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Buying a house with a septic tank: what to check
Off-mains drainage is normal across rural Cornwall — most of it is unremarkable and stays that way for decades. But it is buried, rarely documented, and the one part of a house that a buyer cannot see. These are the things worth establishing before you are committed, because afterwards they are yours.
The eight questions
1. Where does it discharge? The single most important one. To a drainage field (soaking away into the ground) is normal. To a watercourse, ditch or stream means the system must be replaced or upgraded under the General Binding Rules — and a sale is a recognised trigger for dealing with it. If the answer is “we’re not sure”, treat that as a discharge to a watercourse until proven otherwise.
2. What type of system is it? A septic tank, a sewage treatment plant, and a cesspit are three different things with different rules and very different running costs. See septic tank vs treatment plant and cesspit vs septic tank — a cesspit stores rather than treats, and needs emptying several times a year, which is a meaningful ongoing cost.
3. How old is it, and when was it last emptied? Ask for records. Waste transfer notes are the good evidence — a licensed carrier issues one every visit. No records at all is not proof of a problem, but it is a reason to survey.
4. Does it serve only this property? Shared systems are common here. If it is shared, establish who maintains it, how costs are divided, and whether there is anything in writing. See shared septic tank.
5. Where is the drainage field, and is it on land you’re buying? If any part of the system — tank, pipework or field — sits on land you will not own, you need documented rights to access, maintain and replace it. Without them you can end up legally responsible for something you cannot lawfully reach.
6. Is there a pumping station? If the property sits below the road, wastewater may be pumped rather than gravity-fed. That is an extra piece of machinery to maintain — and it may not be yours at all: many private pumping stations transferred to the water company in 2016. Two questions settle it, in is my pumping station mine or the water company’s?
7. Any signs of failure? Wet or unusually green ground over the drainage field, smell, slow drainage indoors, gurgling. Walk the garden, ideally after rain. See common problems and septic tank smells.
8. Has it been extended past its capacity? Extensions, annexes and barn conversions add bathrooms to systems specified for smaller houses. An overloaded system fails early regardless of how well it is maintained.
Get a drainage survey
A mortgage valuation will not tell you any of this — it is a brief visual inspection for the lender’s benefit, and the valuer will not lift a cover. A standard homebuyer’s report will not investigate buried drainage properly either.
A dedicated drainage survey is £250–£500 plus VAT and establishes the system type, discharge point and condition. Against a possible £6,000–£12,000 replacement — see the replacement cost guide — it is the cheapest insurance available in the whole transaction, and it is the one report written for you rather than for the lender.
Commission it before the mortgage valuation if you can. Arriving at your lender with an answer is a very different experience from arriving with a question — the reasons are in will a lender lend on a septic tank property?
”It’s never given us any trouble”
Sellers say this in good faith and it is usually true. It is also not an answer to any of the eight questions above.
A septic tank can run for twenty years without a symptom while discharging somewhere it should not. Absence of trouble is not compliance, and the person selling frequently inherited the system without documentation themselves. Ask for evidence rather than reassurance — and if none exists, that is exactly what the survey is for.
If the answer comes back badly
It is usually manageable, and it is normally a negotiation rather than a deal-breaker. Who bears the cost is not fixed by law — who pays sets out the four ways it typically resolves, and how long a replacement takes covers whether it can realistically be done before completion.
The buyers who get caught out are not the ones who found a problem. They are the ones who never asked.
Sources: GOV.UK — Permits you need for septic tanks · GOV.UK — General binding rules: discharge to a surface water · Water Industry (Schemes for Adoption of Private Sewers) Regulations 2011
Common questions
What should I ask before buying a house with a septic tank?
Establish where it discharges, what type of system it is, how old it is, when it was last emptied, whether it serves only your property, where the drainage field is, and whether any part of the system sits on land you will not own. The discharge point matters most: a septic tank discharging to a watercourse or ditch must be replaced or upgraded under the General Binding Rules, and a sale is a recognised trigger for putting it right — so it can become your bill within weeks of moving in.
Should I get a separate drainage survey when buying?
Yes, on any off-mains property. A mortgage valuation is a brief visual inspection for the lender and a standard homebuyer's report will not investigate buried drainage properly. A dedicated drainage survey typically costs £250–£500 plus VAT and is the only way to establish the system type, discharge point and condition before you are committed. Against a potential £6,000–£12,000 replacement, it is the cheapest insurance in the transaction.
What if the septic tank is shared with a neighbour?
Shared systems are common in rural Cornwall and they need care. Establish who is responsible for maintenance and emptying, how costs are split, whether there is a written agreement, and whether rights of access exist to reach the tank and drainage field. A shared system with no agreement is a future dispute, and it is far easier to sort out before exchange than afterwards. Note also that a shared sewage pumping station connected to the public sewer may have transferred to the water company in 2016.
Can the drainage field be on someone else's land?
It happens, and it is a serious point to check. If the drainage field, the tank or the pipework runs across land you will not own, you need documented rights of access to maintain, repair and replace it. Without them you may be unable to carry out work you are legally obliged to do. Your solicitor should raise it, but only if someone establishes where the system actually runs — which is what a drainage survey is for.
The seller says the tank is fine. Is that enough?
No — not because sellers are dishonest, but because most genuinely do not know. Septic systems are buried, undocumented and often predate the current owner. 'It's never given us any trouble' describes the absence of symptoms, not compliance, and a tank can run for years without complaint while discharging somewhere it should not. Ask for evidence: emptying records, any survey, and a clear statement of where it discharges.
Need this sorting in Cornwall?
Kernow Septic covers Cornwall's towns and rural parishes. Call or run the free tools.