How to build a small rain garden

A rain garden is a shallow depression filled with free-draining media and planted with species that tolerate temporary flooding. Water ponds briefly on the surface, then filters down instead of running off.

Melbourne Water, which has installed these at city scale, gives the most usable sizing rule: rain gardens range from about 1 m² to over 100 m² and are typically sized at 1–2% of the catchment area feeding them, with 200–500 mm of ponding depth above the filter surface and a free-draining filter medium such as a loamy sand.

CIRIA's susdrain platform rates rain-garden performance as good for peak flow reduction and medium for volume reduction and water-quality treatment, with good amenity and ecology potential. It also flags the two common failure modes: clogging where the surrounding landscape sheds sediment, and unsuitability on steep slopes.

There is monitoring evidence that they work. At the Renfrew Close rain gardens in London, University of East London researchers recorded basins attenuating all incoming water through infiltration in many events, with an average lag of 13.2 hours between peak rainfall and peak water level in the primary basin.

Before you dig, test your drainage: fill a 30 cm hole with water and time how long it empties. Melbourne Water advises installing only once the upstream catchment is around 95% developed, since construction sediment clogs new filter media quickly.

We do not publish a single minimum distance from buildings — the figure commonly repeated in garden guides is not one we could trace to an engineering standard. Follow CIRIA's SuDS Manual (C753) or your local authority's guidance, and for anything involving structural drainage or flood protection, involve them directly.