Building Over a Sewer: Build-Over Agreements and Easements

Summary

A "public sewer" — typically the drain from your house that runs across your garden and into the street to the main sewer — is owned and maintained by the local water/sewerage company (Thames Water, Severn Trent, Yorkshire Water, Anglian Water etc.) since the 2011 transfer of private drains to the public sewerage system. This means almost every UK garden has a public sewer running through it, and any extension, conservatory, garage conversion, or garden room placed over or near that sewer requires consent.

Most extensions in the UK are now built over or near a public sewer. The Build Over consent process is the water company's mechanism to: (1) ensure the structure won't damage the sewer, (2) ensure they retain access to the sewer for future repair (or that you've agreed an easement that accepts the consequences if access requires demolition), and (3) ensure your structural detailing (lintels, padstones, foundations) protects the sewer from imposed loads.

For homeowners and tradespeople, the Build Over process is one of the most commonly missed Building Control steps on extension projects — partly because Building Control don't always proactively flag it, and partly because it's a separate process from Planning and from Building Regs. The cost of not getting it is significant: water companies can require the structure to be opened up for inspection, can recover their costs if your build causes a sewer failure, and the title of your property is encumbered with the lack of consent forever (it shows up in conveyancing).

Key Facts

• Primary legislation — Water Industry Act 1991, Section 106 (right to connect) and Section 199 (build-over restrictions); Flood and Water Management Act 2010 (sewer transfer)
• Public sewer definition — A sewer that serves more than one property OR has been transferred to the water company under the 2011 transfer; almost all drains downstream of a property boundary are now public
• Build Over Agreement — Formal consent from the water company; typically required for builds DIRECTLY OVER a sewer or within close proximity
• Self-Cert Build Over — Simplified process for routine proximity work meeting specific criteria (e.g. sewer depth, distance from foundations)
• Easement — A legal right of access for the water company over your land; runs with the title regardless of who owns the property
• Typical "3m rule" — Any structure within 3m of the centreline of a public sewer triggers the consent process; some water companies use a different distance based on sewer depth
• CCTV survey requirement — Most Build Over Agreements require a pre-build CCTV survey of the sewer condition (you can't take over responsibility for an already-broken sewer); £250–£500 typical
• Structural detail — Typically requires a concrete lintel over the sewer to bridge the load, or a granular surround if the sewer is deep
• Access provision — Manholes within the build footprint must remain accessible (typically via a recessed manhole cover under floor finish)
• Consequence of building without consent — Water company can require opening up for inspection at your cost; if sewer fails, you may be liable; title issue for resale
• Response time — Water companies have 21 days to respond to a Self-Cert (silent = deemed approved in some areas); 28 days for full Build Over Agreement
• Cost (homeowner) — Self-Cert £150–£500; Full BOA £350–£1,500+ depending on water company and complexity
• Cost (tradesperson side) — Survey, drawings, engineer's input £400–£2,000; structural reinforcement (lintel, padstones, concrete surround) £500–£3,000+
• Trade name — Often called a "sewer build-over consent" in the trade; same thing

Quick Reference Table

Fees as published 2025–2026; verify with the water company at time of application — they change annually.

Detailed Guidance

When You Need Consent

The trigger is proximity to a public sewer. Public sewers include:

• The drain leaving your property at the boundary, regardless of which side
• Shared drains serving multiple properties (transferred to public ownership 2011)
• Surface water sewers in adopted highway

A drain serving ONLY your property between your kitchen waste and the property boundary is a private drain — no Build Over consent needed.

A drain that crosses your garden but serves another property (e.g. your neighbour's bathroom flows through a pipe under your back garden) is a public sewer — Build Over consent needed.

The "3m rule" is the common threshold: any structure (extension, conservatory, garage) within 3m of the centreline of a public sewer triggers the consent process. Some water companies use 3m, others use a distance equal to 1× or 2× the sewer depth — check with the local company.

Self-Cert vs Full Build Over Agreement

Self-Cert (Self-Certified Build Over Notification) — A simplified process where the builder/homeowner certifies that the proposed works meet specific criteria. Typically allowed where:

• The sewer is <2m deep
• The structure is single-storey
• Foundations don't bear directly on the sewer
• The sewer is in good condition (CCTV evidence)
• No manhole falls within the build footprint

Process: submit form + plans + CCTV survey; water company has 21 days to respond. Silent response = deemed approved in most areas.

Full Build Over Agreement (BOA) — The formal process. Required where:

• The sewer is >2m deep
• A manhole falls within the build footprint
• The structure is two-storey or load-bearing over the sewer
• The sewer is in poor condition
• The detail is unusual (e.g. piled foundations near sewer)

Process: submit detailed application + structural calculations + drawings + CCTV; water company has 28 days; may require an inspection visit, structural detail review, and a signed legal agreement.

The Process — Step by Step

1. Sewer location survey — Identify the sewer route on your property. Water company will provide a Section 199 plan showing recorded sewers in the area. Cost: typically free for a basic plan; £100–£300 for a detailed search.
2. Determine route and depth — Use the plan plus on-site investigation (manholes, drain probing). Sewer depth determines structural detail.
3. Initial CCTV survey — Required for most consents. £250–£500. Confirms condition of sewer pre-build.
4. Structural design — Foundations must not impose new load on the sewer. Options: bridge the sewer with a reinforced concrete lintel; foundations bear on either side of sewer with no contact; deep piled foundations either side.
5. Application — Online or by post to the water company. Pay the fee.
6. Wait for response — 21 days (Self-Cert) or 28 days (BOA). Don't start work in this period.
7. Comply with conditions — Often: provide a recessed manhole cover for any manholes in the floor; provide rodding eyes; submit as-built drawings; final CCTV at completion.
8. Final sign-off — Some water companies require a final inspection or accept the as-built submission as sign-off.

