Drainage in London Homes: Soil Stacks, Sewer Build-Overs, and the Part H Requirements

Above-ground drainage carries the discharge from all sanitary appliances (WCs, basins, baths, showers, kitchen sinks, washing machines, dishwashers) from the point of use to the below-ground drainage system.

Two-pipe systems (pre-1960s London properties): Victorian and Edwardian London properties were typically plumbed on a two-pipe system: a soil pipe (a 100mm diameter lead or cast iron pipe) carrying the discharge from WCs directly to the drain, running externally down the outside of the rear wall; and a waste pipe system (the pipes from basins, baths, and sinks discharging to an open hopper head on the outside wall, or via a trapped gully at ground level). The two-pipe system keeps soil (WC) discharge separated from waste (other appliances) above ground.

When refurbishing an older London property, the existing two-pipe system is often retained for appliances in the original locations. When new bathrooms, en-suites, or WCs are added (in a loft conversion or extension), these new appliances are connected to a new single-stack system.

Single-stack systems (modern standard): A single-stack soil and vent pipe (SVP) carries the discharge from all sanitary appliances — WCs and all waste — in one 110mm (or 100mm) pipe. All branch connections must meet specific conditions to avoid cross-flow and siphonage of the traps (which would allow drain smells to enter the room):

WC connection: must be swept in the direction of flow; the drop from the WC pan to the stack centreline should not exceed 1.5m (to avoid self-siphonage in the WC trap).

Basin connection: 32mm branch pipe, maximum branch length 1.7m for 32mm pipe (to avoid self-siphonage of the 75mm water seal trap). Longer runs require a 40mm pipe.

Bath and shower connection: 40mm branch pipe, maximum 3.0m branch length.

Stack vent: the soil stack must be vented above the highest appliance connection (and must terminate 900mm above the highest openable window within 3 metres). In London terraced properties where the stack runs down the rear wall, the vent pipe terminates above the eaves level.

Air admittance valves (AAVs): where extending the vent pipe to atmosphere is impractical (in a single-storey rear extension, for example, where the stack would otherwise need to run through the new roof), an air admittance valve (AAV) can be used at the head of the branch stack. AAVs allow air to enter the system to equalise pressure (preventing siphonage) but do not allow air — and smells — to discharge from the stack into the room. AAVs must comply with BS EN 12380. Note: AAVs must not be used as the primary vent for the main soil stack — the main stack must still be open-vented to atmosphere at its head.

Below-ground drainage carries the discharge from the above-ground system to the public sewer (or to a private sewage treatment system, cesspit, or soakaway where there is no public sewer connection).

Foul water vs surface water: Foul water: the discharge from sanitary appliances (WCs, basins, baths, showers, sinks). Foul water must not be discharged to the surface water drainage system. Surface water: rainwater from roofs, patios, driveways, and ground surfaces. Surface water must not be discharged to the foul water sewer. London is served by a combined or partially separate sewer system — in most of inner London, the sewer accepts both foul and surface water (a legacy of the Victorian combined sewer system). In outer London and many newer developments, the foul and surface water systems are fully separate.

Building Regulations Part H — drainage and waste disposal: Part H of the Building Regulations sets out the requirements for above-ground and below-ground drainage. Key requirements: Minimum gradients: 100mm foul drainage must be laid at a minimum gradient of 1:80 (12.5mm per metre) — steeper gradients up to 1:40 are recommended where the drain will serve intermittent high-flow appliances (WCs). Shallower gradients lead to inadequate self-cleansing velocity and blockage. Access for clearing: drain runs must include rodding points or inspection chambers at every change of direction, at every junction, and at intervals not exceeding 22m (for 100mm pipes). Underground drainage is not permitted to be routed under the footprint of a building without adequate access provision or an approved build-over. Materials: below-ground drainage in new work is normally uPVC (unplasticised polyvinyl chloride) — vitrified clay or PVC to BS EN 1401-1. Joints must be socket-and-spigot (push-fit) with rubber ring compression seals. Bedding and surround: pipes must be laid in compacted granular bedding (typically 10mm pea gravel) to prevent differential settlement and pipe failure. Class B bedding is standard for residential drainage.

Separate foul and surface water connections: Where the property is in a separate sewer area (foul and surface water sewers are separate), new surface water must not be connected to the foul sewer. Options for surface water disposal in London where a surface water sewer is not available: Soakaway: a rubble or polypropylene cellular soakaway that allows surface water to percolate into the ground. Only suitable where the ground is sufficiently permeable (most of inner London clay is not). A soakaway infiltration test (BRE Digest 365 or similar) must confirm suitability. Attenuation tank with controlled outflow: collecting surface water in an underground tank and releasing it to the sewer at a controlled (restricted) flow rate — used on larger developments where a direct soakaway is not feasible.

The public sewers in London are managed by Thames Water. Building over or adjacent to a public sewer (a sewer that is adopted by Thames Water and vested in them) without consent is potentially serious — if the sewer collapses or needs repair, Thames Water has the right to remove any structure over the sewer to access it for repair, at the owner's cost.

