Can I still install a septic tank in the UK?

Yes, but since 1 January 2020, the General Binding Rules require that septic tanks discharge to a drainage field (soakaway) — not directly to a watercourse. If your site cannot accommodate a drainage field, you must install a sewage treatment plant instead, which can discharge to a watercourse with an environmental permit or under the General Binding Rules.

What is the difference in discharge quality?

A septic tank produces partially treated effluent (typically 200-300 mg/l BOD) which must pass through a drainage field for final treatment in the soil. A sewage treatment plant produces much cleaner effluent (typically 20-40 mg/l BOD), clean enough to discharge to a ditch or watercourse. Treatment plants use aeration or rotating biological contactors to break down waste more effectively.

How much does each system cost to install?

A septic tank costs £2,000-4,000 for the tank plus £2,000-5,000 for a drainage field, totalling £4,000-9,000 installed. A packaged sewage treatment plant costs £4,000-8,000 for the unit plus £1,000-3,000 for installation and discharge pipework, totalling £5,000-11,000. Treatment plants have higher running costs (electricity for the air blower) of £50-80/year.

How often does each system need emptying?

Septic tanks typically need emptying every 12 months (cost £150-250 per empty). Sewage treatment plants usually need desludging every 6-12 months depending on the model and usage, plus an annual service of the mechanical/electrical components (£100-200/year). Some modern treatment plants have very long desludge intervals.

Do I need planning permission for off-mains drainage?

Septic tanks and treatment plants under 7,000 litres are usually permitted development for single dwellings. However, you must comply with the General Binding Rules (England) or equivalent regulations in Scotland and Wales. Building regulations approval is needed for any new installation, and you should consult the Environment Agency if discharging within 50m of a water source.

Septic Tank vs Sewage Treatment Plant

Compare septic tanks with packaged sewage treatment plants under the General Binding Rules 2020 for UK off-mains drainage.

Side-by-Side Comparison

Feature	Septic Tank	Sewage Treatment Plant
Install cost (total)	£4,000-9,000	£5,000-11,000
Discharge quality (BOD)	200-300 mg/l	20-40 mg/l
Discharge to watercourse	Not permitted	Permitted (with GBR/permit)
Drainage field required	Yes — essential	No (can use direct discharge)
Electricity required	No	Yes (air blower ~£50-80/year)
Emptying frequency	Every 12 months	Every 6-12 months
Annual servicing	Visual check only	Mechanical service £100-200
Moving parts	None	Air blower, pump(s)

When to Choose a Septic Tank

Site has suitable ground for a drainage field (percolation test passes)
No nearby watercourse to discharge to
You want a simple, no-electricity system
Budget is tight and soil conditions are suitable
Remote location where electricity supply is unreliable

When to Choose a Treatment Plant

Ground conditions fail percolation test (clay soil, high water table)
Insufficient space for a drainage field
Site is near a watercourse suitable for treated discharge
Environmental sensitivity requires high discharge quality
Replacing a non-compliant pre-2020 septic tank discharging to a watercourse

Related Calculators

Use the Septic Tank Calculator to size a septic system, or the Sewage Treatment Plant Calculator for treatment plant sizing.

How We Calculate This

Frequently Asked Questions

Last updated: April 2026

All calculations are estimates. Verify with your supplier.

Septic Tank vs Sewage Treatment Plant

Side-by-Side Comparison

When to Choose a Septic Tank

When to Choose a Treatment Plant

Related Calculators

How We Calculate This

Frequently Asked Questions

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