A domestic rainwater harvesting tank is generally exempted development if it is ancillary to an existing house and meets the size and siting limits for domestic structures under the Planning and Development Regulations, but larger, non-domestic, or prominently sited tanks can require permission. Check exemption limits and any conditions attached to your house's existing permission before installing one.
Rainwater harvesting systems collect roof runoff, usually into an underground or above-ground tank, for uses such as garden watering, toilet flushing or washing machine supply, and are increasingly used to reduce mains water demand and manage surface water on a site. For a single domestic house, a rainwater harvesting tank is generally treated as an ancillary structure to the dwelling and can fall within the exemptions for domestic structures set out in Schedule 2 of the Planning and Development Regulations 2001 (as amended), provided it stays within the applicable size, height and siting limits for such structures and does not breach any conditions attached to the house's own planning permission.
Whether a specific installation qualifies depends on factors such as the tank's size and whether it is below or above ground, its position relative to the house and boundaries, and whether the overall coverage of ancillary structures on the site (combined with sheds, garages and other exempt structures) stays within permitted limits. Larger tanks intended to serve multiple dwellings, commercial or agricultural use, or tanks installed as part of a wider development requiring permission, would typically need to be included in, or separately assessed as part of, a planning application.
Because exemptions can be affected by protected structure status, conservation area designations, or specific conditions on an existing permission (for example, a condition restricting further ancillary structures), it's worth checking with the local authority's planning section, particularly for larger systems, non-standard designs, or a site with an unusually restrictive existing permission.