Three holes in a field, a measured volume of water and a stopwatch decide whether you can build. That is what a percolation test comes down to, and the number it produces will either move your planning application along or send your engineer back to the drawing board.
If the number comes back slow, you are not automatically stuck. You are looking at a different and dearer system, and at a set of rules that changed in June 2021 in a way that helps low-permeability sites in the West of Ireland more than most people realise.
What a percolation test actually measures
A percolation test is one part of a bigger job called a site characterisation. The assessor is working out how fast water moves through the subsoil at the spot where the percolation area would go.
The result is written down as a percolation value, usually shortened to PV. A low number means water drops away quickly. A high number means it sits there. Both extremes are a problem, for opposite reasons.
That value then decides which kind of domestic waste water treatment system (DWWTS) the site can legally take, and whether the effluent can go to ground at all. It is not a pass or fail stamp on your house. It is a measurement that narrows your options down to one or two.
The rules come from the EPA’s Code of Practice for Domestic Waste Water Treatment Systems (Population Equivalent less than or equal to 10), 2021. It applies to site assessments and installations carried out on or after 7 June 2021. The older 2009 Code can still apply where the assessment or installation started, or planning permission was applied for, before that date, so it is worth knowing which one your file sits under.
If you are at this stage already, the useful first move is to find out where you actually stand before you commit to anything. Tell us the county, the Eircode and what the council or your engineer has asked for, and we will point you to a site assessor who works in your area: find out where you stand or call us.
What happens on the day of the percolation test
The assessor starts before the digger does. A desk study comes first, covering mapping, geology, known water table behaviour and any designations over the site.
Then the trial hole. This is the hole that shows the subsoil profile, how deep it is to bedrock and where the water table sits. Plenty of sites are decided at the trial hole, before anyone times a drop of water.
The percolation tests themselves go in near the proposed percolation area. The Code sets out how the holes are prepared, wetted and timed, and the result used is the average of three tests, not one hopeful hole in the best corner of the field.
Everything is recorded on a site characterisation form. The assessor signs off a recommended system and a discharge route, and that form is what the local authority reads.
Two practical things to know. The water table moves with the season, so an assessor may tell you that conditions on the day mean the result should be treated with caution. And this work is carried out by a qualified site assessor. We do not carry out site assessments, de-sludging or installations ourselves, we put homeowners in touch with the people who do.
Reading the percolation value: what the numbers allow
Table 6.4 of the 2021 Code is the whole game. It sets the permitted percolation value for each system type, and if your PV falls outside the band, that system is off the table.
| System | Permitted percolation value (PV) |
|---|---|
| Septic tank and percolation area | 3 to 50 |
| Secondary treatment system and soil polishing filter, pumped or underlying gravity discharge (Options 1 and 2) | 3 to 75 |
| Secondary treatment, gravity discharge, 500 mm wide trenches (Option 3) | 3 to 75 |
| Secondary treatment, low pressure pipe, 300 mm wide trenches (Option 4) | 3 to 90 |
| Secondary treatment, drip dispersal system (Option 5) | 3 to 120 |
| Tertiary treatment system and infiltration area | 3 to 75 |
Where a system is installed at the surface, the Code requires the subsurface PV to be 3 to 90 for the options marked at 3 to 75.
A PV below 3 is not a win. It means the effluent moves through too fast to be treated, and importing slower soil or subsoil may be considered.
A PV above 120 means the site is unsuitable for a DWWTS discharging to ground. Discharge to surface water can be an alternative, but it needs a Water Pollution Act licence from the local authority, which is a different conversation entirely.
Percolation is also not the only gate. Site improvement works are not acceptable where the slope is steeper than 1:8, where the average PV from the three tests is greater than 120, or where the separation distances cannot be satisfied.
When the percolation test comes back too slow
Take a site in Co. Leitrim with heavy clay. The tests come back too slow for a standard septic tank and percolation area, which needs a PV of 3 to 50, and the owner is told the plan as drawn is dead.
Under the 2009 Code that was often the end of it, because the maximum PV for discharge to ground was 75. The 2021 Code added two routes above that line.
Low pressure pipe distribution puts the effluent through pipes in shallow trenches under pressure and is usable up to a PV of 90. Drip dispersal releases it through pipes just beneath the surface and is usable up to 120.
Neither is a cheap swap. The Code describes drip dispersal as a specialised system requiring expert design and installation, and increased maintenance. You are taking on a system with moving parts and a service schedule, not a concrete tank that sits there for thirty years.
Be careful with anything marketed as a way around the ground conditions. Willow bed evapotranspiration systems can be used for secondary treatment, but monitoring of full-scale systems found that full evapotranspiration was not achieved, so discharging the final effluent to ground still needs an offset tertiary soil polishing filter.
Nobody can tell you from a website which of these your site will take. The PV is one criterion among several, alongside slope, separation distances, depth of unsaturated subsoil above bedrock and the water table, and any SAC, SPA or NHA designation. That is the site assessor’s call on the day.
