The cost of a septic tank installation depends on the tank, the size of the percolation area required by the site's percolation rate, the amount of excavation and imported material needed, and the site characterisation report. Grant assistance is available in specific circumstances for remediating existing systems.
The tank itself is often the smaller part of the cost. The percolation area — its size, the volume of excavation, and the imported sand or gravel where a raised or filter bed is needed — usually drives the total. Sites with slow percolation need larger areas and sometimes engineered solutions.
Grant schemes exist for the repair, upgrading or replacement of existing domestic wastewater treatment systems in defined circumstances, generally linked to inspection under the National Inspection Plan or to particular catchment areas. Eligibility rules change, so check the current terms with your local authority.