Foul Drainage: Connecting to Mains, Build-Over and Adoption

Summary

Foul drainage connections are one of the most procedure-heavy parts of a construction project. Unlike most building work, where a single Building Control submission covers compliance, foul drainage connections involve three separate regulatory regimes: Building Regulations (Approved Document H — Drainage), the water and sewerage company (statutory right to connect, adoption, build-over), and the highway authority (Section 50 street works licence for any trench in the public highway). Failure to engage the water company before starting work routinely costs builders weeks of delay, refusal of sign-off, and in some cases enforcement action requiring the new drainage to be lifted and re-laid.

The legal framework is the Water Industry Act 1991:

• Section 106 — owner's right to connect a private drain or sewer to a public sewer
• Section 104 — adoption of new sewers by the water and sewerage company (developer sewer agreements)
• Section 50 — street works licence for excavation in a public highway
• Section 199 — record keeping; the water company maintains the public sewer map
• Section 102 — adoption of existing private sewers (rare now after the 2011 transfer)

The day-to-day routine for a builder or plumber connecting a single extension or new dwelling to an existing public sewer is: identify the public sewer (sewerage search via the water company's GIS), establish whether the building is "over or within 3m" (build-over agreement needed), submit a New Connection application (consent to connect), arrange Section 50 if trenching in the highway, make the connection in accordance with the water company's witnessing requirements, and hand over a record drawing showing the connection point and as-built invert levels.

This article covers the connection process, build-over agreements, Section 50 street works, the new-connection application, witnessing/inspection by the undertaker, the DCG specification, saddle versus Y-junction connections, rocker pipes at manhole connections, and drop manholes where falls exceed 1m.

Key Facts

• Right to connect — Water Industry Act 1991, Section 106; every owner of premises has a statutory right to connect drainage to a public sewer, subject to consent
• Application required — water and sewerage company "New Connection" or "Section 106 application"; typical lead time 4–8 weeks, fee £400–£1,500 depending on company and scope
• Build-over agreement — required when constructing over or within 3m of the centreline of a public sewer up to 375mm diameter (Section 50 / Section 199); typical fee £400–£1,200; lead time 4–12 weeks
• Build-over distance trigger — within 3m of centreline OR within 1m of the outer face of the pipe (the more conservative of the two applies)
• Sewer adoption (Section 104) — applies to new sewers serving multiple properties intended to become public; requires Design and Construction Guidance (DCG) compliance; bonds and inspections required
• DCG (Design and Construction Guidance) — successor to Sewers for Adoption 7th Edition; published by Water UK; the current specification for adoptable drainage in England (Wales and Scotland have separate frameworks)
• Section 50 street works licence — required for any excavation in a public highway; issued by the local highway authority under the New Roads and Street Works Act 1991; typical fee £200–£800 plus traffic management
• Connection geometry — branch must connect at 45° (or less acute angle) in the direction of flow; never at 90°; never against the direction of flow
• Connection method — purpose-made oblique junction (Y-junction) preferred; saddle connector acceptable on existing clay or PVC-U main sewers
• Pipe materials accepted — PVC-U to BS EN 1401; vitrified clay to BS EN 295; ductile iron to BS EN 598; concrete to BS EN 1916; all acceptable under DCG and Approved Document H
• Rocker pipe at manhole — short (600mm) length of flexible-jointed pipe at every connection into a manhole to allow for differential settlement
• Drop manhole — required where a branch enters a chamber more than 1m above the invert of the through-drain; back-drop pipe internal or external to the chamber, with rodding eye at the top
• Witnessing — the water and sewerage company will witness the connection in many cases (Thames Water, Severn Trent, Anglian Water — varies by region); allow 1–2 weeks notice
• As-built record drawing — required by the water company on completion of a Section 106 connection; must show connection point, invert levels and route of new private drain
• Pre-1937 sewer transfer (2011) — most pre-1937 private drains became public sewers under the Private Sewers Transfer Regulations 2011; check the current sewer map carefully before assuming any drain is private

