Will your site pass a wastewater assessment?

If your house will not connect to a public sewer, find out whether the site can treat its wastewater before you buy the land. A passing percolation test is only one part of the assessment: the soil depth, slope, distances and other site constraints must work too.

Why the council wants it

Ask your council’s planning office what qualifications it requires of site assessors, then find an assessor who meets those requirements. Your architect or engineer may be able to suggest one.

For a house with an on-site system discharging to ground, the treated wastewater soaks into the soil. If the ground is too wet, drains too slowly or has too little separation from groundwater, wastewater can reach wells and streams. Your planning application needs a site assessment that supports the proposed septic tank or treatment system.

The Environmental Protection Agency’s (EPA) 2021 Code of Practice covers systems for up to 10 people. The site characterisation form must be completed in all cases, with photographs, environmental risk maps and site plans. Every planning application for a system should also include cross-section drawings (cut-through side views), design details and the proposed heights of the completed ground floor and surrounding ground. Ask your assessor which edition and council checklist apply to your proposal; councils have their own forms, and the Code can be revised.

If the site is a piece of your parents’ land, see building on family land for the transfer. A rural site must satisfy both the wastewater assessment and the council’s rural housing policy. The systems themselves, and what they cost to run, are in the guide to wastewater treatment systems.

T-test and P-test: the names have changed

A septic tank settles solids first; its percolation trenches are shallow, gravel-filled trenches that spread wastewater into the soil for further treatment.

A polishing filter is a soil treatment area after a treatment unit. Low-pressure pipe distribution uses pumped pipes to spread treated water; drip dispersal uses tubing that releases it in small amounts.

You may hear the older names T-test and P-test. The 2021 Code calls them the subsurface and surface percolation tests. They measure how quickly water drains at different depths.

• Subsurface test. Used when the trial hole shows enough depth of suitable soil for a septic tank with percolation trenches, or a polishing filter discharging at depth (more than 400 mm). It is done at the level of the proposed pipe.
• Both tests. Needed when the system will discharge at or above the ground surface: a raised or mounded percolation area, raised filter, low-pressure pipe distribution or drip dispersal. The surface test checks the soil where the water goes in. The subsurface test checks the ground beneath it.
• Borderline sites. If experience says the site is marginal, both tests are done at once.

You need at least 0.5 m of unsaturated soil to test at all. Unsaturated soil has spaces that are not filled with water; the depth of this layer matters as well as how quickly water drains.

What the assessor does

Check maps and walk the site

Aquifers are underground rock or sediment that holds and carries groundwater. Your assessor uses their mapped classification to decide how deep the trial hole must go. “Poor”, “locally important” and “regionally important” describe the groundwater resource and how much it can supply, not its water quality.

The desk study checks groundwater vulnerability, aquifers, wells, streams, nearby houses and land use. It identifies the groundwater protection response, recorded as a code from R1 to R3² (higher numbers mean greater risk to groundwater and stricter rules). Your assessor works out your code from the EPA’s maps. That code affects which systems and soil depths are acceptable. If an area of high or extreme groundwater vulnerability already has more than six systems per hectare, your assessor must flag it, and it can count against the site.

On site, the assessor checks slope, ditches, vegetation, ground damaged by livestock, old fill and foundations, and nearby wells. Rushes, yellow flag iris, alder and willow suggest a high water table; ferns and ragwort suggest good drainage. These signs help guide the assessment but do not replace the tests.

Dig and inspect the trial hole

The trial hole must be as small as practicable, for example 1 m by 6 m with a sloped way out. It needs to be at least 2.1 m deep, or reach bedrock, where the aquifer is locally important or poor. Over regionally important aquifers, or for groundwater responses R2², R2³, R2⁴, R3¹ or R3², the depth is at least 3 m if possible.

The hole goes beside the proposed percolation area. Digging within that area would disturb the ground and could create an easier route for water to flow through it.

The assessor records the depth to bedrock and the water table, along with the colour, texture, structure, compaction and arrangement of soil layers. Grey or mottled soil indicates seasonal waterlogging. If bedrock, water or mottling occurs within 500 mm of the ground surface throughout the site, it is usually unacceptable for discharge to ground.

The hole stays open for at least 48 hours so the water table can settle. It must be fenced and covered, then backfilled as soon as the assessment is done. View it only from a safe position; these excavations can collapse or hold water.

Measure how quickly water drains

The assessor uses three percolation test holes, each 300 mm by 300 mm by 400 mm deep. For a subsurface test, the top of the hole is as close as possible to the proposed pipe invert, the level of the pipe’s inside bottom, or the relevant level at the base of the system’s gravel layer.

