Where can you buy a septic tank in Cornwall?

You have three main routes to buy a septic tank or sewage treatment plant in Cornwall: national online tank merchants who deliver county-wide (usually the widest choice and keenest supply-only prices), builders' merchants who can order units in, and local installers who supply the tank as part of a supply-and-fit package. Most homeowners end up with the installer route, because the installer then owns the whole job — sizing, warranty, groundworks and commissioning. Supply-only makes sense mainly if you already have a groundworker you trust to do the installation.

Which brands of septic tank and sewage treatment plant are sold in the UK?

The established names in UK septic tanks and domestic sewage treatment plants include Klargester (part of Kingspan), Tricel, Marsh Industries, Graf, Harlequin, WPL and Clearwater. All the mainstream brands sell units certified to BS EN 12566 (Part 1 for septic tanks, Part 3 for packaged treatment plants), which is the certification that matters legally in the UK. Between reputable brands, differences come down to treatment technology, running cost, servicing needs and local installer familiarity rather than one brand being clearly best.

How much does a septic tank cost to buy on its own in the UK?

Supply-only, a domestic GRP septic tank of around 2,800–4,000 litres typically costs about £800–£1,800 including VAT in 2026, with some entry models under £1,200. A packaged sewage treatment plant for a typical household (up to about 6 people) usually costs £2,500–£6,000 supply-only. The tank is the small part of the total project: installation — excavation, bedding, pipework, any electrics, commissioning and reinstatement — usually costs considerably more than the unit itself, which is why full projects run to several thousand pounds.

Why is supply-and-fit so much more expensive than buying the tank on its own?

Because the tank is the cheap part. A domestic septic tank might cost £800–£1,800 to buy, but installing it means excavation (harder and pricier in Cornwall if the digger hits granite), moving spoil, bedding or concrete backfill, pipework, possibly a drainage field, electrical connection for treatment plants, commissioning and putting the garden back. That's plant hire, materials and several days of skilled labour, so full supply-and-fit projects typically land in the £5,000–£12,000 range. You also get one party responsible for the whole job — with supply-only, the tank warranty and workmanship guarantee sit with different people, and a fault can turn into finger-pointing.

What size septic tank or treatment plant do you need?

Domestic systems are sized by population. The convention used across the UK industry, based on British Water's guidance, is to size for the number of bedrooms plus two — so a 3-bedroom house is sized as 5 people, regardless of who actually lives there, because the system must cope with the house at full occupancy. For a typical 3-bedroom home that means a septic tank of roughly 2,700–3,750 litres or a treatment plant rated around 5–6 population. Holiday lets and properties with high occupancy should size generously. Get the installer to confirm the sizing in writing — an undersized system fails early and can put you in breach of the General Binding Rules.

What should you check before buying a septic tank or treatment plant?

Before buying a septic tank or sewage treatment plant in the UK, check four things. First, certification: septic tanks should be certified to BS EN 12566-1 and packaged treatment plants to BS EN 12566-3 — don't buy a unit without it. Second, sizing: confirm the capacity follows the bedrooms-plus-two convention for your property. Third, suitability for your ground: high water tables need anchored or concrete-backfilled tanks, and some units suit shallow digs or rocky sites better than others. Fourth, the paper trail: warranty terms, what commissioning is included, and servicing availability for that brand in your area — a cheap unit nobody local services is a false economy.

Septic tank suppliers in Cornwall: a buyer's guide

If you search “septic tanks Cornwall” you’ll get a wall of suppliers, installers and merchants all pitching at once. This guide sorts the landscape: who actually sells tanks, what the units cost on their own, why supply-and-fit quotes look so different from supply-only prices, and what to check before you order. We don’t recommend specific local companies — this is the map, not a directory.

If you’re at the earlier “what do I even need” stage, start with septic tank vs treatment plant and sewage treatment plants explained.