Manholes Within the Build Footprint

If a manhole sits where you're building:

• Cannot be paved over without access provision (illegal under Water Industry Act)
• Recessed manhole cover (e.g. Saint-Gobain CRH series) sits flush with floor finish and lifts out for access; standard solution
• Manhole CANNOT be relocated without engaging the water company (their asset)
• If the manhole falls inside a habitable room (e.g. a bedroom over the extension), an internal manhole cover is acceptable; if it falls under a bathroom or kitchen, the water company may require relocation or specific detail

Structural Detail — Bridging the Sewer

The typical detail where foundations cross a sewer:

• Sewer protected with a granular surround (typically 150–300mm pea shingle) to allow movement
• Concrete lintel (typically 300mm deep × full wall thickness, RC with engineer's reinforcement) spanning above the sewer, transferring load to padstones either side
• Foundation depths to engineer's design; usually deeper than standard strip foundations to reach below the sewer's influence

For sewers >2m deep, full piled foundations either side of the sewer may be required. Engineer involvement is non-optional at this depth.

Consequences of Not Getting Consent

The water company can:

• Require you to open up the structure for inspection of the sewer at your cost
• Charge you for sewer repairs if the sewer is damaged by your build
• Enter your property for sewer access without notice in some circumstances
• Apply for a court order to require structural alterations

For the homeowner:

• Title is encumbered — any future conveyancing will require disclosure
• Property insurance may exclude sewer-related damage
• Resale is harder; some buyers walk away from non-consented work
• Cost of retrospective consent (where possible) typically 2–3× the original consent cost

Worked Example: Standard 3m × 4m Rear Extension Over Sewer

Job: 1930s semi with 100mm clay sewer running 1.5m from house wall across rear garden. Proposed 3m × 4m single-storey rear extension straddles the sewer line. Sewer depth 1.2m at house wall, 1.5m at garden side. Sewer in good condition per CCTV.

Process: Self-Cert applicable. Customer-side costs:

• Sewer search + Section 199 plan: £100
• CCTV survey: £350
• Application fee (Thames Water example): £196
• Structural engineer (concrete lintel detail): £450

Tradesperson-side additional materials:

• Granular surround (pea shingle, 1m³): £80
• RC lintel (custom precast, 4m long): £450
• Padstones (2× pre-cast): £85
• Additional excavation/labour: £400

Total Build Over cost over and above standard extension: ~£2,100

This is the kind of number that's often missing from "build me an extension" quotes. Include it in your initial pricing or flag it explicitly as a customer-side cost.

Frequently Asked Questions

How do I find out if there's a sewer under my proposed extension?

Request a Section 199 plan from your local water company (usually free online; £100–£300 for detailed search). The plan shows recorded sewers, but it's not always 100% accurate — pre-2011 private drains may not be mapped accurately. Confirm with on-site investigation: locate any manholes in the garden and trace their pipework with a drain rod or CCTV.

Can I move a public sewer to avoid the Build Over process?

Yes, but it's expensive and the water company must approve the diversion (their asset, their consent). Cost is typically £3,000–£15,000+ depending on length, depth, and reinstatement. Used as a last resort; usually a Build Over Agreement is cheaper and faster.

What happens if the sewer breaks under my new extension?

If the build was consented and the structure complies with the consent conditions, the water company is responsible for repair (it's their asset). If the build caused or contributed to the failure, you may be liable for the repair cost AND any opening-up of your structure to access the sewer. CCTV evidence pre- and post-build is your protection against false claims.

Is the Build Over Agreement transferable when I sell the house?

The agreement runs with the property, so yes. The buyer's solicitor will check for consent during conveyancing; producing the BOA is straightforward if you have it. Without it, the buyer will either demand an indemnity policy (insurance against future water company action) or reduce the offer.

What's the difference between Building Control sign-off and Build Over consent?

Two separate processes. Building Control (your local authority or Approved Inspector) checks Building Regulations compliance — structure, fire, energy, etc. The water company's Build Over consent checks sewer-specific risks. Both are needed for an extension over a sewer; one doesn't substitute for the other.

Regulations & Standards

• Water Industry Act 1991 — Sections 106 and 199 (public sewers; build-over restrictions)

• Flood and Water Management Act 2010 — Transferred most private drains to public ownership in October 2011

• Building Regulations 2010 / Approved Document H — Drainage and waste disposal; foul/surface water drainage requirements

• Building (Approved Inspectors etc.) Regulations 2010 — Process for sewer consent within Building Control framework

• Sewers for Adoption — Industry standard for sewer construction (water companies' technical reference)

• NHBC Standards Chapter 5.3 — Drainage; warranty-provider requirements

• CIRIA C737 — Specifying sewers (industry technical guidance)

• Thames Water Build Over guidance — Largest water company; representative process

• Severn Trent Build Over — Process and forms

• Water UK industry portal — Coverage of all water companies

• Water Industry Act 1991 on legislation.gov.uk — Statutory text

• part h drainage — Building Regulations Part H drainage requirements

• building control — Building Control sign-off process

• drain testing — CCTV and drain testing methods

• permitted development extensions — Planning permission interaction