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Sizing is based on the number of people using the property, not just bedrooms. As a rule of thumb, a household of up to 4 people needs a minimum 2,700-litre tank, with roughly 180 litres added per additional person per day. Always check the manufacturer's sizing chart, as building control will want to see this matches occupancy on your application.

You generally don't need planning permission for a domestic septic tank or sewage treatment plant, but you do need Building Regulations approval (Part H) for the installation, and you must register the discharge with the Environment Agency if it isn't going into mains sewer. Discharges to a watercourse or ground need a permit or exemption depending on volume — check current Environment Agency general binding rules before you order.

GRP (fibreglass) tanks are rigid, very strong and well suited to larger capacities, deep burial or unusual shapes, but they're heavier and cost more to install. Polyethylene tanks are lighter, cheaper, and quick to handle on site, making them popular for standard septic and water storage applications. Ground conditions, depth of invert and access for delivery all influence which material makes more sense for your job.

Many water storage and rainwater harvesting tanks are designed for above-ground use, typically sited on a level concrete or slab base and often housed in a shed, plant room or bunded enclosure. Septic tanks and sewage treatment plants are almost always buried, since they rely on gravity flow from the property's drainage. Above-ground tanks are generally easier and cheaper to install but need frost protection in exposed locations.

Larger tanks are usually delivered on a flatbed or hiab-equipped lorry, so you'll need clear, firm access for a large vehicle and space to manoeuvre or crane the tank into position. For underground installations, the excavation should ideally be ready before delivery, as tanks are rarely left standing on site for long unsupported. Let us know about narrow lanes, low branches or soft ground in advance so we can flag any access issues before the day.

A septic tank only settles solids and still needs a properly designed drainage field (soakaway) or discharge point afterwards — it doesn't treat the effluent to a dischargeable standard on its own. If your site has poor percolation, a high water table, or you need to discharge to a ditch or watercourse, a sewage treatment plant is usually required, as it produces a cleaner effluent. A percolation test carried out by a groundworker will normally settle which option your ground actually supports.