Interception made practical: using rainwater harvesting to help meet the “first 5mm” expectation

Standard 2 of the National SuDS Standards focuses on everyday rainfall and interception. The headline is simple: apply a SuDS approach so that at least the first 5mm of rainfall for the majority of rainfall events does not result in runoff from the site to surface waters or piped drainage systems.

There are different ways to achieve interception, but rainwater harvesting is specifically called out. The standards state that surfaces drained to rainwater harvesting systems designed to BS EN 16941 can be assumed to comply with interception requirements, provided the system design is based on regular daily demand for non-potable water from surface water runoff.

That last part is key for developers: it’s not just about having a tank; it’s about having a realistic, regular demand that creates space in storage. That’s why “water butt for garden use” is explicitly not treated the same way in the standards, because it doesn’t guarantee storage will be available unless designed as such.

So the practical developer takeaway is this: if you want rainwater harvesting to do real work in your SuDS strategy, connect it to a predictable demand profile and evidence it with appropriate calculations.