Build Over Agreement for Public Sewers: When You Need One, the Application Process and Design Restrictions

Quick Answer: A Build Over Agreement (BOA) is required from the water authority before building over or within 3m of a public sewer. Applications are made through the relevant water authority (Thames Water, Severn Trent, etc.) before works commence. The agreement specifies restrictions on foundation type, minimum cover to the sewer, and inspection access requirements. Building without an agreement where one is required is a criminal offence under the Water Industry Act 1991.

Summary

Public sewers cross thousands of UK gardens and back plots. Many homeowners are unaware that they have a public sewer beneath their land until they apply for planning permission or commission a drainage survey. Building over or within 3 metres of such a sewer without a Build Over Agreement is illegal and can result in the water authority requiring immediate demolition.

The Build Over Agreement process is managed by the local water authority (e.g., Thames Water in London, Severn Trent in the Midlands, Anglian Water in East Anglia). It protects the water authority's access rights for maintenance and future upsizing of the sewer while allowing development to proceed where structurally sound.

For groundworkers, understanding the BOA process means knowing when to stop and check before laying foundations, and knowing what design restrictions the agreement will impose on the foundation type, depth, and concrete specification.

Key Facts

• Water Industry Act 1991 — Section 109: right to connect to public sewer; protects public sewer infrastructure
• Build Over Agreement — written agreement from the water authority permitting construction over or near a public sewer
• 3m trigger distance — works within 3m horizontally of the centreline of a public sewer require a BOA (applies in most water authority areas; check locally)
• Coincident build — building directly over the sewer always requires a BOA regardless of depth
• Statutory right of access — water authorities have statutory right to enter private land to inspect and maintain public sewers; the BOA preserves this
• Sewer adoption — since the 2011 sewer adoption legislation, many former private sewers (serving more than one property) became public sewers; check with the water authority
• Section 101A adoptions (2011) — the Water Industry (Schemes for Adoption of Private Sewers) Regulations 2011 transferred many private sewers to public ownership; many homeowners are unaware their "private drain" is now a public sewer
• CCTV survey — water authority typically requires a CCTV survey of the sewer before and after construction; provides baseline condition record
• Foundation restrictions — BOA typically requires: minimum 100mm clearance between foundation and sewer crown; no pile caps over the sewer; access manhole not obstructed
• Sewer diversion — as an alternative to BOA, the sewer can be diverted at the developer's cost; requires water authority approval and is typically expensive and disruptive
• Domestic BOA vs commercial — most domestic BOAs are handled via the water authority's standard online process; commercial projects may require a more complex agreement

Quick Reference Table: Build Over Agreement Triggers

Detailed Guidance

Checking for Public Sewers on the Site

Step 1: Water authority sewer records Every water authority maintains a map of public sewers. You can request a copy of the sewer map for a specific address:

• Thames Water: use the online map or submit a form for a formal record
• Severn Trent, Anglian, Southern, etc.: similar online request tools
• These maps show the approximate route and depth of public sewers; they are indicative not exact

Step 2: Drainage survey A CCTV drain survey provides an accurate picture of what is actually in the ground. For development projects, the water authority typically requires a CCTV survey as part of the BOA application.

Step 3: Confirm adoption status Following the 2011 sewer adoption, many drains serving multiple properties became public. If you are dealing with an older property whose drainage survey shows a shared drain, it may now be public. Call the water authority's developer services line to confirm.

The Build Over Agreement Application Process

The specific process varies by water authority, but the general steps are:

Step 1: Pre-application enquiry Contact the developer services team of the relevant water authority. Provide the site address and a brief description of the proposed works. They will confirm whether a BOA is required and provide guidance on the application.

Step 2: Pre-construction CCTV survey Commission a CCTV drain survey of the public sewer in the vicinity of the proposed works. The water authority requires this to establish the baseline condition of the sewer before construction begins. The survey should be carried out by a contractor with CCTV survey capability; submit the report to the water authority.

Step 3: Application submission Submit the BOA application with:

• Site plan showing the proposed building footprint and sewer route
• Structural drawings showing foundation type, depth, and dimensions
• CCTV survey report
• Details of any drainage connection proposed

Most water authorities charge an application fee: typically £300–£800 for a domestic BOA; more for commercial.

