Around 4–5% of UK homes (roughly 1 million properties, per Water UK estimates) run on private drainage rather than mains sewers — but in rural areas of the South West, including Cornwall, that figure rises to 25–30% of properties. Buying one of these properties isn't more complicated than buying a mains-drained house, but it's different in ways that aren't covered in your standard conveyancing pack. This 12-point checklist is the diligence walk-through we recommend every buyer does before exchange. Most of it is conversation; the rest can be done in an hour with the right specialist.
The 12-point checklist
1. Confirm what type of system the property has
Is it a septic tank, a sewage treatment plant, or a cesspit? Each has very different running costs and regulatory requirements. The seller's TA6 property information form should specify, but it often gets it wrong (vendors don't always know themselves). Our comparison guide covers the differences in detail.
2. Find out where the system discharges
Drainage field / soakaway? Direct to ditch? Direct to watercourse? Direct to a soakaway then watercourse? This is the single biggest determinant of compliance status and remediation cost. The vendor should know. If they don't, ask their solicitor to clarify with the previous owner or installer. If still unclear, a CCTV survey can trace it.
3. Check General Binding Rules 2020 compliance
Septic tanks discharging directly to a watercourse have been non-compliant since 2020. Pre-purchase is the point at which non-compliance becomes the buyer's problem — sellers are legally required to fix or disclose. See our GBR 2020 guide. If the system isn't compliant, that's £3,000–£15,000 of remediation work, and you should price it in.
4. Get the emptying history
Ask for waste transfer notes from the last 3+ years. These document who emptied the tank, when, and how much was removed. A consistent annual pattern is reassuring. A 5-year gap is a warning sign. No records at all is a problem — either the tank has genuinely not been emptied (unlikely with a healthy system) or the records were lost/never kept.
5. Confirm tank capacity vs household size
Tanks should be sized to BS 6297. A 4-person home needs at least 2,700L; a 6-person home needs 3,750L+; holiday lets need much larger systems for peak occupancy. An undersized tank fills faster, costs more to run, and overflows more easily. If you're buying to convert a 2-bed cottage into a 6-bed holiday let, the existing tank almost certainly won't cope — see our holiday let guide.
6. Commission a proper septic tank survey
Standard RICS homebuyer surveys are inadequate for septic systems — see our survey guide for what a proper one covers. Budget £250–£450 for visual + CCTV. This is the single highest-ROI spend in the conveyancing — it can flag £15,000 of issues for a £400 outlay.
7. Check permits and registrations
Tanks discharging to a watercourse need an Environment Agency permit. Larger volumes need permits regardless of discharge point. Tanks in Source Protection Zones (drinking water borehole catchments) need permits. Properties near SSSIs may need permits. Ask for current permit documentation — see our registration guide.
8. Check any maintenance contracts
Sewage treatment plants often have annual service contracts (typically £100–£250/year). The contract may transfer with the property or terminate at sale. Find out which, and budget accordingly. Septic tanks rarely have formal contracts, but the seller may have a regular waste carrier — useful continuity for you.
9. Verify tanker access
Drive (or walk) the route a tanker would take from the nearest road to the tank lid. Width? Overhanging branches? Soft ground? Gates? Standard tankers need ~3m width, 4m headroom, and firm ground. If access is restricted, you'll pay 20–50% more per empty for smaller-vehicle access for the life of the property. Worth knowing.
10. Inspect the drainage field
Walk over it. Soggy ground? Lush dark-green vegetation in patches? Reeds or rushes where they shouldn't be? Standing water? Any of these are early signs of soakaway failure — £2,000–£5,000 to rebuild. See our soakaway guide for the warning signs.
11. Sort insurance implications
Confirm with your buildings insurer that they cover properties with septic tanks. Most do, but some specialist insurers offer better coverage for drainage damage (worth checking if you've inherited an older system). Most policies exclude "gradual damage" but cover sudden events. Document the system's condition at purchase as a baseline.
12. Check the TA6 carefully
The TA6 form is the seller's formal declaration of what they know about the property. Check the drainage section in detail:
- System type and location confirmed?
- Maintenance history declared?
- Any known issues or works carried out?
- Permits and registrations referenced?
- Any disputes or notices from the Environment Agency?
If anything's missing or vague, your solicitor should raise enquiries. False declarations can be grounds for post-completion claim, but it's far easier to resolve before exchange than after.
The "deal-breaker" red flags
- Cesspit in a remote / expensive-to-empty location — adds £2,000–£3,500 to annual running costs forever
- Direct watercourse discharge without permit — £8,000–£15,000 to fix
- No drainage field at all — system redesign needed
- Tank in a location now inaccessible (e.g., extension built over)
- Multiple stacked problems — tank cracked + soakaway saturated + non-compliant discharge = full replacement, £10,000+
- Environmental Agency enforcement notice on the property — read carefully; often resolvable but cost depends on the notice
What's it actually going to cost you?
Rough annual cost expectations:
| System | Annual running cost | 20-year total (excl install) |
|---|---|---|
| Septic tank | £150–£400 | £3,000–£8,000 + 1 soakaway rebuild |
| Treatment plant | £300–£600 | £6,000–£12,000 + parts |
| Cesspit | £1,500–£3,500 | £30,000–£70,000 |
If you're choosing between two otherwise-similar properties, this matters. A cesspit property is effectively £25,000–£60,000 more expensive over 20 years than a septic-tank property. Either factor it into the offer or pass.
Negotiating after the survey
If your survey turns up issues, you have three normal options:
- Seller fixes pre-completion. Best for smaller issues. Specify a competent specialist and a sign-off process.
- Price reduction. Most common for bigger issues. 2–3 quotes for the work, present the lowest, expect 60–80% acceptance.
- Retention from completion funds. Solicitor holds back the remediation cost. Cleanest if work needs to happen post-completion.
The short version
Don't buy a property with a septic tank without:
- A proper septic tank survey (£250–£450)
- Emptying records for the last 3 years
- Clear answers on discharge point and GBR compliance
- Photos and notes of the drainage field condition
- An honest budget for the next 5–10 years of maintenance
Get those right and septic-tank properties are no more risky than mains-drained ones — and often considerably cheaper to run for the same square footage.
Considering a Cornwall property with a septic system? Submit your postcode and we'll match you with a licensed specialist for a pre-purchase survey.
Frequently asked questions
Should I avoid buying a property with a septic tank?
No — properly maintained septic systems work fine for decades. The risk is buying a poorly-maintained one without knowing. Proper diligence eliminates almost all the risk. A good survey costs £250–£450 against a property price that's typically £200,000+.
Are septic-tank properties harder to mortgage?
Slightly. Most major UK lenders accept septic systems, but some require evidence of GBR compliance and a recent survey. Specialist or "rural mortgage" products are usually no problem. Your mortgage broker can confirm specifics.
Are septic-tank properties harder to insure?
Not generally — most buildings insurance accepts septic systems as standard. Some policies offer better drainage damage cover (worth comparing). Most exclude "gradual damage" but cover sudden events like tree-falls onto pipework.
Can I switch from a septic tank to mains drainage after I move in?
Only if mains is available. In rural Cornwall, the cost of running a new mains connection can be £10,000–£40,000+ depending on distance — and the water company has to agree. For most rural properties, upgrading the septic system is cheaper than going on mains.
How long do septic tank surveys take?
1–2 hours on site for visual, plus a couple of days for the written report. Add 1–2 weeks lead time for booking. Plan it early in the conveyancing process — don't leave it until exchange week.