The Basics
What is off-mains drainage?
"Off-mains drainage" simply means a property that isn't connected to the public sewer network and instead treats its own wastewater on site. It covers several different system types, each with different rules, costs and maintenance needs.
Roughly 1 in 5 rural UK properties are off-mains, usually because the cost of extending a public sewer connection to a remote property would be disproportionate. Instead, these properties use one of three main system types to treat wastewater on site before it returns to the environment.
The three main off-mains system types
| System | How it works | Learn more |
|---|---|---|
| Septic tank | Settles solids; liquid finishes treatment in a drainage field | Septic tanks |
| Sewage treatment plant | Actively treats wastewater using an aerator | Treatment plants |
| Cesspit | Stores wastewater without treating it, for periodic removal | Cesspits |
Why the system type matters
Each system type has a different relationship with UK regulation — particularly the General Binding Rules, which govern where treated (or partially treated) wastewater is allowed to discharge. This is the single biggest source of confusion for off-mains homeowners, and the starting point for most compliance questions.
If you're new to off-mains drainage
Whether you're buying a rural property for the first time, dealing with a system you've inherited, or planning a new build, a good starting point is understanding which system you have (or will have), then working through the relevant guide:
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