Drainage and Extensions — Sewer Build-Over, Connections, and Drainage Planning

**What is a public sewer?** A public sewer is a drain or sewer that is maintained by the water company (Thames Water for most of London). Since 2011, many private shared drains (serving more than one property) were adopted as public sewers — this significantly increased the number of sewers that are subject to build-over restrictions.

• **The build-over rule**:
• Building Regulations Part H4 requires that where a building is built over or within 3 metres of a public sewer:
• •Building Control must be notified
• •A sewer build-over agreement may be required from the water company
• •The foundation design must be assessed in relation to the sewer

• **Thames Water's position**:
• Thames Water has specific requirements for building over or near their apparatus. The distance thresholds are:
• •Within 3 metres of a public sewer up to 375mm diameter: Thames Water Build-Over Agreement required
• •Within 6 metres of a public sewer over 375mm diameter: Thames Water approval required
• •Over a public sewer: Build-Over Agreement required regardless of pipe diameter

**Types of agreement**: 1. **Self-certified build-over** (low risk, shallow sewer, standard extension, no change to the sewer): in some cases a structural engineer can self-certify the build-over to Thames Water requirements without a separate agreement. Building Control will require confirmation. 2. **Full build-over agreement**: Thames Water reviews the foundation design, inspects before and after construction, and requires access provisions to be maintained. Fee: typically £350–£1,000 depending on scope. 3. **Sewer diversion**: where a build-over is not feasible (deep sewer, large diameter, or water company refusal), the sewer must be diverted around the proposed extension footprint. Diversion costs: £3,000–£20,000+ depending on depth and run length.

**How to check for public sewers**: Thames Water provides a free drainage map search (Build Over mapping tool). Groundsure and other environmental search providers include drainage mapping in their residential searches. Your drainage consultant or structural engineer can access this mapping.

Any new WC, basin, shower, bath, or kitchen waste installed in an extension, loft conversion, or newly added room must be connected to the drainage system. Building Regulations Part H governs drainage.

• **Types of drainage**:
• •**Foul drainage**: carries sewage from WCs, and waste water from basins, baths, showers, and kitchen sinks
• •**Surface water drainage**: carries rainwater from roofs, driveways, and paved areas
• •**Combined drainage**: carries both foul and surface water in the same pipe (common in older London properties where the drainage was installed before the two systems were separated)

**Ground floor drainage connections (extensions)**: For a rear kitchen or dining extension, the most common drainage tasks are: 1. **Connecting the new kitchen sink and appliances**: typically run new waste pipes through the extension floor or wall into an existing inspection chamber or rodding eye 2. **Connecting the roof drainage**: downpipes from the new flat or pitched roof run to the existing stormwater system 3. **Relocating the soil stack if it passes through the extension area**: an existing soil stack that is to be enclosed within the new extension fabric may need to be relocated — a significant cost if the stack is at first-floor height

• **Loft conversion drainage connections**:
• For a loft bathroom:
• •The new toilet must be connected to the soil and vent pipe (SVP) — typically an extension of the existing soil stack upward through the loft level
• •New waste pipes from basin, bath, and shower must be routed to the SVP or to a separate waste discharge point
• •Waste pipes must achieve correct falls (minimum 1:40 for waste pipes) and cannot run horizontally for long distances without careful design

**Inspection chambers and access**: All drainage runs must be designed with appropriate inspection access (rodding eyes or inspection chambers) so blockages can be cleared without excavation. Building Control will check that access is provided at all bends and junctions in the run.

**Sustainable drainage systems (SuDS)** For extensions adding impermeable hard surfacing or roof area, local planning authorities (and Building Regulations Part H) increasingly require surface water to be managed sustainably — not discharged directly to the public sewer. This is particularly relevant in London boroughs where combined sewer systems are at capacity.

**Options for surface water disposal**: 1. **Soakaway**: a pit filled with aggregate or a cellular soakaway crate into which rainwater drains and percolates into the ground. Requires a percolation test to confirm the ground is suitable. Cannot be installed within 5 metres of a building or boundary. The most sustainable and often preferred option. 2. **Attenuation/rainwater harvesting**: collecting roof water in a tank and releasing it to the sewer at a controlled rate (attenuated flow). Required by some LPAs for larger extensions. 3. **Connection to the surface water sewer** (where a separate surface water sewer exists — not all London streets have one): direct connection, simpler, requires water company approval. 4. **Connection to the combined sewer** (where there is no surface water sewer): permissible but not preferred — water companies increasingly restrict new surface water connections to combined sewers.

**Drainage surveys**: For projects where the existing drainage system will be altered, extended, or where a sewer build-over may be required, a CCTV drain survey of the existing underground drainage is strongly recommended before design is finalised. A survey costs £200–£600 and identifies existing drain routes, conditions, and potential issues before excavation begins.