Septic Tanks
Septic Tank Installation
What a new septic tank installation involves, from site assessment and percolation testing through to sign-off, and what it typically costs.
In short: a new septic tank installation is driven by ground conditions as much as the tank itself. A percolation test tells the specialist how well your soil absorbs water, which determines the size and design of the drainage field the tank discharges to — and that, more than the tank itself, is usually what governs cost and feasibility.
When you need a new installation rather than a replacement
- A new build or extension with no existing connection to mains drainage
- A property being converted from another use (barn conversions are a common example)
- A holiday let or annexe being added to a property already on a septic tank
- Land being developed where mains sewer connection isn’t available or is prohibitively expensive
The installation process, step by step
- Site assessment — access, water table depth, proximity to boundaries, watercourses and water sources.
- Percolation test — required before designing a drainage field; see our percolation test guide.
- Design — tank size and drainage field area are calculated from occupancy and soil performance, following BS 6297.
- Permissions — Building Regulations approval is required; an Environment Agency permit may be needed if discharge isn’t to a drainage field.
- Installation — excavation, tank placement, drainage field construction and connection, typically 2–5 days depending on scale.
- Sign-off — building control sign-off and, where applicable, confirmation the discharge meets general binding rules.
What affects the cost
- Ground conditions — clay soils that drain poorly often need a larger, more expensive drainage field, or push the design toward a treatment plant instead.
- Access — tight or long access routes for excavators add time and cost.
- Occupancy — tank and drainage field sizing scale with bedrooms/expected usage.
- Discharge point — discharging to a drainage field under general binding rules is usually simpler and cheaper than needing an Environment Agency permit for discharge to a watercourse.
See our septic tank cost guide for indicative figures. Always get a site-specific quote — ground conditions vary too much for a generic number to be reliable.
New build and off-mains drainage
If you’re planning private drainage for a new build, see our related guidance on private drainage for new builds and off-mains drainage more broadly.
Do I always need a percolation test? + −
If your septic tank will discharge to a drainage field (the normal arrangement), yes — it determines the field's size and design under BS 6297. It's not required if you're installing a sewage treatment plant discharging under a permit or exemption to a watercourse.
How long does a new septic tank installation take? + −
A straightforward domestic installation typically takes 2–5 days on site, though this depends on ground conditions, drainage field size and access.
Can I install a septic tank myself? + −
Building Regulations require the installation to meet defined standards and be signed off, and incorrect design can lead to expensive failures or non-compliance. Most homeowners use a specialist for design and installation even if comfortable with the groundwork itself.
Sources
- Environment Agency — General binding rules: small sewage discharges to the environment
- British Standard BS 6297 — Code of practice for the design and installation of drainage fields