Building Regulations Part H — Drainage Requirements for London Extensions and New Builds

**The above-ground foul water drainage system in a London extension**:

• The above-ground foul water drainage system in a house or extension collects waste from sanitary appliances (WCs; baths; showers; wash hand basins) and from kitchen sinks and dishwashers, and conveys it to the underground drainage system that connects to the public sewer. The system consists of:
• •**Branch pipes**: short-run pipes from individual appliances (waste pipe from basin; bath trap and pipe; shower trap and pipe) connecting to the stack or to a branch pipe that runs to the stack
• •**Stack (soil and vent pipe — SVP)**: the main vertical pipe that collects all the branch pipe connections and carries foul water vertically down to the underground drainage. In a London Victorian terrace, the existing SVP is typically on the rear elevation; in a two-storey extension, a new SVP may be required or the existing SVP extended
• •**Vent**: the SVP is typically extended above the roof level as a vent (to atmosphere) to prevent siphoning of traps and to release sewer gases safely above roof height

**Part H pipe sizing requirements for foul drainage in extensions**:

*Branch pipe minimum diameters (Part H Approved Document H1)*:

| Appliance | Minimum branch pipe diameter | Minimum trap depth | |---|---|---| | WC (washdown or siphonic) | 100mm | 50mm (75mm required if risk of trap loss) | | Bath or shower | 40mm | 75mm seal (bath); 50mm (shower) | | Wash hand basin | 32mm | 75mm seal | | Kitchen sink | 40mm | 75mm seal | | Dishwasher / washing machine | 40mm | Via trap on sink or dedicated 40mm trap | | Bidet | 40mm | 75mm seal |

• *Stack (SVP) minimum diameter for a London house extension*:
• •A foul drainage stack serving a WC must be a minimum of **100mm** internal diameter — typically 110mm OD (outside diameter) solvent-weld or push-fit PVC soil pipe
• •A stack NOT connected to a WC (i.e., a waste-only stack, serving baths and basins only) may be 75mm or 90mm — but 100mm is standard and simpler to connect to

*Branch pipe fall (gradient) requirements*: Branch pipes must fall at a sufficient gradient to ensure self-cleansing velocity — carrying solids away from traps without blockages.

| Pipe diameter | Minimum gradient | Maximum gradient | |---|---|---| | 32mm (basin) | 1 in 12 (1/12) | 1 in 1 (very steep — use a larger radius bend at base instead) | | 40mm (bath; sink; shower) | 1 in 48 (minimum 18mm fall per metre) | 1 in 6 | | 50mm | 1 in 64 | 1 in 6 | | 100mm (WC branch or stack) | 1 in 80 (minimum) — most installations use 1 in 40 or 1 in 60 | 3° (steep gradients on 100mm branches to WC are undesirable as they leave solids behind) |

*Maximum branch pipe lengths (non-vented)*: Branch pipes that are too long without venting risk siphoning the trap — that is, the moving column of water in the long branch pipe creates a suction that drains the trap seal (the water plug in the U-bend that prevents sewer gases from entering the room).

| Appliance | Maximum branch pipe length (no venting) | Notes | |---|---|---| | Basin (32mm) | 1.7m | Longer than 1.7m = risk of trap siphoning; requires vented trap or larger pipe | | Bath (40mm) | 3.0m | | | Shower (40mm) | 3.0m | | | Kitchen sink (40mm) | 3.0m | | | WC (100mm branch) | 6.0m from stack | Longer runs require intermediate venting or back-inlet gully |

*Typical London kitchen extension drainage scenario*: - Kitchen sink in extension: 40mm branch pipe at 1 in 40–1 in 48 fall; maximum 3m to the stack connection or to a new connection to the underground drain via an air admittance valve and 40mm branch - New en-suite in loft conversion above the extension: 100mm SVP run down through the extension (internal or external); branch connections from WC (100mm), basin (32mm), shower (40mm)

**Air admittance valves (AAVs / Durgo valves) — when they are permitted and when they are not**:

An air admittance valve (AAV — trade name: Durgo valve; or MVHK; or HepvO valve for washing machines) is a one-way valve that allows air into the waste pipe when water flows (preventing siphoning) but closes under positive pressure to prevent sewer gases from entering the room. AAVs are a useful alternative to extending the soil pipe vent above roof level.

