Preparing for a new septic tank, from the site checks, percolation test and building regulations to access, the old tank and the paperwork afterwards.
Preparing for a septic tank installation is mostly about the site, not the tank. Before anything is ordered, the ground has to be tested to show it can take a drainage field, the tank and the field have to fit inside the distances the building regulations set, the discharge has to meet the Environment Agency's rules, and a digger and a delivery lorry have to be able to get in. Get those settled first and the installation itself is the simpler part.
This guide goes through each step in the order it happens, so you know what to expect, what to ask and what to have ready. Our new tank installations page covers how we carry out the work itself.
Step one, find out whether a septic tank is the right answer
A septic tank is not the only choice, and on many plots it is not the right one. It settles the solids and lets the liquid out to a drainage field in the ground, where the soil finishes the treatment. That only works where the soil drains at the right speed and sits far enough above the water table. It must never discharge to a ditch, stream or other watercourse.
If the ground will not take a field, the usual answer is a sewage treatment plant, which cleans the water well enough to go to a stream that flows all year. Where nothing can discharge at all, a sealed cesspool may be the only option. How to choose between a cesspool, septic tank or treatment plant compares all three, and do I need a sewage treatment plant helps if a plant looks likely.
Before any of that, check whether a public sewer is near. The binding rules expect a new discharge to connect to the sewer if one is within 30 metres of the boundary of the property, and your water company can tell you where the nearest one runs.
Step two, test the ground
The building regulations expect a trial hole and a percolation test before a drainage field is designed. The trial hole shows what the soil is and how high the water table rises, and the percolation test measures how quickly the soil takes water.
The answer has to fall inside a window rather than simply being fast. Soil that drains too slowly, such as heavy clay, cannot cope with the water, and soil that drains too fast, such as gravel or fissured chalk, lets the liquid through before the ground has treated it. The bottom of the field also has to sit far enough above the highest winter water table, which is why a test dug in a dry August can give the wrong answer for January.
The design of the field itself follows BS 6297, the British Standard code of practice for drainage fields. Replacing a failed drainage field explains the test in more detail, and why ground across the Weald clay so often fails it.
Step three, check the distances
Approved Document H, the building regulations guidance on drainage, sets out where a tank and a field can go.
- A septic tank should be at least 7 metres from the habitable parts of any building.
- It should be within 30 metres of where a tanker can park, so it can be emptied.
- A drainage field should be at least 10 metres from any watercourse and 15 metres from any building.
Those distances rule out more gardens than people expect, especially on a small plot or where a ditch runs along the boundary. It is worth marking them out on a plan of the property before you go any further.
Step four, check the discharge rules
A new septic tank that discharges to ground has to meet the General Binding Rules or have an environmental permit from the Environment Agency. For a new discharge, that means more than the volume limit. It cannot use the rules in or near certain protected sites, such as ancient woodland or a Site of Special Scientific Interest. It needs a permit if it is within 50 metres of a spring, well or borehole that supplies drinking water. And since October 2023 it needs one if it is within 50 metres of another system discharging under the rules, which matters on any plot with a neighbour's septic tank or treatment plant close by.
Ask your neighbours about their systems and their water supply early. Where the rules cannot be met, the Agency's guidance on the permits you need for a septic tank explains how to apply, and a permit takes time to come through.
Step five, building regulations and planning
A new septic tank needs building regulations approval, so the council's building control team, or an approved inspector, is told before work starts and inspects it as it goes in. Planning permission may also be needed, depending on the property and the work, and your council will confirm it. A listed building, a conservation area or a site in a National Landscape can each add steps, so ask before you set dates.
Step six, choose the size and the tank
A septic tank is sized on how many people the property can sleep, not how many live there now. A house with five bedrooms needs a tank sized for that, even if two people live in it today, because the next owners may fill it. A tank that is too small sends solids out to the field and shortens its life.
New factory-made tanks come in glass reinforced plastic and polyethylene, and they should meet BS EN 12566. Your installer should tell you which tank they propose and why, and give you its manual.
Step seven, plan the access and the day
The installation itself needs room for a digger, a lorry delivering the tank and stone, and somewhere to put the spoil. Think about these before the day.
- Getting in. A narrow gate, a tight drive or a soft field may need a smaller machine or ground protection matting. Overhead cables and buried services need marking.
- Where the old tank is. An old septic tank or cesspool has to be emptied, then removed or filled in and made safe. The binding rules expect a system that is no longer used to be decommissioned properly.
- The drains during the work. Most installations need the drains off for part of the work. We will tell you when, and for how long.
- Power. A septic tank needs none, but if the tank or field sits higher than the house a pump will be needed, and that needs a supply and a high level alarm.
- The garden afterwards. Trenches are backfilled, but the ground over a new field settles and grass takes time to come back.
Step eight, the paperwork afterwards
When the work is finished, keep everything together in one place.
- The building control completion certificate
- The tank's manual and its make and size
- A plan showing where the tank, the field and the outlet are
- The waste transfer note for emptying the old system
- A plan for emptying, usually once a year
The binding rules expect you to pass all of this to the buyer in writing when you sell. Selling a house with a septic tank in Kent explains what a buyer's solicitor will ask for.
After it is in
A new tank needs nothing for its first months except normal, careful use. Keep wipes, fat and strong chemicals out of it, keep vehicles off the tank and the field, and book the first empty for about a year after it goes in. Our septic tank emptying and septic tank servicing can be booked on the same visit, which keeps the field in good order for years.
Planning your installation
The work that decides whether a new septic tank is a success happens before the digger arrives. Test the ground, mark out the distances, check the discharge rules and the neighbours' systems, get the approvals in hand and plan the access, and the installation itself is straightforward. If the ground says no, it is far cheaper to find out at the trial hole than after a field has failed. We install tanks across Kent and East Sussex, and what is involved in new tank installations covers the work on the day.
KP Cesspool Services has over twenty years' experience of installing and looking after tanks, and you can read our reviews on Checkatrade. Call 01233 720725, email kpcesspoolservices@hotmail.com, or contact us to arrange a site visit.
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