Drainage for House Extensions: What Builders and Clients Need to Know

**The four main drainage scenarios in rear extension projects**:

**Scenario 1 — No public sewer crossing the site**:

• The simplest scenario. The existing soil and waste drainage from the house connects to a private drain that runs within the property boundary to a public sewer at the boundary or in the road. The rear extension does not cross any public sewer. In this case:
• •The existing soil vent pipe (SVP) position is confirmed early in design (to confirm the extension does not cover the SVP access)
• •The extension's rainwater drainage (gutters, downpipes, rainwater pipe) is designed to connect to the existing surface water drainage system or a new soakaway
• •No Thames Water or other utility involvement is required for the private drainage within the boundary
• •Drainage within Building Regulations scope: the drainage design is assessed as part of the Full Plans Building Regulations application (Part H of the Building Regulations covers drainage and waste disposal)

*Cost implication*: Minor — drainage redesign and connection within the footprint adds typically £800–£2,500 to the groundworks package.

**Scenario 2 — Build-over agreement required**:

The most common drainage complication in London rear extension projects. A public sewer (previously adopted by Thames Water, typically a brick or concrete sewer installed pre-1937 but potentially newer) runs beneath or close to the proposed extension footprint.

*What a public sewer is*: A public sewer is owned and maintained by Thames Water (in London) and carries the combined or separate foul and/or surface water discharge from multiple properties. Thames Water has statutory rights to maintain, repair, and replace this sewer — meaning any structure built over or too close to the sewer may need to be removed at the landowner's cost if Thames Water needs access.

• *The Build-Over Agreement*: To build over or within 3m of a public sewer, the homeowner must obtain either:
• •A Build-Over Agreement from Thames Water (for structures built directly over a public sewer)
• •Or a Sewer Diversion Agreement (to move the sewer away from the footprint — more expensive but sometimes necessary where the sewer position makes build-over impractical)

*How to check*: Thames Water's 'Asset Map' (available online via the Thames Water website) shows the approximate location of public sewers. Any sewer shown within or near the proposed extension footprint should be treated as a potential build-over issue until confirmed otherwise. Thames Water also issues a 'section 105A report' — a formal search of their records — which solicitors and architects use.

*Build-Over Agreement process*: 1. Submit a build-over application to Thames Water with structural drawings and drainage details showing the proposed foundations and their proximity to the sewer 2. Thames Water reviews the application and may require a CCTV survey of the sewer to confirm its current condition (cost: £400–£800) 3. Thames Water approves or refuses the application (typically 4–6 weeks determination) 4. If approved, the Build-Over Agreement is issued with conditions — typically minimum foundation depths, structural continuity across the sewer, and access requirements

*Cost of a Build-Over Agreement*: Thames Water application fee: £355 (2025 rate). CCTV survey (if required): £400–£800. Additional foundation design by structural engineer: £500–£1,500. Total additional cost: £1,255–£2,655.

*What happens without a Build-Over Agreement*: Building over a public sewer without approval is a contravention of the Water Industry Act 1991. Thames Water can require demolition of the structure and may charge the cost of making the sewer accessible back to the landowner.

**Scenario 3 — Sewer diversion required**:

Where a public sewer runs directly through the proposed extension footprint and build-over is not feasible (typically where the sewer is too large, too shallow, or Thames Water objects to the build-over), the sewer must be diverted. Sewer diversions are expensive and time-consuming:

*Cost*: Typically £8,000–£25,000 depending on sewer depth, length, and access requirements. Thames Water must approve the diversion design and may carry out the diversion themselves. *Programme impact*: 6–14 weeks from application to completion, depending on Thames Water's workload and the complexity of the diversion.

In some cases, a sewer diversion makes the proposed extension commercially unviable — particularly for smaller extensions where the cost of diversion exceeds the added value. The design should be reviewed to see whether a smaller extension footprint can avoid the sewer entirely before a diversion is committed to.

**Scenario 4 — New drainage connection required**:

• Where the extension creates a new WC, shower, basin, or utility room with drainage requirements, a new connection to the existing soil drainage must be designed. In London rear extensions, this is particularly relevant where:
• •A downstairs WC or utility room is being added as part of the extension
• •The kitchen drainage is being rerouted as part of the open-plan redesign
• •A new drainage access chamber (inspection chamber) is required to serve the new connection

*What is required*: New drainage runs to BS EN 1401 (uPVC plastic drain pipe), minimum 1:40 fall for 100mm foul drain; new access chamber at junction and any change of direction; Building Control inspection at drain-laying stage.