• *When AAVs are permitted under Part H*:
• •AAVs are permitted on branch pipes and on stacks where the stack is an internal stack (i.e., it does not need to terminate above roof level for any other reason)
• •An AAV can replace the open roof vent on a secondary stack serving a WC, provided the drainage system is adequately vented at another point (e.g., the main stack on the existing house remains open to atmosphere)
• •AAVs are appropriate for loft conversions where running an external soil pipe vent above the roof ridge is aesthetically undesirable

• *When AAVs are NOT appropriate*:
• •On the main (and only) soil vent stack for a building — the main SVP must have a vent open to atmosphere above roof level; an AAV cannot replace the only external vent
• •Where there is a known risk of positive pressure in the drain (e.g., close to a sewer inspection chamber or at the base of a tall stack) — positive pressure could force the AAV open, allowing sewer gases into the building
• •Where the drainage authority (Thames Water in London) requires an open vent as a condition of drainage design — confirm with Building Control

• *AAV installation requirements*:
• •Install the AAV above the highest trap connection on the branch or stack it serves
• •Minimum 200mm above any trap or appliance connection it serves
• •Must be accessible (not permanently concealed behind fixed plasterboard with no access panel)
• •Must be BS EN 12380 compliant (the European standard for air admittance valves)
• •Must be installed in a ventilated space — if in a cupboard, the cupboard must have ventilation to allow air to enter through the AAV

**Below-ground drainage design principles for London extensions**:

• The below-ground drainage system for a London house extension collects the discharge from the above-ground stack and from any gulley connections (kitchen waste gullies; yard gullies) and conveys it underground to the connection with the public sewer. In London, the public sewer is typically a combined sewer (carrying both foul water and surface water in the same pipe) for Victorian-era areas, or a separate sewer system in newer areas. The below-ground drainage must:
• •Have adequate pipe size for the anticipated flow
• •Fall at a gradient that provides self-cleansing velocity (avoiding deposition of solids)
• •Be accessible for rodding (clearing blockages) at intervals not exceeding the maximum unrodded length
• •Be bedded and haunched appropriately to prevent ground movement from crushing or misaligning the pipes

**Part H below-ground drainage pipe sizing and gradients**:

| Pipe diameter | Typical application | Minimum gradient | Recommended gradient for London extensions | |---|---|---|---| | 100mm (4") | Single house or extension drain serving up to 1 dwelling | 1 in 80 (minimum; self-cleansing only at high flow rates) | 1 in 40–1 in 60 (standard for London house drains) | | 150mm (6") | Drain serving 2–5 dwellings or a significant commercial flow | 1 in 150 (minimum) | 1 in 80–1 in 100 | | 225mm (9") | Drain serving 6–20 dwellings | 1 in 300 (minimum) | 1 in 150 |

*Gradient practical rule for single house extensions (100mm drain)*: design for 1 in 40–1 in 60. This means a fall of 16–25mm per metre of drain run. On a 5m run from the extension stack connection to the inspection chamber at the boundary, this gives a total fall of 80–125mm — readily achievable in most London terraced house rear garden drainage layouts.

**Pipe material for below-ground drainage in London house extensions**:

| Pipe material | Notes | |---|---| | Vitrified clay (VC) — e.g., Hepworth; Naylor | Traditional material; very durable; used in all pre-1980s London drainage. New clay drain connections should use flexible-jointed clay pipes (rubber ring joints or slip-ring joints) rather than rigid cement mortar joints — rigid joints crack on ground movement. | | PVC-U (plastic) — e.g., Osmadrain; Marley Civils | Most commonly used for new-build and extension below-ground drainage. Lightweight; easy to cut and join; push-fit ring-seal joints allow slight angular deflection to accommodate ground movement. Min 200mm depth of cover in a garden; 450mm under a road or paved area. | | HDPE (polyethylene) | Used for aggressive ground conditions or chemical waste — rarely needed in standard London residential extensions. |

