Last updated: September 2026
If your property in Surrey, Sussex or Kent is not connected to mains sewerage, you will at some point have to choose between a septic tank and a sewage treatment plant. Both do the same basic job, which is to deal with the wastewater from your home. They go about it in very different ways, and the wrong choice can leave you with a system that is not compliant, or with a bill for a permit or a replacement that you did not need.
We have been installing both types across the South East for over 40 years. This guide sets out how they differ, roughly what each costs to install and run, and which one suits most rural properties we visit.
How a Septic Tank Works
A septic tank is a simple, passive system. Wastewater flows into an underground tank where the solids settle to the bottom and fats and grease float to the top. The liquid in the middle leaves the tank and passes to a drainage field, sometimes called a soakaway, where it soaks slowly into the ground and is treated naturally by the bacteria in the soil.
There are no moving parts and nothing electrical. Gravity and nature do the work. That keeps installation costs down and, when the tank and drainage field are working properly, keeps maintenance simple too. The trade-off is that the liquid leaving the tank is only partly treated. That matters a great deal under the current rules in England.
How a Sewage Treatment Plant Works
A sewage treatment plant, often called a package treatment plant, uses a mechanical process alongside the biological one. Most modern domestic plants pump air into the tank so that oxygen-loving bacteria break the sewage down far more thoroughly than the conditions inside a septic tank allow. The Klargester BioDisc and BioTec models we fit most often both work on this principle.
The treated water that leaves a plant is much cleaner than septic tank effluent. In England that cleaner discharge can, in most cases, go to a ditch, stream or surface water drain, which is not allowed for a septic tank.
The Legal Position: the General Binding Rules
This is where the decision becomes more than a matter of price. The General Binding Rules for small sewage discharges came into Force in England on 1 January 2020. Under those rules a septic tank must discharge to ground through a drainage field. A septic tank that discharges to a surface water drain, ditch or watercourse does not meet the rules, and the Environment Agency required those systems to be replaced or upgraded by 1 January 2020, or sooner if the property was sold. That deadline has long passed. In 2026 any septic tank still discharging to surface water is operating outside the rules, and the only lawful ways forward are to upgrade to a treatment plant, install a proper drainage field where the ground allows, or apply for an individual permit from the Environment Agency, which carries an application fee and ongoing conditions.
For a new installation serving up to 10 people, a septic tank discharging to a correctly designed drainage field is still permitted under the rules. A sewage treatment plant that meets BS EN 12566-3 and is sized correctly can discharge to surface water under the same rules in most situations.
Which Is Cheaper to Install?
As a guide for 2026, a septic tank with a new drainage field typically comes in somewhere between £3,500 and £7,000 installed, depending on tank size and ground conditions. A sewage treatment plant is usually in the region of £7,000 to £15,000 installed. The difference is mostly the plant itself, the electrical supply it needs and the commissioning work.
Ground conditions can settle the question on their own. Much of the Weald has heavy clay, and parts of Kent and Sussex have a high water table in winter. On those sites a drainage field may not be viable at all, which makes a treatment plant the only practical option whatever the price difference. A percolation test tells us this before any money is spent.
Which Is Cheaper to Run and Maintain?
On paper the septic tank wins. A well-functioning tank needs emptying every 12 to 18 months at roughly £150 to £300 a time, and that is about it. A treatment plant needs an annual service, typically £150 to £250, more regular desludging, and a small amount of electricity to run the air pump, which most owners find costs between £50 and £100 a year.
Those sums change quickly if a septic tank needs an Environment Agency permit, or if a tired drainage field has to be rebuilt, which can run to several thousand pounds. Many of the homeowners we work with decide that the modest extra running cost of a plant is a fair price for knowing the system complies.
Long-Term Property Value
A compliant, well-installed sewage treatment plant is generally seen as the more future-proof option. When a property is sold, the buyer’s solicitor will ask about the drainage, and a septic tank that does not meet the General Binding Rules can hold up a sale or knock money off the price. We are asked to survey systems for exactly this reason several times a month.
Making the Decision
For a property with free-draining ground, room for a drainage field and no need to discharge to surface water, a correctly installed septic tank remains a sensible, economical choice that meets the rules.
A sewage treatment plant is almost always the better answer, and usually the cheaper one over its life, where the effluent has to go to a ditch, drain or watercourse, where the ground will not support a drainage field, where the property sits close to a watercourse or within a Source Protection Zone, or where you simply want a fully treated discharge and a system that will stay compliant for as long as you own the house.
Get a Professional Assessment
Ground conditions can vary from one end of a garden to the other. A percolation test and a drainage survey are the only reliable way to choose between the two systems. Country Drainage offers free site surveys across Surrey, Sussex and Kent, and we will give you a fixed-price quote for the right system for your property. Contact us to arrange a visit.
Frequently Asked Questions
Can I upgrade a septic tank to a sewage treatment plant?
Yes. In most cases the existing tank is decommissioned and a treatment plant installed in its place. Sometimes the old tank chamber can be kept as a settlement chamber ahead of the new plant, which reduces the cost of the work.
How long does installation take?
A straightforward replacement is usually finished in three to five working days. Difficult access, poor ground or a long electrical run can add to that.
Do I need planning permission?
Planning permission is not normally needed for the drainage system itself, but Building Regulations approval is. If your property is listed, in a conservation area or in a flood zone, check with your local planning authority first. Our planning permission guide covers this in detail.