Percolation tests: how your ground decides what system you can have
A percolation test measures how fast your soil accepts water, and that single figure sets the size of your drainage field — or rules one out entirely.
Read the guideThe unit is often the smallest part of the bill. Groundworks, ground conditions, access and the outfall move the number. How a quote is built up.
The plant itself is usually a minority of the cost. Excavation, ground conditions, access for machinery, the electrical supply and what happens at the outfall end are what move the figure. That is why a sensible quote follows a site visit rather than a phone call.
People ask for the price of a treatment plant and expect one number. In practice a quote is five costs stacked together, and the unit is rarely the biggest.
Change any one of those and the total moves. That is why two neighbours can be quoted very differently for what looks like the same job.
Digging a hole in workable soil is routine. Digging the same hole in rock, in running sand, or below a water table that fills it as fast as you empty it is a different job with different equipment. On a site with a high water table the tank itself may need anchoring, which adds concrete and time.
If an excavator and a delivery lorry can reach the working area, the job proceeds normally. If everything has to come through a narrow gate or be craned over a building, the cost reflects that. This is the single biggest difference between an open rural plot and a cottage in a village centre.
This is where quotes diverge most. A short run to an existing ditch is inexpensive. A full drainage field sized from a percolation test is a significant piece of groundworks in its own right — often more than the plant. If the levels mean the discharge has to be pumped, add a pumping station, its chamber, its electrics and its own maintenance thereafter.
Plants are sized on the population the property can support, not the number of people in it today. A four bedroom house is specified on that basis whether two people or six live there, and lets are sized on how many they sleep.
The old tank has to be emptied, cleaned and either removed or broken and filled so it cannot collapse later. It is a modest cost, but it is a real one and it should be on the quote rather than discovered afterwards.
Comparing drainage quotes is difficult because they are rarely written to the same scope. Before comparing numbers, check that each one includes: the percolation test and field design, spoil removal, the electrical supply and connection, commissioning, decommissioning the old tank, reinstatement of the ground, and building control notification.
A quote missing three of those will always look cheaper. It is not; it is just shorter. The commonest omissions we see are spoil removal, the electrical connection and reinstatement — and each of them is real money.
A treatment plant costs more to own than a septic tank: electricity for the blower, an annual service, and de-sludging. Against that it needs far less ground and is tolerant of sites where a drainage field is impossible. Over ten years the gap is smaller than the installation prices suggest, particularly if the alternative was a large field on difficult ground.
What costs the most of all is the wrong system: a plant too small for the property, or a field built without a test. Both fail within a few years, and the second installation costs more than the first because somebody has to undo the first one.
We survey the site, establish where the existing system goes, carry out trial holes and percolation testing where a field is in play, and then quote against a written scope so you can see what is and is not included. If the ground or the access means the job is more involved than it looks, you will hear that at the survey rather than halfway through.
More on how we handle installations, or call 07884 111 641 and describe the property.
It helps to picture the shape of a straightforward domestic installation. Roughly speaking, the plant and its delivery is one part of the bill, the excavation and groundworks another, and the outfall arrangement — field, ditch connection or pumping station — the third. Electrics, commissioning, decommissioning the old tank and reinstating the ground make up the remainder between them.
What surprises people is how much of the labour has nothing to do with the plant. Getting a machine to the right place, digging accurately, bedding and backfilling correctly, and carting spoil away is most of the work, and it is the part that varies with your particular site rather than with what you buy.
Groundworks in February on heavy clay is slower, messier and harder on the site than the same job in June. Where there is no urgency it is worth planning for drier months, both for cost and for the state of the garden afterwards.
The exception is a sale. If the drainage has to be resolved before completion, the calendar is not yours to choose, which is the argument for dealing with a known problem early rather than at the point a buyer’s solicitor finds it. We have written separately about how drainage comes up in a sale.
We can tell you which end of the range a job is likely to sit in once we know the property, the ground and the outfall. A firm number follows a site visit, because the variables are in the ground rather than in the catalogue.
Sometimes. On ground where a field would need to be very large, a plant with a small field or a watercourse discharge can work out cheaper overall, even though the unit costs more.
It must be emptied, cleaned and made safe — either removed or broken and filled. Leaving a redundant tank intact underground is how voids collapse years later.
Yes, though we would want to confirm the specification suits the site first. We also supply systems on their own, UK-wide and into Europe, for people with their own groundworker.
Septic tank emptying, installations, servicing and drainage across Derbyshire, Sheffield and Yorkshire — 24/7.
A percolation test measures how fast your soil accepts water, and that single figure sets the size of your drainage field — or rules one out entirely.
Read the guideA waterlogged drainage field is the most expensive part of an off-mains system to fix. What causes it, how to spot it early, and what a rebuild costs.
Read the guideThe two look similar from the garden but work differently, need different maintenance and face different rules. How to tell which one you own.
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