**Pipe bedding and haunching — critical for preventing pipe crushing**:

Below-ground drainage pipes must be bedded and haunched (surrounded with fine material) correctly to prevent the surrounding ground load from crushing the pipe. The four standard bedding classes (per BS EN 1295-1) for drain pipes:

• *For PVC-U pipes in London residential extension drainage (the most common configuration)*:
• •Use Bedding Class N (loose-soil bedding) or Class F (granular material — pea gravel or coarse sand, 20mm nominal size) for pipes in good ground away from heavy loadings
• •Use Class B (granular material around and below the pipe; concrete surround not required) for most London residential drainage
• •Use Concrete Surround (encasement in concrete) where pipes are under driveways, under slabs, or in poor or unstable ground — this is a common requirement in London where garden drainage runs near or under extension slabs

**Inspection chambers and rodding access requirements**:

Part H requires that every below-ground drain run is accessible for rodding (clearing blockages) and inspection. The requirements:

• *Rodding eyes*: a rodding eye is a capped pipe access point at ground level (or near-ground level) that allows drain rods to be inserted for clearing a blockage. Required at:
• •The head of each drain run (where the above-ground stack discharge connects to the below-ground drain)
• •At each bend or junction in the below-ground drain
• •At maximum 45m intervals on straight runs (though in practice, London residential drains are rarely longer than 10–15m)

• *Inspection chambers*: a full inspection chamber (with an open-top benched base and removable cover) is required:
• •Where more than one drain joins (a junction between foul and a second connection)
• •Where a bend of more than 45° occurs (a 90° bend underground without an inspection chamber is not acceptable — use two 45° bends with an inspection chamber between, or a purpose-made channel bend in an inspection chamber)
• •At the boundary of the property (the property boundary inspection chamber allows Thames Water or the contractor to rod the public sewer from the private drain in an emergency, and vice versa)
• •At a maximum of 22m unrodded length between access points on a straight run

• *Standard London residential inspection chamber sizes*:
• •225mm diameter plastic (PVC-U or HDPE) shallow access chamber — suitable for shallow drains up to 600mm deep to invert
• •300mm diameter plastic chamber — for medium depth drains
• •450mm × 450mm brick or precast concrete inspection chamber — for deeper drains (600mm+ to invert) or where access is required for full rod clearing
• •Cover and frame: must be able to carry the applied load at that location (A15 loading for lightly trafficked areas; B125 for driveways; C250 or D400 for roads)

**Separation of foul and surface water drainage**:

• Building Regulations Part H and current planning policy both require that foul water (WC waste; kitchen and bathroom waste) and surface water (rainwater from roofs; paving; driveways) are kept separate and discharged separately:
• •Foul water: to the foul sewer or combined sewer
• •Surface water: ideally to a soakaway (infiltration); if not feasible, to a watercourse (if available); if not feasible, to a surface water sewer (if available); if no other option, to the combined sewer (with agreement from Thames Water)

• *Why separation matters for London extensions*:
• •If surface water from the new extension flat roof is discharged to the foul sewer instead of the surface water system or a soakaway, it adds hydraulic load to the foul sewer during rainfall events — contributing to combined sewer overflows (CSOs) — which Thames Water and the LPA will object to
• •The Building Control inspector will check and verify the drainage layout at inspection stages — connecting surface water to the foul sewer is a non-compliance

• *Soakaway design for extension surface water drainage*:
• A soakaway (an underground void or rubble-filled pit into which surface water drains and from which it infiltrates gradually into the surrounding soil) is the preferred disposal route for surface water from a London house extension roof. However:
• •Soakaways are only feasible where the underlying soil is permeable enough to accept the design infiltration rate — London clay (which underlies most of south London, much of central London, and parts of west London) is NOT permeable enough for soakaways
• •A percolation test (BRE Digest 365 method) should be carried out before designing a soakaway — bore a 300mm diameter hole to the proposed base of the soakaway; fill with water; measure the drainage rate; if the percolation rate Vp exceeds 10 minutes per 25mm depth, the soil is too impermeable for a soakaway
• •If a soakaway is feasible: design per BRE Digest 365; minimum 5m from the building foundation; minimum 2.5m from the property boundary
• •If a soakaway is NOT feasible (clay soil): surface water from the extension roof must be piped to the combined sewer (with Thames Water agreement) or to a rainwater harvesting tank

