Foul drainage assessment

Last Updated: 24 April 2023

Used to ensure a satisfactory standard of foul drainage.

When required

Mains foul drainage assessment

For Major developments incorporating foul drainage into the public sewer, we advise early discussions with United Utilities to determine whether or not a load or flow assessment should be submitted with the planning application, in order to demonstrate the impact of the development on the public sewer infrastructure.

Non-mains foul drainage assessment

Development involving either the installation of new non-mains drainage or the use of existing non-mains drainage (package treatment plants, septic tanks, cesspools). This includes domestic extensions if the foul sewage will be treated by existing or proposed non-mains system.

Please use this where the application site is served by a non mains drainage and/or non mains water supply.

Non mains foul drainage assessment form (PDF 214KB / 6 pages)


Check the government legislation the installation of non-mains drainage systems.

Where new waste water treatment infrastructure or the replacement or upgrade of a septic tank is proposed, consideration must be given to the proximity of existing systems and the need to avoid adverse impacts on the operation of those systems.

In order to demonstrate that the development can be effectively served by a non-mains foul drainage system without inadvertently affecting the environment, amenity or public health, a foul drainage assessment form should be submitted.

The applicant must provide details of the responsibility, means of operation and management of the non-mains foul drainage system for its lifetime to ensure the risk to the environment is low.

If a new non-mains foul drainage system is proposed, its position together with any associated soakaways and pipework must be shown within the application site (the red line) on the site location plan.

Percolation tests

Percolation test results will not be required where a new  treatment plant will discharge to a watercourse.

Percolation test method to calculate area of drainage field for Septic tanks or sewage treatment systems:

  1. Excavate a test hole 300mm square x 300mm deep below proposed invert level of the drainage field trench bottom.
  2. Fill the test hole with water and allow drain away over night.
  3. Refill to a depth of 300mm and note time taken in seconds to drain away from 75% full to 25% full (that is 150mm drop in level from 225mm to 75mm).
  4. Repeat the procedure in two more test holes and calculate the average of the three results as follows. Test one + test two + test three = average three time taken.
  5. Calculate the Vp (average time in seconds for the water to drop 1mm) as follows:

For example: If average time above took 2,700 seconds

(i) Divide 2,700 seconds by 150mm depth of water
(ii) 2,700 = 18 Vp* (see note below*)
(iii) Area of trench = number of persons to use property x Vp x 0.25.Therefore: 5 persons X 18 X 0.25 = 13.5m2 of effluent drain required.
(iv) To calculate actual length of trench divide 13.5 by width of the trench required therefore 13.5m2 = 22m (minimum permitted area is 30m 0.6m wide long x 0.6m wide).

* Vp should range between 12 and 100 to be successful; otherwise the system should be designed by a drainage specialist.

If connection to non-mains drainage requires pipework to cross land outside the applicant’s ownership, other than the public highway, the correct ownership certificate B, C or D should be completed and a notice served on the owner(s) of that land.


Policy DM6 South Lakeland Local Plan Development Management Policies Development Plan Document; Flood Risk Management and Sustainable Drainage Systems.

Policy DM7 South Lakeland Local Plan Development Management Policies Development Plan Document; Addressing Pollution, Contamination Impact and Water Quality.

Policy AS12 Arnside and Silverdale Area of Outstanding Natural Beauty (AONB) Development Plan Document (DPD); Water Quality, Sewerage and Sustainable Drainage.