Septic tank survey
A septic tank survey is a specialist inspection of a property's off-mains drainage: the tank or treatment plant, where it discharges, the drainage field and the paperwork. A general homebuyer survey rarely covers any of this beyond noting that the house is not on mains drainage, yet a failed or non-compliant system can cost £10,000 or more to put right.
Published guides put a septic tank survey at about £250 to £300, rising to about £500 where percolation testing is added. Sellers who commission one early avoid a renegotiation late in the sale.
Survey for a sale or purchase£250 to £500a guide, not a quote
When to get a septic tank survey
- You are buying a house that is not on mains drainage, and your homebuyer survey has flagged it.
- You are selling, and want to know before a buyer does whether the system meets the rules.
- The tank's outlet is unknown, or may run to a stream or ditch.
- Nobody can find the paperwork: age, make, size, emptying records.
- There are signs of failure: smells, slow drains, wet ground over the field.
What a survey typically costs
| Item | Typical price |
|---|---|
| Septic tank survey for a sale or purchase Visual inspection, emptied tank checked, outlet traced | £250 to £300 |
| Survey with percolation testing Where a new drainage field may be needed | £250 to £500 |
| Emptying before inspection Often needed so the tank walls can be seen | £120 to £350 |
Where our information comes from
Installed prices are typical ranges from published UK price guides, autumn 2026, summarised by us and read in November 2026. The guides rarely say whether VAT is included, so ask every installer for prices on the same basis. A guide for budgeting, not a quote.
What a good survey looks at
Ask for a written report that answers each of these, with photographs.
The system
Type (septic tank, treatment plant or cesspit), material, approximate age, capacity against the number of bedrooms, and its physical state: walls, baffles or tees, lids, inlet and outlet. For a treatment plant, whether the blower and alarm work and when it was last serviced.
The outlet
Where the effluent goes is the question that decides whether the system is legal. A dye test or a camera can trace it. A septic tank discharging to a stream, river or ditch in England or Wales has to be dealt with, and the Environment Agency guidance says buyer and seller should agree in writing who will do it as part of the sale.
The drainage field
Whether it exists at all, whether it is a proper field or an old soakaway pit, and whether the ground over it is wet or sunken.
The paperwork
Under the binding rules in England, the seller must give the new operator written details of the system: what it is, where its parts are, changes made, maintenance requirements and records. In Scotland the SEPA registration and in Wales the NRW registration should be passed on too.
Questions to ask the seller
Ask through your conveyancer and get the answers in writing.
- When was it last emptied?
- The rules expect at least once a year. Ask for the waste carrier's receipts.
- Where does it discharge?
- Ground or watercourse. "Not sure" is a reason to survey before exchange.
- Is it shared?
- Shared systems need an agreement on access, emptying and repairs between the properties.
- Is it registered?
- In Scotland with SEPA, in Wales with NRW. England has no register for systems under the binding rules.
How a survey runs
Book and empty
The tank is often emptied first so its walls, baffles and outlet can be inspected.
Inspect
Lids lifted, chambers checked, plant equipment tested, outlet traced by dye or camera.
Check the field
Ground over the field walked and probed; test holes dug if a new field may be needed.
Report
A written report with photos, whether the system meets the rules, and the cost of any work needed.
Buying or selling a house off mains drainage? Get the system checked before exchange.
Ask for a site visitQuestions about septic tank surveys
How much does a septic tank survey cost?
Published guides put it at about £250 to £300, or up to about £500 with percolation testing. Emptying the tank first, if needed, is extra.
Does a homebuyer survey check the septic tank?
Usually not in any depth. A RICS homebuyer survey will normally note that the property has private drainage and recommend a specialist inspection.
Who is responsible for a non-compliant septic tank when a house is sold?
In England the Environment Agency says buyer and seller should agree in writing who replaces a septic tank discharging to a watercourse, as a condition of sale. Your conveyancer will put it in the contract.
What paperwork should the seller hand over?
Under the general binding rules in England: a description of the system and drainage, where the parts are, any changes made, the maintenance manual and requirements, and any maintenance records.
Can I get a mortgage on a house with a septic tank?
Generally yes. Lenders and their valuers may ask for a specialist survey or a retention if the system looks non-compliant or failing.
Keep reading
- Septic tank replacement
When the old tank has to go, and what goes in its place.
- Sewage treatment plant installation
Powered treatment for a stream outlet or poor ground.
- Septic tank regulations
England, Scotland and Wales side by side.
- Septic tank installation cost
Estimator with tank sizes and drainage field areas.
Ask for a septic tank survey
Three short steps: where the property is, what the system needs, and how to reach you. About two minutes.
A free matching service, not a off-mains drainage installer. We look for one who works in your area and name them to you first. Nothing is shared until you say yes.
Thanks, your enquiry is in.
- We look for an off-mains drainage installer who works in your area and takes on this kind of job, then email or call to tell you who it is.
- Your details reach that firm only if you say yes.
- They arrange a site visit, test the ground if needed and give you a written price. No marketing from us, ever.