**Why Thames Water Build Over Agreements (BOAs) are relevant to drainage planning for London extensions**:

As covered in the `build-over-agreement-guide`, Thames Water requires a Build Over Agreement (BOA) or self-certification where any construction work comes within 3m of a public sewer (or 6m of a combined or surface water public sewer of more than 225mm diameter). In London, the presence of public sewers under rear gardens is extremely common — the Victorian-era combined sewer network runs through the rear of many terrace properties, particularly in older London stock (Victorian and Edwardian terraces in inner London Boroughs).

*Checking for public sewers before designing drainage for a London extension*: 1. Search the Thames Water Sewer Records portal (available via the Thames Water website) or obtain a CON29DW (drainage and water enquiry) as part of the conveyancing search pack — this identifies the location of public sewers on or near the property 2. If a public sewer is within 3m of the proposed extension footprint (3m from the centreline of the sewer to the nearest face of the extension foundation), a BOA or self-certification is required before construction begins 3. The drainage design for the new extension must take into account the presence of the public sewer — new drainage connections and underground drainage runs must be routed away from the sewer protection zone where possible

• *Connecting new extension drainage to the existing property drain vs. the public sewer*:
• In London, the standard approach for an extension kitchen drain is to run the new drain from the extension back to the existing property drain inspection chamber, rather than making a new direct connection to the public sewer. This:
• •Avoids the need for a Thames Water new connection application (which is more complex than a BOA)
• •Uses the existing drain capacity and the existing sewer connection point
• •Is simpler for Building Control approval

New direct connections to public sewers in London require a Thames Water 'Request for a new sewer connection' application — a separate process from the BOA.

**Drainage in London — common issues for extensions in older stock**:

*1. Shared drains*: In many London Victorian terraces, the foul drain in the rear garden is a 'shared private sewer' — historically owned and maintained by the adjacent properties, but adopted by Thames Water as a public sewer in many cases under the Water Industry Act 2011 (the sewer transfers of 2011 brought a large number of shared private drains into public ownership). Checking the CON29DW drainage search will reveal whether the rear garden drain is a public or private sewer.

If the drain is public: a BOA process applies for the extension. If the drain is private and shared between two or more properties: drainage easement rights (shared access, maintenance rights, and access for clearing) are typically implied or formally recorded, but new connections to a shared private drain require agreement from all the drain owners — not just Building Control sign-off.

• *2. Earthenware (clay) drains with rigid joints in old London houses*:
• Pre-1970s London drainage is typically earthenware (vitrified clay) pipe with rigid cement mortar joints. These rigid-jointed clay drains are prone to joint failure and root ingress over time. Before making any new connection to an existing old drain for an extension, it is good practice to:
• •CCTV survey the existing drain (a 'push-rod CCTV' survey) to check for existing root ingress, joint defects, and collapse sections
• •If the existing drain has significant defects, repair or replace the defective sections before connecting the new extension drain

*3. Interceptor traps in older London drains*: Many Victorian and Edwardian London properties have an interceptor trap (an old regulatory requirement now no longer required by Part H) in the inspection chamber nearest the public sewer. This is a sealed U-bend that was intended to prevent sewer gases from the public sewer entering the private drain, with a rodding arm above it for clearing blockages. Interceptor traps can cause blockages (they catch debris at the U-bend) and should be maintained or removed as part of a drainage works project. Building Control and Thames Water accept the removal of interceptor traps.

**Part H compliance sign-off and CCTV survey**:

• Building Control for a London house extension typically requires:
• •Approval of the drainage layout plan and specifications at plan-check stage
• •Inspection of the open drainage (before it is backfilled) at the drainage stage inspection — the inspector will check pipe size, gradient, bedding, and access points
• •A satisfactory CCTV survey of any drainage affected by the extension works at or after completion — required where drainage runs under or near new foundations, or where a BOA is in place
• •A completion certificate for the drainage works (as part of the overall extension completion certificate)