If you are working through a slow result, our percolation testing page sets out what the assessment covers, and what to do after a failed percolation test goes through the options in order of cost. If the outcome is a new system rather than a repair, start with septic tank replacement.
Inspections, grants and the paperwork behind the test
Most people do not go looking for a percolation test. It arrives attached to something else.
A household near Boyle gets a letter from the council. Under the National Inspection Plan the notice is at least 10 working days, and local authorities inspect over 1,000 systems a year nationally. Typically half fail, and a quarter are a risk to human health or the environment.
If it fails, an advisory notice follows within 21 days, setting out the reasons, the works needed and the deadline. If you disagree with the result, a re-inspection costs €20. Before anyone panics: the official position from gov.ie is that there is “no question of imposing modern standards on older systems”, and no question of householders having to buy extra land to fix a system after an inspection.
That advisory notice is also the gateway to money. The grant for works arising from a National Inspection Plan inspection covers 85 per cent of cost up to a maximum of €12,000, under S.I. No. 562 of 2023, and it is paid after the works are completed to the local authority’s satisfaction. Two other schemes exist for Prioritised Areas for Action and High Status Objective Catchment Areas, and both require a letter from the local authority before you can apply. Routine maintenance, servicing and de-sludging never qualify under any of the three. The detail is in the septic tank grant in Ireland.
The other common trigger is a sale. A house sale in Co. Sligo slows to a halt because the solicitor has asked for the registration certificate and nobody can find it.
Registration costs €50 through protectourwater.ie, it applies to every system, and failing to register is an offence carrying a fine of up to €5,000 on conviction. The five yearly re-registration requirement has been removed, so an existing certificate stays valid. We cover the process in septic tank registration.
Keep the de-sludging receipts as well. The homeowner must obtain evidence of de-sludging from the authorised contractor each time and retain it for five years, and the contractor has to hold a waste collection permit for septic tank sludge. Any operator can be checked on the National Waste Collection Permit Office register, which has a search function for septic tank de-sludging.
Not sure which of these applies to you? Tell us what the letter says and we will tell you which door you are standing at.
What a percolation test costs
We are not going to give you a figure, and here is why.
The ranges circulating on Irish sites for percolation tests and site assessments cite no source, and no assessor we could find publishes a rate. Quoting an unsourced number back at you would be worth nothing when the real quote arrives. Get two written quotes from site assessors covering your county, and ask each one what is included: desk study, trial hole, the tests, and the written site characterisation report.
The figures we can stand over are the statutory ones. Registration is €50 through protectourwater.ie, and a re-inspection, if you dispute a result, is €20. The grant is 85 per cent of cost up to a maximum of €12,000 under S.I. 562, 563 and 564 of 2023. The fine for failing to register is up to €5,000 on conviction.
On emptying we can put a number on Galway and nowhere else. A long-established Galway operator publishes a fixed town by town list at €299 for inner towns and €329 for outer towns, based on a standard 3 m³ tank, with larger sizes liable to cost more. A second Galway operator states an average of €240 to €270, marked valid as of February 2025.
So roughly €240 to €330 for a standard 3 m³ domestic tank in Co. Galway. We publish no emptying price for Mayo, Roscommon, Sligo or Leitrim, because no operator there publishes one.
One last thing worth saying plainly, because it catches people out. There is no grant for a percolation test on a new build. All three schemes require an existing house built at least seven years ago, permanently occupied, with no DWWTS grant received in the last seven years, and they are tied to an inspection or a council letter. If the test is for planning permission on a new house or an extension, it is your own cost.
Frequently Asked Questions
How long does a percolation test take?
That depends on the assessor and on the ground. The Code requires the holes to be prepared and wetted before the timed test, so the work is not always finished in a single visit, and the written site characterisation follows afterwards. Ask the assessor for their own timeline before you book, particularly if a planning deadline is sitting over you.
Can a percolation test fail?
The test itself does not fail, the site does. If the average percolation value from the three tests is greater than 120, the site is unsuitable for a system discharging to ground, and discharge to surface water would need a Water Pollution Act licence from the local authority. A slope steeper than 1:8 or separation distances that cannot be met will also rule out site improvement works.
Does the septic tank grant cover a percolation test?
Not on its own. The three grant schemes cover remediation works that arise from a National Inspection Plan inspection and advisory notice, or from a council letter under the Prioritised Areas for Action or High Status Objective Catchment Area schemes. Routine maintenance, servicing and de-sludging are excluded outright. Your local authority’s Rural Water Liaison Officer is the person who can tell you whether your works come under any of them.
Do I need a percolation test if I am only replacing the tank?
Ask the local authority before you order anything. A replacement system does not need to be re-registered, but works must be carried out in accordance with the 2021 Code, and the Code sets the permitted percolation values for each system type and discharge route. In practice that means the site data has to exist and be current enough to size and site the percolation area properly.