Selling a Cornwall property with a septic tank is straightforward — most of the time. The 10% of cases that get messy are almost always preventable: an undisclosed direct discharge to a watercourse, an unrecorded soakaway failure, a non-compliant system that the buyer's solicitor flags during conveyancing. By that point you've lost negotiating leverage and you're racing the chain. This guide covers what you legally must disclose, what a pre-sale check costs, and how to handle the most common issues that come up.
The TA6 form — what you must declare
The Law Society's TA6 Property Information Form (6th edition, 2025) is the seller's formal disclosure of what they know about the property. Section 12 (Services) requires you to disclose drainage details. For a property with a septic tank, you must state:
- System type — septic tank, small sewage treatment plant, or cesspit
- Whether shared with neighbouring properties (and how many)
- Discharge type — drainage field/soakaway, watercourse, or other
- Emptying frequency and who the regular contractor is
- Permit status — whether the property has an Environment Agency permit or relies on the General Binding Rules
- Any known issues — past problems, repairs, ongoing concerns
- Any notices from the Environment Agency or local authority
The form has been strengthened in recent years; the 6th edition (2025) requires more detail on drainage than older versions. False or misleading declarations can be grounds for post-completion claims against the seller.
The compliance gotcha: GBR 2020
Since 1 January 2020, a septic tank discharging directly to a watercourse (stream, ditch, river) without an Environment Agency permit is non-compliant. Sellers must declare this on TA6, and buyers' solicitors will require it to be remedied before completion. Remediation cost: £3,000-£15,000 depending on what's needed.
The trap: many older Cornwall sellers don't actually know where their tank discharges. The previous owner installed it; nobody asked; "it's always worked." Then the buyer's solicitor raises an enquiry, and you have weeks to figure it out under chain pressure.
See our GBR 2020 guide for the full picture.
The pre-sale check — your best £250
Before listing, commission a basic septic tank survey. Cost: £200-£500. What it gives you:
- Confirmation of discharge type (drainage field or watercourse)
- Visual inspection of tank condition
- CCTV of pipework (recommended addition, £150-£250)
- Drainage field walk-over
- Written assessment you can present to buyers
If issues turn up, you have time to address them on your terms — getting 2-3 quotes for any remediation, choosing your contractor, scheduling around your timeline. If issues only emerge during conveyancing, you have none of those advantages.
What buyers' solicitors typically ask
Once the property is on the market and an offer is accepted, expect enquiries like:
- "Please confirm the location and type of the drainage system"
- "Please provide waste transfer notes from the last 3 years"
- "Please confirm GBR 2020 compliance"
- "Please clarify whether the system is shared with neighbouring properties"
- "Please provide any Environment Agency correspondence or permits"
- "Please confirm any treatment plant service contract terms"
The better you've prepared, the faster you can answer. Photocopies of waste transfer notes, an old installation invoice, a recent service certificate — these all reassure the other side and speed completion.
Common pre-completion remediation scenarios
Direct discharge to watercourse
Two solutions:
- Replace septic tank with sewage treatment plant (which can legally discharge to watercourse with EA permit). Cost: £5,000-£12,000.
- Install a drainage field. Cost: £3,500-£6,500.
Negotiation: typically you either fix before completion (and price the work in) or accept a price reduction reflecting the cost (usually 60-80% of the quoted work).
Soakaway near end-of-life
Survey shows saturation, advanced biofilm, or evidence the field is failing. Likely £2,000-£5,000 to rebuild. Most reasonable buyers will accept either pre-completion repair or a price reduction.
Tank cracks or structural issues
Visible damage; effluent leaking. Repair £400-£2,500 or full replacement £3,000-£8,000. Often resolvable with a price adjustment.
Undocumented / unknown system
You don't actually know what kind of system you have. Pre-sale survey clarifies and provides documentation. Sometimes this turns out to be a cesspit pretending to be a septic tank (which materially affects the property's running costs and resale value).
Documents to gather before listing
- Waste transfer notes for last 3 years (from your waste carrier)
- Treatment plant service records (if applicable)
- Original installation documentation (if available)
- Any EA correspondence or permits
- Recent CCTV survey or condition report
- Any guarantee certificates from past repairs
If you don't have these, your waste carrier and (if applicable) treatment plant service company can often reconstruct them from their records.
Pricing the property fairly
If your system is in good condition with clear records, your property's septic tank shouldn't reduce its value vs comparable mains-drained properties. If there are issues:
- Minor issues (small repairs needed): typically priced in by £500-£2,000 below comparable
- Moderate issues (soakaway rebuild or major repair): £3,000-£8,000 reduction expected
- Major issues (full system replacement, GBR non-compliance): £10,000-£20,000 reduction or remediation pre-completion
Cornwall's high proportion of off-mains properties means buyers are generally familiar with the issues — they'll discount fairly for problems, but they don't reject properties just for having a septic tank.
The estate agent conversation
Tell your estate agent about the septic system upfront — they'll need to disclose it in marketing materials. Push back if they suggest hiding it; modern buyers research drainage before viewing and an undisclosed septic tank is a major trust issue if discovered mid-process.
Selling a Cornwall property with a septic tank? Submit your postcode for a pre-sale survey — typically £250-£450 — and avoid the conveyancing surprises.
Frequently asked questions
What do I have to declare about my septic tank when selling?
On the TA6 Property Information Form (Section 12), you must disclose: system type (septic tank/treatment plant/cesspit), whether shared, discharge type (soakaway or watercourse), emptying frequency, EA permit status or GBR compliance, any known issues, and any agency notices. The 6th edition (2025) requires more detail than older versions.
Do I need to fix septic tank issues before selling?
Legally, you must disclose honestly on TA6 — you're not legally required to fix everything. In practice, buyers' solicitors typically require GBR 2020 compliance before completion. Other issues are usually negotiable as either pre-completion fixes or price reductions.
How much will septic tank issues knock off my Cornwall property's value?
Minor issues (small repairs): £500-£2,000 below comparable. Moderate (soakaway rebuild): £3,000-£8,000 reduction. Major (full replacement, GBR non-compliance): £10,000-£20,000 or remediation before completion. Well-maintained systems with records don't reduce property value vs comparable mains-drained properties.
Should I get a pre-sale septic tank survey?
Yes, ideally before listing. £200-£500 for visual survey, £250-£450 with CCTV. Catches issues on your terms (your contractor, your timeline, your negotiating position) rather than during conveyancing under chain pressure. Highest-ROI piece of pre-sale preparation for rural Cornwall properties.
What if my buyer's solicitor raises septic enquiries I can't answer?
Don't guess. Contact your waste carrier (waste transfer notes), past contractors (installation records), or commission a quick survey (£200-£400, 1-2 weeks). Buyers' solicitors accept reasonable timescales for honest answers; they reject guesswork.