The site assessor fills in the trial hole, writes an average percolation value on the site characterisation form, and that single number decides what you are allowed to install. If it comes back above 50, a septic tank with a standard percolation area is off the table for that site. A failed percolation test is where most one-off house projects in the West of Ireland stall, and it is usually the moment a homeowner gets told something that has not been correct since June 2021.
On the heavy clay and low permeability subsoils that run through Leitrim, north Roscommon and parts of Mayo, a slow result is ordinary. It says something about the ground, not about you or your builder. Under the Code of Practice that has applied since 7 June 2021, it is not automatically the end of the project either.
What a failed percolation test actually means
Nobody stamps the form “failed”. The test is one part of a site characterisation, which also takes in a desk study and a trial hole, and the site assessor uses the whole picture to recommend a system and a discharge route.
The test itself produces a percolation value, usually written as a PV. It is a measure of how long water takes to fall in a test hole, averaged across three tests, so a high number means slow ground and a low number means fast ground.
What people call a failure is normally one of three things. The PV sits outside the range the Code permits for the system you had planned. The PV is above 120, which rules out discharge to ground altogether. Or the PV was fine and something else on the site was not, such as slope, separation distances, or the depth of unsaturated subsoil above bedrock or the water table.
A very fast result counts too. Below a PV of 3 the effluent is not held in the soil long enough to be treated, and the Code allows importing slower soil or subsoil to be considered.
If you have the assessor’s form in front of you and cannot tell which of those applies, send us the figures or give us a ring. We will read what the form actually says and come back with a quote from an operator we work with for the work it points to. We do not carry out site assessments ourselves, and no website can tell you what your ground will take.
Your percolation test result, and what the 2021 Code allows on it
Table 6.4 of the EPA 2021 Code of Practice sets the permitted percolation values for each type of system. It is the table your assessor is working from, and it is worth seeing in full before anyone quotes you for anything.
| System | Permitted percolation value (PV) |
|---|---|
| Septic tank and percolation area | 3 to 50 |
| Secondary treatment system and soil polishing filter, pumped or gravity discharge (Options 1 and 2) | 3 to 75 (3 to 90 subsurface if installed at the surface) |
| Secondary treatment, gravity discharge, 500 mm wide trenches (Option 3) | 3 to 75 (3 to 90 subsurface if at the surface) |
| Secondary treatment, low pressure pipe, 300 mm wide trenches (Option 4) | 3 to 90 |
| Secondary treatment, drip dispersal (Option 5) | 3 to 120 |
| Tertiary treatment system and infiltration area | 3 to 75 (3 to 90 subsurface if at the surface) |
Read down that table and the pattern is clear enough. A plain septic tank needs the fastest ground of anything on the list, and every option that works on slower ground involves treating the effluent before it reaches the soil.
That is why a slow result so often turns into a conversation about replacing the tank rather than extending the percolation area. Our septic tank replacement page sets out what that involves, and there is more on the difference between a tank and a treatment system in our guide to septic tanks versus treatment systems.
Slow percolation: the two options the 2021 Code added for difficult ground
Under the 2009 Code, discharge to ground stopped at a PV of 75. Anything slower was simply unsuitable, and plenty of sites in the five counties were written off on that basis.
The 2021 Code added two routes that did not exist before. Low pressure pipe distribution puts effluent through pressurised pipes in shallow 300 mm trenches and is permitted up to a PV of 90. Drip dispersal releases it through pressurised pipes just below the surface and is permitted up to a PV of 120.
Both sit downstream of a secondary treatment system with a soil polishing filter. They are not a swap of pipework on an existing septic tank, they are an upgrade of the whole system, and the EPA is blunt about what they need: specialised design, specialised installation and more maintenance than a gravity system. A pump means power, a control panel and something that can fail while you are away.
A site in south Leitrim makes the point. Heavy clay, three tests averaging a PV of 94, which is outside the range for a septic tank, outside it for a gravity soil polishing filter, and outside it for low pressure pipe distribution at 90. On paper the only ground discharge route left is drip dispersal, and only if slope, separation distances and depth to bedrock all stack up as well.
That last clause matters. The percolation value is one criterion out of several, and an assessor can pass you on the PV and stop you on the water table. Nobody, including us, can tell you from a desk that your site will take a particular system.
Willow bed evapotranspiration systems come up a lot on slow ground in the West. They can be used for secondary treatment, but monitoring of full-scale systems found that full evapotranspiration was not achieved, so final effluent going to ground still needs an offset tertiary soil polishing filter. They are not a way around the percolation problem.
If the assessor has ruled out a standard percolation area, the useful next step is a costed scope rather than more reading. Tell us what the form says and we will get you a quote for the upgrade it points to.
When a site is genuinely unsuitable for discharge to ground
There is a hard edge, and it is worth knowing where it is. Site improvement works are not acceptable where the slope exceeds 1:8, where the average percolation value from the three tests is above 120, or where the separation distances cannot be satisfied.
