Every UK development has three distinct water-related networks: the clean water supply, the foul drainage system, and the surface water drainage system. They are separate networks with different design rules, different adoption routes, and different regulatory regimes. Understanding the differences is fundamental to planning any new water connections package.
This guide covers what each network is, who regulates it, and how they interact.
Clean Water Supply
The clean water supply brings potable water into the site. It consists of:
A supply main from the water company’s network to the site boundary.
A distribution main through the development to reach each plot or building.
Individual plot services and meters.
Fire hydrants at agreed spacing for fire service use, sized and located as set out in our commercial water and fire hydrants guide.
Clean water is regulated by the Water Supply (Water Quality) Regulations, which set out standards for water treatment, pipe materials, and commissioning procedures. All clean water pipes must be WRAS-approved materials, and the system must be disinfected before being brought into service.
The network operator is the local water company (or a Self-Lay Provider or NAV on adopted networks).
Foul Drainage
The foul drainage system removes waste water from toilets, sinks, baths, washing machines, and commercial processes to a treatment works. It consists of:
Gravity-driven foul sewers running to a receiving main or treatment works.
Pumping stations where gravity flow is not possible.
Rising mains from pumping stations to the next gravity section.
Manhole access for maintenance.
Foul drainage is regulated under the Water Industry Act 1991. Section 104 agreements are the standard adoption mechanism. Design must comply with Sewers for Adoption (SfA) 7 in England and Wales.
The network operator is the local sewerage undertaker (usually the same company as the water supplier).
Surface Water Drainage
The surface water drainage system removes rainwater from roofs, roads, and hard landscaping. It typically drains to:
A surface water sewer (adoptable by the water company).
A watercourse (river, stream, or open water).
A soakaway or infiltration system (for on-plot disposal).
A SuDS (Sustainable Drainage System) with attenuation and treatment.
Surface water is regulated by the Environment Agency, the local planning authority’s SuDS approving body, and (for adopted sewers) the water company. The design must meet drainage rates set by the receiving authority and include appropriate water quality treatment.
Increasingly, surface water is managed through SuDS rather than connected to combined or surface water sewers. This is driven by flood risk management and environmental water quality policy.
Why Separation Matters
Combining foul and surface water sewers was common until the 1970s. Combined systems cause environmental problems because storm overflows discharge untreated sewage to rivers during heavy rain. Modern UK policy is to separate the two systems wherever possible.
On new developments, foul and surface water must almost always be separated. Connecting them together is rarely permitted and requires specific justification.
Typical Design Hierarchy
A well-designed site works through the following hierarchy for each type of water.
For clean water supply: connect to the nearest adequate main. If none, extend the main, following the new water mains process for residential developments.
For foul drainage: use gravity if possible. If not, use a pumping station. Adoption follows the Section 104 adoption route.
For surface water: discharge to ground via soakaway if feasible. If not, discharge to a watercourse. If not, discharge to a surface water sewer with appropriate SuDS. Connection to a combined sewer is a last resort.
Coordination
The three networks interact on site in several ways.
All three need trenches through the same site. Coordination of depths, crossings, and spacing is essential.
Surface water and foul sewers sit deeper than clean water mains. The clean water must cross over without excessive depth.
Fire hydrants on the clean water main must be positioned for fire service access, which interacts with road layout and SuDS features.
Pumping stations for foul drainage require electricity supply and may be designed alongside the LV electrical network.
Regulatory Interfaces
Planning consent typically requires:
Pre-application consultation with the water company on clean water supply and foul drainage capacity.
A Flood Risk Assessment for surface water.
A Drainage Strategy Report covering all three systems.
SuDS approval by the relevant approving body.
Section 104 agreement for foul sewer adoption.
Self-Lay Agreement or equivalent for clean water main adoption.
Common Developer Errors
Treating all three as a single “drainage” package and missing the different regulatory regimes.
Assuming surface water can connect to a combined sewer without checking capacity and consent.
Ignoring SuDS requirements until late in the design, then having to rework the site layout.
Positioning clean water hydrants without fire authority consultation.
Failing to separate foul and surface water on sites where separation is mandatory.
What Good Looks Like
Good development design integrates all three systems from RIBA stage 2. A Drainage Strategy Report sets out the hierarchy for each. Water company consultation is completed by stage 3. SuDS design is developed by stage 4. Adoption agreements are in place by stage 5. Construction integrates all three under a single site logistics plan.
The Bottom Line
Clean water, foul drainage, and surface water are three separate systems with three separate regulatory regimes. Treating them separately in design but coordinating them in execution is the key to a smooth site. Conflating them in design or sequencing them poorly in execution is where problems start.