The three ways to buy

National online merchants. Several large UK merchants sell tanks and treatment plants online and deliver nationwide, Cornwall included. Widest choice, most transparent pricing, and useful for benchmarking even if you buy elsewhere. Delivery of a bulky GRP tank to a narrow-lane property is a real consideration — check the delivery vehicle and whether offloading is included.

Builders’ merchants. Local branches can order in tanks from the main brands. Convenient if you have a trade account, but off-mains drainage is a sideline for them — don’t expect sizing advice beyond the brochure.

Installer-supplied. The most common route: a drainage or groundworks contractor quotes for the whole job and supplies the tank within it. You pay a margin on the unit, but the installer owns sizing, warranty registration, groundworks and commissioning — one throat to choke if anything goes wrong.

The brands you’ll actually see

The established UK names include Klargester (part of Kingspan, probably the most recognised), Tricel, Marsh Industries, Graf, Harlequin, WPL and Clearwater. All the mainstream brands certify to the standards that matter (more below), so between reputable makes the honest differences are treatment technology, electricity use, servicing needs, and — underrated — which brands your local installers know well. An installer who fits one brand weekly will commission it properly and fix it quickly; the obscure import saved £400 and costs it back the first time it needs a part.

We deliberately don’t rank brands. Fit for your site and local servicing matter more than the badge.

What the units cost, supply-only

Ballpark 2026 figures, including VAT:

Septic tank, 2,800–4,000 litres (typical domestic): roughly £800–£1,800. Entry-level 2,800-litre units from major brands start around £1,100–£1,500.
Packaged treatment plant, up to ~6 population: roughly £2,500–£6,000 depending on brand and technology.
Larger plants for big houses or multiple dwellings run higher again.

Prices move around, so treat these as ballpark and check current listings. The more useful number is the next one.

Why supply-and-fit costs so much more

The tank is the cheap part of the project. Full supply-and-fit typically lands at £5,000–£12,000 for a domestic system — see the cost guide for the breakdown — because installation means excavation, spoil away, bedding or concrete backfill, pipework, possibly a new drainage field, electrics for a treatment plant, commissioning and reinstatement.

Cornwall adds its usual seasoning: granite close to the surface makes digging slower and dearer, high water tables near the coast can require anchoring the tank so it doesn’t float, and narrow-lane access complicates both the excavator and the delivery lorry. This is why buying the tank yourself saves less than it appears — the unit might be 15–25% of the project cost, and a supply-only purchase leaves you coordinating delivery, warranty registration and a groundworker, with nobody owning the job end to end.

Supply-only makes sense in one situation: you already have a groundworker you trust, and the manufacturer or merchant will support commissioning. Otherwise, get supply-and-fit quotes and compare those. The full process is covered in new septic tank installation.

The four checks before you order

1. Certification. Septic tanks should be certified to BS EN 12566-1, packaged treatment plants to BS EN 12566-3. Every mainstream UK brand complies; the standard exists to filter out the ones that don’t. No certificate, no purchase — an uncertified unit can leave you unable to show compliance when you sell.

2. Sizing. UK industry convention, from British Water’s guidance, is bedrooms plus two: a 3-bed house is sized for 5 people whoever lives there now, because the system must cope with full occupancy. That’s roughly a 2,700–3,750 litre septic tank or a 5–6 population treatment plant for a typical 3-bed. Holiday lets should size generously — they run at full occupancy all season. Get sizing confirmed in writing.

3. Ground suitability. Tell the supplier about your site: water table, rock, dig depth, distance from the drains. Some units are designed for shallow digs or wet ground; matching the unit to the site is cheaper than fighting the site.

4. The paper trail. Warranty length and what voids it, whether commissioning is included, and — for treatment plants — who services that brand in Cornwall and what an annual service costs. A plant is a machine; buy one someone nearby can maintain.

Next step

Shortlist two or three supply-and-fit quotes, make each state the brand, model, capacity and certification in writing, and run the totals against our cost calculator to see if anything looks off.

Sources: GOV.UK — Permits you need for septic tanks · GOV.UK — General binding rules: discharge to a surface water · British Water — Code of Practice: Flows and Loads 4 (PDF)