**Why surface water drainage matters for extensions**:

A rear extension adds new impermeable roofscape — a flat roof of 25m² or a pitched roof of 35m² both generate significant rainwater runoff that must go somewhere. In London, the surface water drainage infrastructure (drains and public sewers) is under significant pressure — Thames Water and the Mayor of London's planning policy actively require new development (including domestic extensions) to manage surface water drainage on-site where possible, using Sustainable Drainage Systems (SuDS).

**The options for surface water disposal**:

*Option 1 — Connect to existing surface water sewer*: Where a separate surface water sewer is accessible within the property boundary, the new rainwater downpipes from the extension can connect into it. Check whether the existing sewer is a combined (foul and surface water) or separate (surface water only) system — Local Authority Building Control (LABC) guidance specifies that new surface water connections should go to the surface water sewer, not the combined foul sewer, where a separate system exists.

*Option 2 — Soakaway*: Where the ground is permeable and there is sufficient space, a soakaway (a rubble-filled or crate-type buried pit that allows rainwater to percolate into the ground) is the preferred SuDS option. Soakaway design is governed by BRE Digest 365; a minimum percolation test must be carried out to establish the ground absorption rate before a soakaway can be designed. Not all London soils are suitable — heavy London clay soils (which cover much of inner London) typically have insufficient percolation to make a soakaway viable. Cost of a crate soakaway: £600–£1,500 installed.

*Option 3 — Rainwater harvesting or attenuation*: For larger extensions or where planning requires it, a rainwater harvesting tank (collecting rainwater for garden use or WC flushing) or a rainwater attenuation tank (holding peak flows and releasing them slowly to the public sewer) may be specified. Cost: £1,500–£5,000 for a small domestic system.

*Option 4 — Green or blue roof*: For flat roof extensions, a green roof (planted with sedum or wildflower mat) or blue roof (designed to retain and slowly release rainwater) provides surface water attenuation and potentially helps meet planning SuDS requirements in London. A basic sedum green roof adds approximately £40–£70/m² to the roof specification. See rooflight-types-guide and flat-roof-guide for more on flat roof construction.

**When planning requires a SuDS drainage strategy**:

For extensions in London over 100m² in footprint (major development threshold — unlikely for most domestic extensions but possible for large wraparound extensions), a full SuDS drainage strategy is typically required as part of the planning application. For smaller extensions in sensitive drainage areas (Critical Drainage Areas in London, as designated by the EA and GLA), a simple surface water strategy demonstrating that the extension will not increase peak surface water runoff may be required. Check the LPA's local planning documents and the GLA's London Sustainable Drainage Action Plan for site-specific requirements.

**Drainage in the planning application**:

• For most domestic extensions (under 100m² footprint), drainage is not a primary planning consideration. However:
• •A planning condition may require drainage details to be submitted and approved before development starts (discharge of condition application: £34)
• •The London Plan Policy SI 13 (Sustainable Urban Drainage) requires all major development to manage surface water drainage; some LPAs apply a looser version of this to householder applications in high-risk areas
• •A sewer build-over affecting a public sewer may be noted in the planning decision as requiring Thames Water consent separately from planning

**Drainage in the Building Regulations process**:

• Part H of the Building Regulations (Drainage and Waste Disposal) governs drainage in all new construction and material alterations, including extensions. Part H requires:
• •New drainage to be laid to adequate falls (minimum 1:40 for 100mm pipes) with rodding access at junctions and every 12m on straight runs
• •Adequate ventilation of the drainage system (soil and vent pipe extending above eaves level)
• •Surface water disposal to be via an adequate system — soakaway, sewer connection, or other approved system
• •No surface water to connect to a foul sewer (in a separate drainage area)

Building Control will inspect the drainage at the drain-laying stage (before backfilling) as one of the mandatory inspection stages. This is the most important compliance check for drainage — do not allow the contractor to backfill drainage without a Building Control inspection.

**Practical drainage checklist for extension clients**:

• •Before design is finalised: check the Thames Water asset map for any public sewers crossing or near the proposed footprint
• •At design stage: confirm the proposed surface water drainage strategy (soakaway, sewer connection, or attenuation)
• •When receiving quotes: ensure drainage works (new connections, soakaway, any build-over application) are explicitly included or excluded with a clear sum — do not leave drainage as a provisional sum if the sewer position is known
• •During construction: do not allow drainage to be backfilled before Building Control inspection
• •At completion: ensure drainage CCTV survey is available if required by Thames Water; ensure all drainage connections are shown on the 'as-built' drawings issued at practical completion