Septic Tank Survey: What It Checks and When You Need One

A septic tank survey is an inspection of the tank, the pipe that feeds it and the place its effluent goes, carried out to find out whether the system works, whether it is in sound condition and whether it meets the rules that apply to it. In London it usually comes up when someone is buying a property in the green-belt fringes of Bromley, Havering, Hillingdon or Enfield and discovers the house is not on mains drainage.

What does a septic tank survey check?

A proper survey looks at three things: the tank itself, the drains running into it, and the drainage field or discharge point it empties into. A report that covers only the tank misses the parts that most often fail and cost the most to put right.

At the tank, the surveyor records the type (brick or concrete chambers, precast rings, or a moulded glass-fibre or plastic tank), its approximate capacity, the condition of the walls and lids, and the inlet and outlet arrangements. The dip pipes or T-pieces at the inlet and outlet matter more than people expect. If the outlet fitting is missing or broken, floating scum and solids pass straight out of the tank and clog whatever sits downstream.

The inlet drain from the house is normally checked with a CCTV camera, because root ingress, displaced joints and back-falls on that run are what cause slow toilets and smells inside the property. The outlet pipe is checked the same way where it can be reached.

Beyond the tank, the surveyor looks for where the effluent actually ends up. That might be a drainage field of perforated pipes, an old-style soakaway pit, or a pipe to a ditch or stream. Signs of trouble include waterlogged or unusually lush ground over the field, standing effluent, smells in the garden and a tank that fills back up quickly after emptying.

Why the tank should be emptied before it is inspected

A full tank hides most of its defects. The walls, the base and the inlet and outlet fittings are under liquid and a crust of scum, so a survey of a full tank is little more than a look through the lid.

Emptying first lets the surveyor see cracks, missing render, root entry through the walls and broken baffles, and check whether groundwater runs back in through the structure once the level drops. It also gives a useful clue about the drainage field: a tank that was brim full before emptying, with the outlet backing up, often points to a field that no longer takes water. Where a tank is emptied as part of the job, our septic tank and cesspit emptying service records the condition before and after on camera, so the emptying and the inspection are one visit rather than two.

Septic tank, treatment plant or cesspit: which one is it?

The survey should establish which kind of system the property has, because the three work differently and carry different obligations. It is surprisingly common for sellers, and even estate agents, to use the wrong name.

System	What it does	Where the liquid goes
Septic tank	Settles solids; liquid leaves partly treated	Must go to a drainage field in the ground
Sewage treatment plant	Treats effluent mechanically, usually with a pump or air blower	Drainage field, or a watercourse where allowed
Cesspit (cesspool)	A sealed holding tank with no outlet	Nowhere: it is emptied by tanker

A treatment plant also needs its electrical and mechanical parts looked at, and its service history asked for. A cesspit survey is mostly about whether the tank is watertight and whether an alarm or level indicator works, because a cesspit that leaks is failing at its only job.

The rules a survey should check against

In England, small domestic systems are covered by the Environment Agency's general binding rules, and a good survey report says whether the system appears to meet them. The most important point for buyers: a septic tank must not discharge directly into a ditch, stream or other watercourse.

A septic tank that discharged to surface water had to be replaced with a treatment plant, or have its outlet diverted to a drainage field, by 1 January 2020. A system that still discharges to a watercourse is in breach of the rules, and a sale is often when that comes to light. Sellers are also required to give buyers written information about the system. In practice, conveyancers ask for a survey partly to see whether that information is accurate.

Other points the report should cover are whether the tank is emptied regularly by a registered waste carrier (the general binding rules require regular emptying, in line with the manufacturer's guidance), whether the system appears to be sized for the property, and whether it is maintained. Discharges above the small-system limits need an environmental permit rather than falling under the general rules. A surveyor can flag that a permit may be needed. The Environment Agency decides whether it is.

Is a septic tank survey worth it when buying a house?

Yes, if the property is off mains drainage. A standard homebuyer survey does not open the tank, and a conveyancing drainage search from the water company will only tell you the property is not connected to the public sewer. Neither tells you whether the system works.

The costly failures are not usually the tank. They are a drainage field that has silted up and has to be relaid, or a discharge to a watercourse that has to be replaced with a treatment plant. Either can change what you are prepared to pay, so the survey is most useful before exchange, while there is still room to negotiate or to ask the seller to put it right. The same logic applies to the ordinary drains; our guide on whether to get a drain survey when buying a house covers the mains-connected side.

One case where it is the wrong answer: if you are not certain the property is off mains, check that first. Some older houses on the edge of London were connected to the public sewer decades ago and the old tank was simply abandoned in the garden. Surveying a tank nobody uses is wasted money. A drainage search, plus a dye test from a sink or toilet, settles it.

What the written report should include

The report is the part you will use, so it should be clear enough for a solicitor or a buyer's lender to read without a site visit. Look for these items:

• The system type, approximate capacity and location, with a simple sketch plan showing the tank, the inlet run from the house, and the drainage field or discharge point.
• The condition of the tank structure, lids, and inlet and outlet fittings, with photographs taken after emptying.
• CCTV findings for the inlet drain, with defects recorded by position and graded the same way as a standard CCTV drain survey.
• What was found at the drainage field or outfall, including any evidence that it is not taking water.
• A plain statement of whether the system appears to comply with the general binding rules, and what would need to change if not.
• What was not inspected and why, for example a drainage field under a patio that could not be reached.

That last point matters. A survey that silently skips the drainage field reads as a clean bill of health when it is not. If you are unsure what you need, tell us about the property and we will say what can be inspected and what cannot before anything is booked.