Quick Reference Table

Detailed Guidance

Identifying the Public Sewer

Before any design work, identify what sewers exist on or near the site:

• Free public sewer maps — most water companies offer a free online viewer (e.g. Thames Water "Sewer Records", Severn Trent "Asset Locations")
• Commercial drainage search — formal CON29DW search ordered through a search company or solicitor; typically £40–£80; required for conveyancing
• On-site inspection — manhole covers, gullies, vent stacks; lift covers to inspect direction of flow (always replace covers safely afterwards)
• CCTV survey — for any uncertain or distant connection, commission a CCTV survey to confirm pipe condition and invert levels at the proposed connection point; typically £350–£800

The 2011 Private Sewers Transfer brought most pre-1937 shared drainage into public ownership. A drain that was private before 2011 may now be public — always confirm with the current water company map.

Build-Over Agreement Process

Where a building or extension sits over or within 3m of a public sewer, the water company's Build-Over Agreement (BOA) is required. The process:

1. Submit application with site plan, proposed elevation, sewer location, construction method statement
2. Water company reviews — may request CCTV survey of the sewer to confirm condition before agreeing
3. Structural conditions imposed — typically: lintel over the sewer to bridge any future excavation; granular surround to the sewer beneath the building footprint; foundations either side of the sewer (not on it); minimum cover maintained
4. Building Control coordination — the BOA conditions become part of the Building Regulations approval
5. Witnessing on completion — water company inspector witnesses the structural detail before backfill

Typical fees: £400–£1,200; lead time 4–12 weeks. Failure to obtain a BOA before building leaves the property without legal authority to occupy the structure above the sewer, and the water company can require removal at the owner's cost.

The 3m centreline rule is a default trigger — some water companies set a more conservative limit on larger-diameter sewers. Always check the specific water company's BOA policy.

Section 50 Street Works Licence

Any excavation in a public highway requires a Section 50 licence under the New Roads and Street Works Act 1991. This covers trenching across or along a road to make a connection — common where the public sewer runs in the road and the new drain must cross the verge and carriageway to reach it.

The application requires:

• Site plan showing extent of excavation
• Method statement covering excavation, support, backfill and reinstatement
• Traffic management plan (often a separate fee — TM company quotes can be £500–£3,000 for road closures or stop-and-go)
• Public liability insurance £5m minimum
• Proof of competence (NRSWA streetworks unit qualifications — Street Works Operative and Supervisor cards)

Reinstatement standards are dictated by the Specification for the Reinstatement of Openings in Highways (SROH). Substandard reinstatement carries a 2-year defect liability period — failures during this period must be re-excavated and remade at the contractor's cost.

The New Connection Application

A New Connection application (often badged as a "Section 106 application" by water companies) is submitted online via the water company's portal. Typical content:

• Property address and plot reference
• Type of drainage (foul, surface water, combined)
• Number of new connections (single dwelling, multiple)
• Proposed connection point (public sewer reference, manhole or junction)
• Proposed materials and pipe size
• Site plan showing route of new drain
• Builder/contractor details

The water company reviews and either:

• Grants consent — sends connection conditions; arranges witnessing
• Refers to Build-Over team if construction within 3m of sewer
• Refuses or requires modifications — typically if connection point is unsuitable, capacity is constrained, or surface water cannot be discharged to the foul system (most common refusal)

Consent typically issued within 4–8 weeks. The connection must be made within the consent validity (typically 12 months).

Connection Geometry: Saddles, Y-Junctions, Manholes

The hydraulic principle is straightforward: never disturb the flow in the main sewer with a connection that opposes or perpendicularly intercepts the flow.