The build-over requirements:

Building within 3 metres of a public sewer (not directly over it): A Build Over application must be submitted to Thames Water (via their online portal at developers.thameswater.co.uk). Thames Water will confirm whether the application is acceptable and may impose specific conditions (minimum foundation depth, additional reinforcement, installation of an inspection chamber adjacent to the sewer within the footprint, use of flexible couplings at the point where the private drain connects to the sewer).

Building directly over a public sewer: A more rigorous build-over agreement is required. Thames Water may require: A CCTV survey of the sewer before and after construction (to confirm the sewer is undamaged). The installation of an access chamber over the sewer within the structure (a manhole or rodding point accessible from inside the building). Specific foundation details that protect the sewer from loading (pile and beam foundations, or reinforced strip foundations bridging the sewer). Insurance provisions.

Cost of a Thames Water build-over application: there is currently a fee (check the Thames Water website for the current fee schedule). The CCTV survey before and after is a significant additional cost (£300–£800 for each survey).

What is a public sewer? A public sewer is one that has been adopted by Thames Water. Prior to July 2011, only sewers serving multiple properties were typically public sewers; since the transfer of private sewers in July 2011, many shared private sewers that serve more than one property were adopted by Thames Water as public sewers. The Sewer Adoption Map on the Thames Water website (or the Sewer Records search) shows the position of adopted sewers. Always check the Sewer Records before designing an extension that involves groundworks close to the site boundary.

What is a lateral drain? A lateral drain (sometimes called a branch connection) is the section of the private drain from the property boundary to the point where it connects to the public sewer. Since July 2011, lateral drains in England and Wales were transferred to the sewerage undertaker (Thames Water in London) as public sewers. This means that the section of drain running from your property boundary to Thames Water's sewer is now Thames Water's responsibility to maintain.

A CCTV drainage survey involves inserting a camera into the drainage system to inspect the internal condition of the below-ground drains — identifying defects, blockages, tree root ingress, offset or broken joints, and the location of the drain relative to any proposed construction.

When to commission a CCTV drainage survey:

Before any extension groundworks: to confirm the position of all underground drains within the footprint and within 3 metres of the proposed foundation, before the design is finalised. A drain that is not identified until ground is broken can stop the project for 2–4 weeks while the build-over agreement is obtained and the foundation design is revised.

Before buying a property if any ground-floor extension is intended: many older London properties have drainage defects that are not visible above ground — cracked clay pipes, root ingress, offset joints. Identifying these before purchase allows the cost of repairs to be factored into the acquisition.

As part of a Party Wall Award: some party wall surveyors recommend a CCTV survey as part of the Schedule of Condition process — to record the condition of any shared drain or lateral drain adjacent to the party wall before works begin, so that any subsequent damage to the drain can be attributed to the building works.

What a CCTV survey report typically includes: A plan showing the drain layout, the direction of flow, and the location of each inspection chamber or access point. A schedule of defects — each defect is recorded by distance from the start of the survey, type of defect (root ingress, crack, offset joint, intruding connection, grease build-up), and severity (WRC-JE2 standard — A, B, C, or D severity categories). Video footage (typically provided on USB or as a video file download) of the camera traversal through each run.

Cost: £250–£600 for a standard residential property CCTV drainage survey (4–6 drain runs, 1–2 hours on site).

CCTV survey recommendations: Commission the survey from a drainage specialist who provides a full WRC-coded report, not just a video. The coded report is what is required for a Thames Water build-over agreement application and for a Building Control drainage pre-commencement condition.

Shared private drains (drains that serve two or more properties but have not been adopted as public sewers) are a common source of confusion and dispute in London, particularly in Victorian terraces where the original builder constructed a single shared drain along the rear of a run of properties.

Maintenance responsibility: The owner of the land through which the drain runs is generally responsible for maintaining it, unless there is a specific legal agreement (in the title deeds or a separate drainage easement) that creates a shared maintenance obligation between the connected properties. This is often unclear in older London properties where the original drainage documentation has been lost.

The 2011 transfer: The public sewer transfer of July 2011 brought many shared drains into public ownership — but only where the drain served two or more properties and was not already adopted. Drains serving only one property (even if they run under multiple properties' land) were not transferred. The Sewer Records should be consulted to confirm whether a shared drain has been adopted.

Common issues with shared private drains in London building projects: The neighbour's refusal to allow diversion works on the shared drain — requiring the building owner to redesign the foundation to bridge the drain. Discovering that the shared drain is in very poor condition (cracked clay pipes, significant root ingress) — raising the question of who is responsible for the repair cost. The shared drain runs under the neighbour's property as well as the building owner's — requiring a drainage easement to be confirmed in the deeds before any works affecting the drain can proceed.

Practical advice: Before any project involving groundworks adjacent to or over shared private drains: obtain the deeds to confirm the drainage easements; carry out a CCTV survey to confirm the drain's condition; consult a solicitor if the maintenance responsibility is unclear; and discuss the proposed works with the affected neighbours early.