Foul Sewage Assessment
A Foul Sewage Assessment addresses one of the most fundamental requirements of any new development — how human waste and wastewater will be safely collected, treated, and disposed of. Local planning authorities require evidence that adequate foul drainage arrangements exist or can be provided before granting planning permission, and failing to resolve sewage matters early can cause significant delays or outright refusal.
Typical Cost
£300 – £5,000+
Turnaround
1 – 6 weeks
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What is a Foul Sewage Assessment?
A Foul Sewage Assessment is a technical report that evaluates how a proposed development will manage foul water — that is, wastewater from toilets, sinks, baths, showers, kitchens, and washing machines. The assessment identifies the most appropriate method of foul drainage for the site, whether that involves connection to an existing public sewer, use of a private treatment system such as a package treatment plant, or in some cases a septic tank. It also considers the capacity of existing infrastructure to accommodate the additional flows generated by the development.
When is a Foul Sewage Assessment required?
A Foul Sewage Assessment is required for virtually all planning applications that involve the creation of new dwellings, commercial premises, or any development generating foul water discharge. The requirement stems from Paragraph 20 of the standard planning application form, which asks applicants to describe how foul sewage will be dealt with. Most local authorities require a standalone assessment for developments of five or more dwellings, or where connection to the public sewer is not straightforward. Sites in rural areas without access to mains drainage will always need a detailed assessment covering non-mains options.
What does a Foul Sewage Assessment include?
A thorough Foul Sewage Assessment includes an appraisal of the existing foul drainage infrastructure serving the site and surrounding area, confirmation of the proposed method of foul water disposal, calculations of anticipated foul water flows based on occupancy rates and Building Regulations Part H guidance, evidence of sewer capacity from the relevant water company or a capacity assessment, details of any proposed pumping stations or rising mains, and confirmation that the proposed approach complies with the General Binding Rules or Environmental Permit requirements where non-mains drainage is proposed.
How much does a Foul Sewage Assessment cost?
For a straightforward residential development connecting to an existing public sewer, a Foul Sewage Assessment typically costs between £300 and £800. Sites requiring detailed capacity analysis, percolation testing for non-mains drainage, or liaison with the water company can expect costs of £1,000 to £2,500. Larger developments involving new sewer infrastructure, pumping stations, or complex negotiations with water companies may require assessments costing £3,000 to £6,000 or more.
Who can prepare a Foul Sewage Assessment?
Foul Sewage Assessments are typically prepared by civil engineers, drainage consultants, or environmental consultants with expertise in foul water management. For non-mains drainage proposals, the assessor should have experience with Environment Agency permitting requirements and the General Binding Rules. Many consultancies hold membership of organisations such as the Chartered Institution of Water and Environmental Management (CIWEM) or are registered with the British Water trade body.
How long does a Foul Sewage Assessment take?
A basic assessment for a site with an available public sewer connection can be completed within 1 to 2 weeks. Assessments requiring percolation testing, sewer capacity confirmation from the water company, or Environment Agency consultation typically take 3 to 6 weeks. Water companies have a statutory obligation to respond to capacity enquiries within 21 days, though in practice responses can take longer during busy periods.
Frequently Asked Questions
What is the difference between foul sewage and surface water?
Foul sewage is wastewater generated by human activity — from toilets, sinks, baths, showers, dishwashers, and washing machines. Surface water is rainwater that falls on roofs, driveways, and other hard surfaces. These two types of water must be drained separately in most modern developments, as combining them can overwhelm sewer systems and treatment works during heavy rainfall.
Can I connect to the public sewer without permission?
You have a right to connect to the public sewer under Section 106 of the Water Industry Act 1991, but you must give the relevant water company at least 21 days notice before making the connection. The water company can specify the manner and location of the connection. For developments generating significant additional flows, the water company may require infrastructure upgrades before connection is permitted.
What are the General Binding Rules for sewage?
The General Binding Rules replaced the need for an Environmental Permit for small sewage discharges in England from January 2015. They allow the use of septic tanks and package treatment plants without a permit, provided certain conditions are met — including that the discharge is no more than 2 cubic metres per day, the system is properly maintained, and the discharge does not enter a protected watercourse. Discharges that cannot meet these conditions still require an Environmental Permit from the Environment Agency.
Do I need a Foul Sewage Assessment for a single house?
Most local authorities require at least basic foul drainage information for any new dwelling. If you are connecting to an existing public sewer and the connection is straightforward, the information on the planning application form may suffice. However, if the site is not served by mains drainage, or if there are known capacity issues with the local sewer network, a standalone Foul Sewage Assessment will be needed.
What happens if the existing sewer does not have enough capacity?
If the water company confirms insufficient capacity in the existing sewer network, they will typically require the developer to fund or contribute to network upgrades. This might involve upsizing existing pipes, constructing new sewers, or building a pumping station. These requirements are usually secured through a Section 104 adoption agreement or planning conditions. The cost of such upgrades falls on the developer.
Can I use a septic tank for a new development?
Septic tanks are permitted only where connection to a public sewer is not reasonably practicable and where the discharge meets the General Binding Rules. Since January 2020, septic tanks discharging directly to a watercourse are no longer permitted — all septic tank effluent must drain to a soakaway system. For new developments, package treatment plants are generally preferred over septic tanks as they produce a cleaner effluent.
What is a package treatment plant?
A package treatment plant is a self-contained sewage treatment system that processes foul water on site to a standard where the treated effluent can be safely discharged to a watercourse or soakaway. Unlike septic tanks, treatment plants use biological processes to break down waste and produce a much higher quality effluent. They are the most common non-mains drainage solution for new developments in areas without public sewers.
Do I need percolation tests for my Foul Sewage Assessment?
If the proposed drainage solution involves a soakaway — either from a septic tank or a package treatment plant — percolation tests will be required to demonstrate that the ground conditions are suitable for infiltration. The tests must be carried out in accordance with BS6297 and involve digging trial holes, filling them with water, and measuring the rate at which the water drains away. Results that fall outside acceptable limits may mean a soakaway is not feasible.
Who is responsible for maintaining private drainage systems?
The property owner or management company is responsible for maintaining private drainage systems including septic tanks, package treatment plants, and private drains. Failure to maintain these systems properly can result in pollution incidents, which are criminal offences under the Environmental Permitting Regulations. Regular desludging of septic tanks and servicing of treatment plants is essential to ensure continued compliance.
Will a Foul Sewage Assessment delay my planning application?
If your application requires a Foul Sewage Assessment and one is not submitted, the application will not be validated, causing immediate delay. Even with an assessment submitted, delays can occur if the water company takes time to confirm sewer capacity or if percolation tests reveal unsuitable ground conditions requiring alternative drainage strategies. Starting the assessment process early is the best way to avoid holding up your application.