Oblique (Y) junction:

• Cut the existing main with a portable saw (suit pipe material)
• Insert a purpose-made 45° Y-junction with two flexible couplers
• Restrain pipework on both sides to prevent lateral movement
• Backfill with granular bedding to maintain support

Saddle connection:

• Suitable for existing clay or PVC-U mains (not for concrete or asbestos cement — these require specialist methods)
• Position saddle so that branch enters at 45° in the direction of flow
• Use a hole-saw to cut through the wall of the main pipe (do not snap or hammer)
• Bond the saddle to the main with the manufacturer's resin or strap clamp
• Allow cure time (typically 1–2 hours for resin) before testing

Manhole connection:

• Preferred over saddle/Y wherever a manhole is reasonably accessible
• Core-drill the chamber wall with a diamond core (typically 110mm for a 100mm pipe — 5mm clearance for sealing)
• Insert a flexible socket (manhole socket, e.g. Hepworth or Osma)
• Connect branch with a rocker pipe — short 600mm length with flexible joints both ends — to allow for differential settlement between the manhole and the surrounding ground
• Benching inside the chamber re-formed to direct the new flow toward the through-drain

Drop Manholes

Where a branch enters a manhole more than 1m above the invert of the main through-drain, Approved Document H1 (paragraph 2.39) requires either:

• Internal back-drop — vertical drop pipe inside the chamber, with rest-bend at the bottom and rodding eye at the top; pipe diameter at least the diameter of the branch
• External back-drop — vertical drop pipe outside the chamber, with rest-bend at the bottom and the connection brought through the chamber wall below the through-drain invert; preferred for new construction because it keeps the chamber clear for rodding

The drop is necessary to prevent the branch flow plunging onto the through-flow at high velocity, which would cause surcharging and damage. The rest-bend at the bottom redirects the flow horizontally into the chamber.

DCG: Design and Construction Guidance (Adoption)

For new drainage intended to be adopted by the water company under Section 104 — typically on housing developments — the Design and Construction Guidance (DCG, successor to Sewers for Adoption 7th Edition) sets the technical standard.

Key DCG requirements:

• Minimum pipe size — 150mm for adoptable foul sewers (100mm permitted only for laterals serving up to two dwellings)
• Materials — typically vitrified clay (BS EN 295), PVC-U (BS EN 1401) or ductile iron (BS EN 598); concrete (BS EN 1916) for larger sizes
• Gradient — 1:60 minimum for 150mm; 1:150 maximum for self-cleansing
• Bedding — Class B (concrete bed) or Class S (full concrete surround) for adoptable sewers under roads
• Manholes — pre-cast concrete (BS 5911) at every change of direction, change of gradient, junction, and at maximum 90m centres on straight runs
• Cover — minimum 1,200mm under adoptable highway
• CCTV survey of completed works — mandatory; report submitted to water company
• Bonds — developer posts a financial bond (typically 10% of construction cost) to cover defects in the first 12–24 months after adoption

Adoption agreements are typically negotiated by the developer's drainage consultant; smaller projects (single extension, single dwelling) do not normally involve adoption — they remain private drains connecting to an existing public sewer.

Witnessing and As-Built Records

Most water companies require witnessing of the actual connection. The notice period is typically 5–10 working days. The connection cannot be backfilled until the inspector has signed off the work.

Required at witnessing:

• Connection geometry correct (45°, in direction of flow)
• Saddle/junction sealed to manufacturer's specification
• Pipe bedding correct class for cover depth
• Flexible joints used throughout
• Drain water-test or air-test completed (typically 100mm water gauge for 5 minutes, max 25mm pressure drop)

After completion, an as-built drawing is submitted showing:

• Connection point (chainage or grid reference)
• Invert levels at connection and at any new manholes
• Route of new private drain
• Pipe material and size

This drawing becomes part of the water company's records and the property's drainage record.

Frequently Asked Questions

What's the difference between a public sewer and a private drain?

A public sewer is one vested in (owned by) the water and sewerage company — it serves multiple properties and is maintained at public expense, funded from sewerage charges on the bill. A private drain (sometimes called a "lateral drain" or "private sewer" depending on configuration) serves a single property and is the owner's responsibility to maintain. The 2011 Private Sewers Transfer brought most pre-1937 shared drainage into public ownership, so a "shared" drain serving two or more properties is now almost always a public sewer. The current public sewer map (held by the water company) is the authoritative record.

