Site suitability assessment

  • What information may a City or County Council request in relation to the suitability of a site?

    Under Article 22 (2)(c) of the Planning and Development Regulations 2006, the applicant must submit information on the type of septic tank system proposed and evidence of suitability of the site for the system in the planning application. Contact your City or County Council if further clarification is required. 

  • Who can carry out site suitability assessments and install septic tank systems?

    The Code of Practice and Building Regulations guidance require that site assessment and the design, installation and commissioning of septic tank systems should be carried out and/or supervised by a suitably qualified person or persons. Specific qualifying criteria are not specified and there are no registration requirements, so it is not possible to provide a list.

  • How can I apply for the site suitability assessment training course?

    This course is run by the Water Services Training Group which is part of the Local Authority Services National Training Group.

  • How far must the septic tank system be from wells, rivers, neighbours etc.

    These requirements are set out in tables  6.2 and E2 in the Code of Practice.

    For septic tank systems and wells, the minimum requirement varies from 15-60m depending on whether the well is up, across or downgradient, the percolation value, soil type and depth.

  • What can I do if I have a high water table?

    The water table is the level of groundwater in the soil. Much of the treatment of the liquid effluent from septic tank systems takes place naturally in the soil, so the soil needs to be deep enough for this to happen before the liquid gets to the groundwater. A ‘high water table’ is one that is near the surface. This is checked during site suitability assessment by digging a hole with a digger. If the water table is high, there are some types of systems that are raised and/or do not require deep unsaturated soils.  Consult the Code of Practice for more details or seek advice from a qualified person.

  • Is it permissible to have more than one house sharing a septic tank?

    This is a planning matter, contact your City or County Council.

  • I want to use a constructed wetland as my treatment system. What should I do?

    A site suitability assessment is required prior to selecting any system and will have to show that the site is suitable for a constructed wetland. A constructed wetland used in a secondary capacity is preceded by a septic tank and followed by a polishing filter. Used in a tertiary capacity, it is preceded by a secondary system and followed by an infiltration area. Any discharge from a constructed wetland to surface water requires a water pollution discharge licence in accordance with the Water Pollution Acts 1977-1990. Specific information can be obtained from the City or County Council.

Percolation tests

  • What is a percolation test?

    A percolation test assesses how fast water will drain down through soil. In simple terms, the time taken for the water to drop in a small hole is measured. The test is part of a wider assessment including trial pitting. The assessment and percolation test must be done by a suitably qualified person.

  • What is the difference between subsurface/T and surface/P tests?

    Both of these are percolation tests that assess the ability of the subsoil to allow water to percolate to the water table. The main difference is that they are carried out at different levels. A subsurface/T test is used to test the suitability of the subsoil at depths greater than 400mm below the ground level. A surface/P test is carried out at ground level where there are limiting factors such as high water table or shallow rock.

  • What is the modified test method? When is it to be used?

    The modified percolation test may be used to obtain a percolation test result in areas with low permeability soils.  The modified test may be used where the initial drop T100 >210 minutes or where the site assessor expects that the percolation test result will be above 75.

  • What is the minimum recommended distance between two percolation test holes?

    Test holes should be located at either side of the proposed percolation area (but not within it), to make sure that the percolation characteristics are assessed across the percolation area.

  • I have got very different percolation test values from my site – what do I do?

    Three percolation test holes are required for all sites.  Where there is a high degree of variability in the soil characteristics then there may be very different percolation test values and more tests could be carried out.  The results of these additional tests should be examined in light of the other three test results.  A detailed examination of the soil types within the trial hole and the individual test holes should be undertaken to determine the dominant site conditions.  The dominant subsoil type in the test holes and trial hole should be taken as representative of the site conditions and therefore the percolation test results that equate to that should be used.

Percolation areas and polishing filters

  • What is a percolation area?

    A percolation area is a network of sub-surface perforated pipes in stone filled trenches through which the final waste water is distributed into the soil and where it receives further treatment.

  • Is a reserve percolation area required?

    The Code of Practice does not require a reserve percolation area but requires a thorough site assessment, correct installation and proper maintenance of the system.

  • How do I construct a percolation area on a sloping site?

    Mound systems can be constructed on a sloping site, if the slope of the site does not exceed 12%. They must be constructed carefully along the contour to make sure that minimum installation thickness is maintained and to assist the even distribution of waste water.

  • Can I have trench widths greater than 500mm in my percolation area?

    The recommended trench width is 500mm. The percolation trench lengths given in the Code of Practice depend on this trench width.

  • Can I build over my septic tank system

    Under no circumstances should you build over a septic tank or percolation area. Access to the tank is needed for regular maintenance and the percolation area should not be compacted.

    Even with a hard-standing area located above a percolation area, traffic may damage percolation pipes and result in ponding or escape of inadequately treated waste water.

  • Can I place a car park or driveway over the percolation area or polishing filter?

    Roads, driveways, paved areas or any underground services must not be located over the percolation area or polishing filter.  This is due to the need to have easy access to the site for maintenance, preventing damage to the pipework and compaction of the filter materials.

  • What is a polishing filter?

    Polishing filters consist of either soil or sand and are used to treat waste water from secondary systems and discharge it to ground. Soil polishing filters comprise in-situ or improved or imported soil, whereas sand polishing filters are comprised of layers of sand. The rules for installation of soil or sand filters are in the Code of Practice.

    We do not have a list of suppliers for the materials used in the construction of sand filters. 


  • Who installs domestic waste water treatment systems?

    A suitably qualified installer should install on-site waste water treatment systems. The supplier will usually recommend a person who is suitable to install the system, or they may install it themselves.  The installer is then responsible for testing the system to make sure that it is working effectively. The installation of the system should be certified.  Contact your City or County Council for more details as to their specific requirements.

  • Who certifies domestic waste water treatment systems?

    You can find out this in the Construction Products Regulation part of the Department of Housing, Planning and Local Government website. The manufacturer must draw up a declaration of performance and affix a CE marking to the product, following testing by a notified third party body. There are also obligations on importers and distributors to ensure the products they handle are compliant. You will find information on building product control and certification on the Department of Housing, Local Government and Heritage and the National Standards Authority of Ireland websites