The assessor scratches the hole surfaces to remove soil smeared smooth by digging, which could affect drainage. The holes are then pre-soaked twice and timed as the water drops. The standard method averages repeated timed falls and converts them to minutes per 25 mm. The modified method uses the Code’s conversion factors. The site value is the average of the three hole values. These are procedures for your assessor, not a test to attempt yourself.

If the first drop of 100 mm (400 mm down to 300 mm) takes more than 8 hours, the value is above 120 and the test stops: that location fails. If it takes up to 210 minutes, the standard method continues. If it takes longer than 210 minutes, the assessor switches to the modified calculation method, which is how clay sites are tested without waiting days.

Complete the report

The assessor records the findings on the EPA site characterisation form. If the site passes, the report recommends a system and discharge route. Get the report and layout to your engineer or architect before fixing the house position.

The five checks, and the numbers

A slope of 1 in 8 rises or falls 1 metre over 8 metres horizontally; 1 in 20 in the worked example is gentler. All five checks must pass. A suitable percolation value cannot make up for a well too close to the treatment area or too little soil above groundwater.

Check	Requirement
Slope	Natural slope no steeper than 1 in 8
Distances	Table 6.2 distances to wells, streams, houses and other features
Depth of soil	Minimum unsaturated soil or subsoil above bedrock and water table (Table 6.3)
Percolation value	Within the range the system allows (Table 6.4)
Other constraints	Underground services, protected ecological areas, floodplain and archaeology

Protected ecological areas include special areas of conservation (SACs), special protection areas (SPAs) and natural heritage areas (NHAs). Ask the assessor how any designation affects your layout.

Separation distances

A karst feature is a sinkhole or an opening in limestone that can carry water underground. A slope break is a sharp change in the steepness of the ground. A surface-water soakaway is a pit or underground structure that lets rainwater drain into the soil. A group water scheme is a shared community water supply. The foreshore is the shore between high- and low-tide marks.

Measure from the relevant outside edge of the tank or plant and the infiltration or treatment area. These are selected distances from Table 6.2; your assessor must check the full table and its notes for the layout.

Feature	Distance
Public or group scheme water source	60 m
Down-gradient domestic well (or unknown flow direction)	30 to 60 m, depending on percolation value and depth to bedrock
Alongside a domestic well	25 m
Up-gradient domestic well	15 m
Karst feature	15 m
Neighbour’s wastewater system, including its tank/plant and infiltration area	10 m
Lake or foreshore	50 m
Stream	10 m
Open drain or ditch	10 m
Your own house	7 m from tank or plant, 10 m from infiltration area
Neighbour’s house	7 m from tank or plant, 10 m from infiltration area
Surface water soakaway	5 m
Road, slope break	4 m
Trees, site boundary	3 m

Up-gradient means groundwater flows from the well towards the system; down-gradient means it flows from the system towards the well. Alongside means neither is up-gradient of the other. Ask your assessor to establish groundwater flow: the surface slope alone does not settle it. Well distances apply to the entire system, including its tank and infiltration area, and to wells on neighbouring land.

Constructed wetland treatment systems with water exposed at the surface have separate dwelling distances: 10 m from the on-site house and 25 m from a neighbouring house. Surface-water soakaways should be down-gradient of the infiltration area; check neighbouring soakaways too. Protected ecological and heritage features need specific advice.

If any requirement cannot be met on a new site, the Code says the site is not suitable. Wells are the usual trouble: the down-gradient distance depends on soil permeability and depth, and can reach 60 m, so a small plot with a neighbour’s well nearby can fail on distances alone.

Depth and percolation value

The minimum unsaturated depth is measured below the point where water enters the soil, not from ground level. Table 6.3 sets these depths for percolation trenches after a septic tank:

• R1 and R2¹ groundwater responses: at least 1.2 m.
• R2², R2³, R2⁴ and R3¹: at least 2.0 m.
• R3²: these trenches are not acceptable.

Tertiary systems add a further treatment stage before water enters the ground. These can need less soil depth than septic-tank trenches. Drip dispersal and some tertiary systems need less again, down to 0.6 m in the best case. If you have at least 0.5 m of suitable soil but not the full required depth, a raised infiltration area may work. With less than 0.5 m of suitable soil, ask your assessor whether ground works could make the site suitable, what those works would involve and what evidence the Code requires. Have the assessor match the depth to the particular system and groundwater response.

Typical results by soil, from the Code’s Table 5.2 (minutes per 25 mm, for 500 tests): gravel 2 to 8, sand 5 to 18, silt 11 to 31, silt/clay 18 to 43, clay over 41. Soil description alone does not establish the measured value. The allowed ranges by system are in the wastewater systems guide: in short, 3 to 50 for a septic tank, and up to 75, 90 or 120 for treatment systems depending on the distribution method.

What a passing result can look like

An illustrative case (invented numbers): a 0.4-hectare field, ditch on the north boundary, neighbour’s well 50 m east and slightly up-gradient.