My extension is within 3m of a sewer — can I just notify the water company instead of getting a build-over agreement?

No. The Build-Over Agreement is a formal application that requires the water company's written consent and typically imposes structural conditions (lintels, granular surround, foundation positions). The "notification only" option applies only where the building is between 3m and 6m of the centreline — and even then, the water company may impose conditions on the excavation. Within 3m, a full BOA is required. Building without one carries enforcement risk and routinely causes problems on resale because the property's drainage will not show as compliant on the conveyancing search.

How long does a Section 106 application take?

Typically 4–8 weeks from submission to consent issued, depending on the water company. Some companies (Thames Water in particular) have been running longer turnaround times — 10–12 weeks at peak. Build it into the project programme. The application cannot be submitted retrospectively in any meaningful sense — connecting first and applying after is technically a breach and will trigger an "unauthorised connection" investigation. Submit at the same time as Building Control to keep the project moving.

Can I connect surface water (rainwater) to the foul sewer?

No, almost never. Approved Document H3 requires surface water to be disposed of by, in order of preference: (1) infiltration (soakaway), (2) watercourse, (3) surface water sewer, (4) combined sewer (only where no separate surface water sewer exists). The water company will refuse new surface-water connections to the foul system in almost all cases — sewage treatment works are not designed for storm-water peak loads, and cross-connecting causes pollution discharges and capacity overload. Some older properties have legacy combined systems but new work must separate the systems.

Who pays for the public sewer side of the connection?

The applicant (property owner or developer) pays for: the New Connection application fee (£400–£1,500); the saddle, Y-junction or manhole entry components; the excavation in the highway under Section 50; the reinstatement of the highway; the witnessing visit. The water company does not normally charge for the actual physical work of making the connection (which the contractor does), only for the application/consent and witnessing. Some water companies operate "Self-Lay Providers" — accredited contractors who can make the connection without separate water-company witnessing — which can be faster.

Regulations & Standards

• Water Industry Act 1991 — primary legislation; Sections 106 (right to connect), 104 (sewer adoption), 199 (records), 50 (street works), 102 (private sewer adoption)

• New Roads and Street Works Act 1991 — Section 50 street works licences and reinstatement requirements

• Specification for the Reinstatement of Openings in Highways (SROH) 4th Edition — reinstatement materials and methods after street works

• Approved Document H1 (Foul Water Drainage) — Building Regulations requirement for foul drainage design and installation

• Approved Document H3 (Rainwater Drainage) — surface water disposal hierarchy

• Sewerage Sector Guidance — Design and Construction Guidance (DCG) — Water UK guidance for adoptable sewers, successor to Sewers for Adoption 7th Edition

• BS EN 752:2017 — drain and sewer systems outside buildings; design and hydraulic performance

• BS EN 1610:2015 — construction and testing of drains and sewers

• BS EN 1401:2019 — plastic piping systems (PVC-U) for non-pressure underground drainage

• BS EN 295 — vitrified clay pipe systems for drains and sewers

• BS 5911 — precast concrete chambers, manholes and pipes

• The Private Sewers Transfer Regulations 2011 — transferred most pre-1937 private sewers to water company ownership

• Water UK: Sewerage Sector Guidance — DCG, build-over and adoption framework

• Approved Document H: Drainage and Waste Disposal — MHCLG official guidance

• Ofwat: Sewer Adoption — regulatory framework for sewer adoption and developer charging

• Thames Water: Building Over a Public Sewer — example water company process

• HSE: Construction (Design and Management) Regulations 2015 — CDM 2015 application to drainage construction

• Street Works UK — NRSWA street works qualifications and reinstatement standards

• underground drainage — below-ground drainage pipe sizes, gradients and Part H summary

• below ground pipe bedding and gradient — pipe bedding classes, gradient and cover for foul drains

• manhole construction — inspection chamber and manhole construction, cover loading classes

• build over agreement sewers — build-over agreement process for foundations near sewers

• building over a sewer build over agreement — compliance overview of build-over under Water Industry Act 1991