Foul drainage strategy
A strategy for how foul water from a development will be collected and connected to the sewer network.
About this service
Every new home creates wastewater from toilets, kitchens, showers and washing machines, and that flow has to go somewhere. A foul drainage strategy explains how it will be collected on site and connected to the public sewer, and shows that the network can take the extra load. Councils often ask for one with a planning application, and Thames Water may need the same information when you apply to connect.
We prepare foul drainage strategies for new houses, small blocks of flats, mixed-use schemes and conversions where a single house becomes several units. The strategy estimates the foul flows from the development, proposes a pipe layout with inspection chambers and gradients, identifies the connection point and notes any sewer capacity constraints. Where gravity drainage is not possible, for example from a basement, we set out the pumped option and what it involves.
A foul strategy usually sits alongside a surface water drainage strategy, because councils want to see both halves of the drainage picture. Our guide to drainage strategies for planning explains how the two documents fit together and what councils typically look for in each. Every strategy goes through our three-step quality control process, with final sign-off by our Head of Engineering, before it is issued.
When you need it
- A planning application needs a foul drainage strategy
- You're building new homes or converting a building
- The water company needs connection details
What's included
- Foul flow calculations
- Proposed layout and connection points
- Sewer capacity considerations
How it works
- Gather site informationWe review the proposed plans, existing drainage records, sewer maps and site levels, and identify the nearest public sewers and whether they are combined or foul-only.
- Estimate foul flowsWe calculate expected flows from the number and type of homes or commercial units, using recognised design guidance, so the connection and pipe sizes can be checked.
- Design the layoutWe propose pipe runs, falls, inspection chambers and connection points, avoiding foundations and trees where possible, and set out any pumping or basement drainage needed.
- Check sewer capacityWhere the scale of development warrants it, we advise on a pre-development enquiry to Thames Water and incorporate any capacity comments or constraints into the strategy.
- Issue the strategyWe issue a concise report with a drainage layout drawing, ready to submit with the planning application or to support a connection application to the sewerage company later.
What you receive
Combined sewers, basements and conversions
Much of inner London is served by combined sewers, carrying foul and rainwater in the same pipe. Even so, councils and Thames Water usually expect new developments to keep foul and surface water separate on site, joining only at the final connection if they must. This keeps options open if a separate system is ever built and makes it easier to limit rainwater flows. Designing separate systems from the start is far cheaper than changing later.
Conversions and basements bring their own problems. Splitting a house into flats can overload an old shared drain that was sized for one household, and new kitchens on upper floors may need new stacks. Basement toilets and showers sit below the sewer, so they need a pumped system and protection against sewer surcharge. Where new drains run close to a public sewer, a build over or near sewer package may also be needed.
Frequently asked questions
What is a foul drainage strategy?
It is a report explaining how wastewater from toilets, sinks, showers and appliances in a new development will be collected and taken to the public sewer. It usually covers estimated flows, the proposed pipe layout, connection points and whether the sewer network has capacity. Councils often request one for new homes and conversions, and it supports the application to connect to the sewer.
Do I need a foul drainage strategy for a planning application?
It depends on the council and the scale of the scheme. Many London boroughs ask for drainage information on new homes, conversions creating extra flats and mixed-use developments, and some make it a validation requirement. Small householder extensions rarely need one. Check your council's local validation list, or ask us and we will advise based on your scheme and borough.
How do I connect a new development to a Thames Water sewer?
A new connection to a public sewer needs an application to the sewerage company, which in most of London is Thames Water. You provide details of the development, drainage layout and connection point. For larger schemes, a pre-development enquiry first confirms whether the network has capacity. Our strategy provides the technical information these applications require.
Can a basement toilet drain by gravity?
Usually not, because the basement floor is often below the level of the public sewer. In that case a pumped system lifts wastewater up to a gravity drain. The design needs a suitable pump chamber, a backup in case of failure and protection against sewage backing up from the sewer in heavy rain. Our basement conversion guide covers this in more detail.