• Desk study: groundwater response R2¹, so trenches need 1.2 m of soil above bedrock and water.
• Trial hole, 2.1 m deep, left open 48 hours: brown silty subsoil to 2.1 m, no water, no mottling. Bedrock not reached.
• Three percolation holes, average fall times of 33, 38 and 40 minutes for 25 mm: site value about 37.
• Checks: slope 1 in 20, passes. Assessor confirms the well is hydraulically up-gradient at 50 m against the 15 m minimum, passes. Ditch 25 m away against 10 m, passes. The proposed infiltration point is 0.85 m below ground, leaving at least 1.25 m of proven unsaturated soil beneath it: above the 1.2 m minimum.
• Result: value 37 is within the 3 to 50 range, so a septic tank with percolation trenches is a potential option, provided the full design also meets the requirements for the expected amount of wastewater, all other distances and the site’s groundwater protection rules. A passing test does not itself grant planning permission.

If the three holes had averaged 70 instead, a septic tank would be ruled out but a secondary treatment unit (further treatment after solids settle) with a polishing filter could remain an option if the selected distribution method and all other criteria permit it; compare the full design and operating costs.

How big does a site need to be?

There is no single national minimum plot size for a one-off house in Ireland. Councils set their own expectations in the county development plan and in the way they assess each application, so ask the planning office rather than rely on a figure from another county. In practice the size is driven less by a rule than by what has to fit on the land:

• Wastewater. On-site treatment needs room for the tank or treatment unit, the percolation area or polishing filter, and the separation distances in the EPA Code to wells, streams, ditches, the house and boundaries. This is the usual reason a small plot fails, because the house, the percolation area and your neighbour’s well all compete for the same ground.
• Your own and neighbours’ wells. The distance to a well applies from the tank or plant and the infiltration area, and a well on the next field can rule out part of your site.
• Access and sightlines. The entrance needs visibility along the road in both directions. A short road frontage, or a hedge or wall you do not own, can make this impossible.
• Planning policy. Councils may also ask for a house that fits the character of the area and does not over-develop a larger holding, so a very large or very small plot can raise questions.

Instead of asking “how many acres?”, ask the assessor and your designer to draw the percolation area, the house, the well and the entrance on a scale plan of the land. If they all fit with the distances respected, the site is probably big enough. If not, ask whether a different layout or a treatment system with a smaller footprint is allowed. A pre-planning meeting with the council will confirm what they expect.

Mistakes to avoid

• Buying first, testing later. Agree access and testing with the seller, and ask your solicitor how to protect the purchase if suitability or permission is not established.
• Testing in the driest week and trusting it. A dry-weather hole can miss a winter water table. Ask the assessor how they checked for mottling.
• Using a decades-old report. The 2021 Code applies from 7 June 2021, with exceptions for some earlier assessments, installations or planning applications. Ask your assessor and council whether an older report qualifies under those exceptions and whether the layout or conditions have changed.
• Placing the house first. Decide where the percolation area and the well go before you fix the house position. The distances to your own house and boundary apply.
• Digging your own trial hole. The Code treats it as a safety risk: deep, steep-sided, and likely to hold water. A collapsed wall is a real hazard.
• Ignoring the neighbour’s well. Check for wells on adjoining land. They set the distances.

What to ask the assessor

Professional indemnity insurance covers claims arising from mistakes in their professional work. Ask to see evidence of it alongside their training.

• Are you trained and qualified under the Code, and can I see the certificate and your insurance?
• Which version of the Code will you use?
• Will you test both the surface and subsurface values, and why?
• What is your fee, does it include the written report and drawings, and what if the first location fails?
• Can you explain the trial-hole findings from a safe viewing position, photographs and the report?
• Which systems will the site allow, and roughly what does each cost to install and run?

Check your council’s current forms and any per-test fee. In Laois, for example, the explanatory form specifies a fee per test and site layout plans at a minimum scale of 1:500 showing percolation areas and wells. Inaccurate maps or conflicting information will be returned. Those are Laois’s instructions; check what your own council requires before commissioning the drawings.

Checklist before you rely on the result

• Assessor is trained and qualified, and the certificate is on file
• The report is on the 2021 EPA site characterisation form with photographs, maps and site plan
• Trial hole was open at least 48 hours and went deep enough for the aquifer
• Three percolation holes, with times and dimensions recorded
• Every separation distance in the EPA Code’s Table 6.2 (see Sources) is met on your house layout
• The recommended system and discharge route are named
• Your engineer or architect has the assessor’s report and wastewater layout before preparing the house’s planning site plan

For the application, see planning permission for a one-off house and the planning application step by step.

If there is no mains water supply, see water supply for a one-off house.