Past that point, discharge to surface water may be an alternative, but it requires a licence under the Water Pollution Act from the local authority. That is a council decision, made on the merits of the site, and no installer can promise it.
This is the stage where you need a qualified engineer or site assessor standing on the ground, ideally before contracts are signed on a site purchase. A refused percolation result on land you already own is a much more expensive problem than one you found during a search.
An existing house is a different question from a bare site. If your system is already in place and serving a lived-in house, the standard applied at inspection is whether it poses a risk to human health or the environment. The Department is explicit that there is “no question of imposing modern standards on older systems”, so a failed inspection on a 1990s system does not mean you are being held to the 2021 Code for a new build.
Who pays for the remedy, and the paperwork that decides it
The grant is 85 per cent of the cost of the works up to a maximum of €12,000. It rose from €5,000 on 1 January 2024, and the old requirement to have registered the tank by 1 February 2013 was dropped at the same time. Plenty of pages still repeat the old deadline, so check the date on anything you read against the Citizens Information page, which carries the current position.
What almost nobody explains is that there are three separate schemes under S.I. 562, 563 and 564 of 2023, and you cannot simply apply because your system is old.
- The inspection scheme requires that the works arise directly from an inspection under the National Inspection Plan and the advisory notice the local authority issues afterwards.
- The Prioritised Area for Action scheme requires a letter from the Local Authority Waters Programme Office confirming that you are eligible.
- The High Status Objective Catchment Area scheme requires a letter from the local authority, and you can check an Eircode against the catchment map on gov.ie.
Two of the three you cannot apply for unless the council writes to you first. The third you cannot apply for unless your system has been inspected and failed. The conditions then stack on top: the house must be at least seven years old, permanently occupied rather than a holiday home, and must not have had a system grant in the last seven years. Payment comes after the works are finished to the local authority’s satisfaction, not before.
Routine maintenance, servicing and de-sludging never qualify, under any of the three schemes. For scale on that, Walsh Waste publishes a fixed Galway town list at €299 and €329 for a standard 3 m3 tank, and CES Environmental puts the average at €240 to €270 as of February 2025. Useful figures to know, but not grant work, and we hold no published rate for Mayo, Roscommon, Sligo or Leitrim. There is more detail in our guide to the septic tank grant.
A house near Boyle shows how the two halves connect. The owner got a letter giving ten working days’ notice of an inspection, which is the minimum the council must give. If that inspection fails, an advisory notice follows within 21 days setting out the reasons, the works required and the deadline, and that notice is also the document that opens the S.I. 562 grant route. If you think the result is wrong, a re-inspection costs €20.
Registration is the other piece of paper. It costs €50 through protectourwater.ie, it applies to every domestic waste water treatment system, and failing to register is an offence carrying a fine of up to €5,000 on conviction. The five year renewal requirement has been removed, so an existing certificate stays valid.
That is what catches sellers. A sale in Sligo slowed to a crawl because the solicitor asked for the registration certificate and evidence of de-sludging, and neither could be found. De-sludging has to be carried out by a contractor holding a waste collection permit, the receipt must be kept for five years, and a cash job with no paperwork leaves you with nothing to hand over. You can check an operator on the National Waste Collection Permit Office register at nwcpo.ie before you book anything, and we check the same register before we pass anyone a lead.
If you are somewhere in this and want to know where you stand before you spend money, tell us what has happened so far. More background on the test itself is in our guide to percolation testing in Ireland, and our percolation testing service page covers what an assessment involves.
Frequently Asked Questions
Does a failed percolation test mean I cannot build?
Not on its own. If the average percolation value is above 120 the site is unsuitable for a system discharging to ground, and any alternative involving discharge to surface water needs a Water Pollution Act licence from the local authority. Below that, the 2021 Code allows low pressure pipe distribution up to a PV of 90 and drip dispersal up to 120, subject to the rest of the site characterisation. Only the site assessor who carried out the tests can tell you which of those applies to your ground.
Will the grant cover a new percolation area after a failed inspection?
It can, at 85 per cent of the cost up to €12,000, but only if you qualify under one of the three schemes. The inspection route requires that the works arise directly from an inspection under the National Inspection Plan and the advisory notice issued afterwards. The house must be at least seven years old, permanently occupied, and must not have received a system grant in the last seven years. The money is paid after the works are completed to the local authority’s satisfaction.
How much notice does the council have to give me before an inspection?
At least 10 working days. The inspector is appointed by the EPA and should show identification if you ask, and they will check that the system is registered, so have the registration certificate and any maintenance or de-sludging receipts to hand. If the system fails you will get an advisory notice within 21 days setting out the reasons, the works needed and the deadline for carrying them out.
Can I just have the tank de-sludged more often instead?
De-sludging keeps solids out of the percolation area, which is worth doing, but it will not fix ground that is too slow to accept the effluent. The 2021 Code replaced the old annual advice with a frequency based on occupants and tank volume, so many households are on a two to five year cycle rather than a yearly one. Whoever empties it must hold a waste collection permit, and you must keep the receipt for five years. De-sludging is never covered by the grant.