The current standard for domestic effluent treatment and disposal from single dwelling houses is set out in Recommendation SR6:1991, which was drawn up by the National Standards Authority of Ireland and which was issued by the Department by way of circular letter to each planning authority on 8 January 1992.
Recommendation SR6 indicates that, in general, site sizes for the proposed dwelling should be such that the septic tank effluent can be disposed of within the boundary of the site, without recourse to disposal in adjoining land, even in the event of failure of both the main and reserve percolation areas. Where a site has no mains supply, SR6 indicates that the basis for determining the site size must be the nature of the soil and subsoil and the depth to water table and-or fissure rock. Where groundwater is afforded good natural protection, for example by a thick clay-rich soil and subsoil, the recommendation is that that the site be no less than 2,000 square metres, approximately half an acre. Where the natural protection is limited, for example gravel rich soil and subsoil, the site should be no less than 3000 square metres. SR6 indicates that the overall density of septic tank systems in a locality may also need to be limited if there is likely to be a build up of contaminants, such as nitrates in the water supply. Sites with mains water supply, i.e. which do not depend on a groundwater supply within the curtilage may be slightly smaller. The minimum site size recommended for sites with a mains water supply is 1,500 square metres.
The Environmental Protection Agency, EPA has published a draft manual on treatment systems for single houses in 2000 which was designed to help planning authorities, builders and others to deal with the complexities of on-site systems, including packaged systems. Instead of recommending a minimum site size, the draft EPA manual makes recommendations about matters such as separation distances from water courses and wells, site boundaries etc.