UndergroundWater Tanks

This varies by tank model and manufacturer, so always check the specific installation guide before digging. Most polyethylene tanks have a maximum cover depth stated (often around 600mm-1m of soil above the tank), and exceeding this can risk structural damage. If you need greater burial depth, look at models specifically designed and reinforced for deeper installation, or discuss ground conditions with the manufacturer's technical team.

Most underground water tanks for domestic rainwater harvesting or non-potable use don't require planning permission, but this can depend on your local authority and the size/location of the installation. If the tank is feeding into a potable water system, or forms part of a drainage or sewage arrangement, building regulations are likely to apply. It's always worth checking with your local building control department before installation, particularly for larger commercial systems.

A high water table can cause an empty or partially empty tank to float or shift within the ground, which can damage inlet/outlet connections and pipework. In these conditions, anti-flotation measures are essential — typically a concrete anchor slab or ballast collar, sized according to the tank manufacturer's guidance. Always get a groundwater survey done beforehand if you're unsure, especially on clay or low-lying sites.

Installation typically involves excavating a hole larger than the tank to allow for a bedding layer (usually pea gravel or sharp sand), lowering the tank in, then backfilling in stages while filling the tank with water simultaneously to equalise pressure. This staged backfill-and-fill approach prevents the tank walls from distorting or collapsing under the weight of the surrounding soil. Full manufacturer installation instructions should always be followed, as procedures vary between GRP and polyethylene tanks.

Capacity depends on what the water is being used for — a domestic garden irrigation system might only need a few thousand litres, while a household greywater or whole-house rainwater harvesting setup typically needs upwards of 5,000 litres depending on roof catchment area and household size. As a rough guide, consider average daily consumption, expected rainfall, and how many dry days of storage you want as a buffer. Our Guide to Underground Water Tanks has more detailed sizing calculations if you're unsure.

Standard underground tanks are generally not designed to take vehicle loading directly above them, as the structure and covers aren't rated for that weight. If you need a tank under a trafficked area, you'll need a model specifically designed for traffic loading, along with a reinforced, load-bearing cover or manhole to distribute the weight away from the tank itself. Always confirm load ratings with us before installing in driveways, car parks or access roads.