long-term value, success and risk minimisation, whilst not exacerbating risk for others.

The Six Stages of the CIRIA C790 Code of Practice

1. Risk Assessment
2. Survey
3. Options Development & Design
4. Construction
5. Commissioning & Handover
6. Operation & Maintenance

It may feel like additional regulation, or red tape, but each stage exists because of problems that occurred when they were skipped.

Why Skipping Stages Doesn’t Work (the tea test)

A good way to think about this is as simple as making a cup of tea.

1. Fill the kettle.
2. Boil the water.
3. Put a teabag in the mug.
4. Pour the water in.
5. Let it brew and remove the bag.
6. Add milk (or sugar, if that’s your thing).

Take one stage out, and what happens? Forget the teabag, you’ve got hot water. Skip boiling, you’ve got a cold cup of tea. Miss the milk, you’ve got a different drink altogether.

It’s the same with flood resilience. Each stage might feel small or frustrating, and you could save time by skipping it, but together they work to achieve the correct outcome. Skip one, and the result is rarely what you hoped for.

This is exactly what happens when people skip straight to Stage 4 and ask a builder to “just build a flood wall.” The builder reassures them it’s “as strong as an ox”, and it looks great, until the next flood comes. Then they wonder why it’s still flooding and realise the drainage from next door flows straight into the site, bypassing the wall completely, and the house floods anyway.

That’s why the stages matter, and why I’ve put together a short video on this very point when building a flood wall: Watch on Youtube.

Real-World Lessons from Stages 1 & 2

Stages 1 & 2: Risk Assessment and Survey

These are often bundled together as a “Flood Survey.” But they’re not a fix in themselves, and that’s where many misconceptions start.

I saw this first-hand during the 2020 Flood Grants. The communication around the grants led homeowners to believe there was a £5,000 solution waiting for them. In reality, it was a contribution towards assessing risk and considering management options. For some smaller homes, it went a reasonable way, barriers, repointing, a non-return valve, a pump. But for large six-bedroom farmhouses, it didn’t stretch far at all.

I also remember a DEFRA Grant case where a homeowner wanted me to approve digging a ditch across a farmer’s field. Creative, yes, but completely beyond scope, and it would have needed complex modelling to prove any benefit. And certainly, we’d need to evidence this to approve the spend of taxpayers’ money.

Stages 1 & 2 are about clarity: what’s the risk, what’s the property like, and what’s realistic. If flooding depths are significant, for example, it may be better to focus on recoverability, making sure the property bounces back quickly, rather than trying to keep water out, which can cause structural problems.

It has been known, where recoverability is the best option, that frustrations arise. Homeowners can often feel frustrated because the water can’t be stopped at the door. But this isn’t a failure of the process, it’s actually a positive outcome. Recognising when exclusion isn’t safe or feasible means the focus shifts to measures that make recovery faster, safer, and ultimately more cost-effective.

Sometimes the best flood defence isn’t a barrier or a wall at all, it’s being properly informed of the risk. With that understanding, homeowners can make better decisions about insurance, maintenance, and the right resilience measures for their property, alongside the long-term goals and future for the property. In more extreme cases, resilience can even mean rebuilding with raised floor levels or designing a ground floor to be sacrificial. I have surveyed properties along the Thames where this approach has been adopted.

Stage 3: Options Development & Design

This is where frustration often builds. Homeowners want action. One told me: “I don’t want any of this, I want people with shovels digging trenches and building walls.” I completely understand that sentiment. But without design work, walls fail, drains get missed, and neighbours suffer knock-on effects.

What Stage 3 Can Involve

• CCTV drainage surveys.
• Drainage authority searches (CON29DW).
• Topographical surveys.
• Floodplain displacement calculations.
• Structural and foundation design.
• Planning permissions or Environmental Permits.
• Material selection and product specification.
• Pump selection, friction loss calculations and even new drainage.
• Modelling or drainage calculations.
• Input from Ecologists, Quantity Surveyors, Structural Engineers.
• Design Risk Assessments.

That list sounds costly, but it’s what ensures positive outcomes.

A Nottinghamshire project sticks in my mind. The client was unhappy with a previous company’s flood survey and often lived abroad, meaning deploying PFR would be difficult. On top of that, land levels had changed, altering risk. Instead of rushing to install measures, I undertook both the Risk Assessment and Survey (known as a Flood Survey) and explored design options having discussed with the client their goals, and budget. With MicroDrainage modelling, we showed a new drainage system could reduce the risk to a 1:100 + climate change storm event, meaning the property was no longer expected to flood in most events.

Yes, that meant extra design fees, but the result was far stronger than a quick fix. You can read the full case study here: PFR Survey and Drainage Design for Nottinghamshire Property.

Sometimes Stage 3 is simple: picking the right flood door, barrier, or non-return valve. But even then, Design Risk Assessments ensure solutions can be stored, maintained, and accessed long-term.

From Construction to Maintenance: Stages 4, 5 & 6

Stages 4 & 5: Construction, Commissioning & Handover

Once designs are in place, construction follows. This isn’t just about building but doing so safely and properly, with CDM regulations, structural checks, and permissions where necessary.

Commissioning and handover are equally important. Measures should be tested, and homeowners need clear instructions: manuals, training, sometimes even videos. A flood barrier is only useful if it can be deployed correctly at the right moment. For commercial properties these can be included in the H&S file and stored suitably.

Stage 6: Operation & Maintenance

Flood resilience isn’t “fit and forget.” Seals need inspecting, pumps need running, walls may need repointing. A little maintenance goes a long way and neglecting it can undo all the investment made earlier.

Utilising the CIRIA C790 Code of Practice

At FPS Environmental Ltd, we follow the CIRIA C790 CoP because it protects homeowners, their neighbours, and ultimately the investment being made. We work on stages 1, 2 and 3a (which includes the options development but not full design) under our Environment Agency frameworks, and we can collaborate with our partners on the structural design or construction stages. We also can complete the full stage 3 section on direct instructions.

The key point I want to emphasise is this: while it might be tempting to jump straight to building, the six stages exist for good reason. Even the most detailed and thorough Flood Survey will not give you a fully designed flood wall with permits and storage calculations, but they are the right starting point. Following the process gives far better outcomes than skipping ahead.

In the coming weeks I’ll be creating a video on this topic, but in the meantime, if you’d like to discuss how the Code of Practice applies to your property, please get in touch.