Step 4: Review and conditions The water authority reviews the application and issues the BOA with conditions. Common conditions:

• Minimum clearance between foundation and sewer crown (typically 100mm)
• No piling within specified distance of sewer centreline
• Foundation design to ensure sewer can be accessed for maintenance
• Post-construction CCTV survey required
• Construction method statement for works close to the sewer

Step 5: Construction Carry out works in accordance with the BOA conditions. The water authority inspector may attend site during critical phases.

Step 6: Post-construction CCTV survey A follow-up CCTV survey after construction confirms the sewer was not damaged during works. The water authority compares pre- and post-construction footage. If damage is found, the developer must repair it at their cost.

Foundation Design Restrictions Near Public Sewers

The BOA will specify restrictions on how close foundations can be to the sewer and what types are permitted. Key restrictions:

Clearance:

• Minimum 100mm clearance between the soffit (underside) of the foundation and the crown of the sewer pipe
• Where the foundation is above the sewer (building over), a bridging slab or specially designed raft may be required

Foundation type:

• Strip foundations extending over the sewer are generally not permitted
• Where a strip foundation passes within 1m of the sewer, the strip must be deepened below the sewer invert level to avoid any load transfer to the sewer
• Raft foundations may be permitted over sewers if designed to span the sewer without bearing on it
• Piling within 1.5m–3m (varies by authority) of sewer centreline is typically restricted; check with authority

Load transfer: The foundation design must not transfer loads to the sewer or increase ground pressure around the sewer pipe. The structural engineer must demonstrate this in the drawings.

Access: If there is a manhole (access cover) within or adjacent to the building footprint, the BOA will require it remains accessible. This may require a manhole to be relocated, encased in a chamber accessible through the floor, or the building to be redesigned to avoid obstruction.

Sewer Diversion as an Alternative

If the BOA conditions make the proposed development impractical (e.g., the sewer runs directly through the building footprint and bridging is not feasible), the developer can apply to divert the public sewer under Section 185 of the Water Industry Act 1991.

The sewer diversion:

• Is carried out by the water authority or an approved contractor
• Is paid for entirely by the developer
• Typically costs £5,000–£50,000+ depending on length, depth, and urban constraints
• Requires planning permission for the diversion route
• Takes 3–6 months from application to completion

Given the cost and programme implications, sewer diversion should be considered only where a BOA cannot achieve an acceptable design.

Frequently Asked Questions

What happens if I build without a Build Over Agreement?

Building within 3m of a public sewer without a BOA is an offence under the Water Industry Act 1991. The water authority can serve a notice requiring the unauthorised works to be removed or modified to provide access to the sewer. In extreme cases, the developer can be required to demolish part of the building. Additionally, when the property is sold, the solicitor's drainage search will reveal the absence of a BOA, which can prevent the sale from completing.

Does the BOA apply to my roof overhang or only to foundations?

The BOA specifically concerns construction that could affect access to or the structural integrity of the public sewer. Roof overhangs that don't load the ground are not typically covered. The 3m rule applies horizontally from the sewer centreline to the foundation or any load-bearing structural element.

My drain survey says the sewer is 4m deep. Does the 3m rule still apply?

Yes. The 3m rule is horizontal, not related to depth. However, the water authority may take depth into account when specifying conditions — a very deep sewer may have fewer restrictions on overlying foundation design than a shallow one. Confirm with the water authority developer services team.

Can I check for public sewers myself using online tools?

Yes. Most water authorities provide an online public sewer map tool accessible from their websites. The level of detail varies; some provide approximate routes only. The Environment Agency's Flood Map and the Land Registry title plans do not show sewers. For accurate sewer data, use the water authority's tool or commission a full drainage survey.

Regulations & Standards

• Water Industry Act 1991 — Section 109: right to connect; Section 185: sewer diversion; protects public sewer access

• Water Industry (Schemes for Adoption of Private Sewers) Regulations 2011 — sewer adoption; many private sewers became public

• Building Regulations Approved Document H — drainage coordination requirements

• BS EN 752:2017 — Drain and sewer systems; design near existing infrastructure

• Water Industry Act 1991 — legislation.gov.uk — Section 109 and 185

• Thames Water — Build Over — example water authority BOA process

• Approved Document H — GOV.UK — drainage near existing sewers

• building regs part h drainage — drainage system design including sewer connections

• drainage falls and gradients — drainage gradient design coordinated with sewer connections

• cdm regulations groundworks — CDM obligations for drainage groundworks

• strip foundation design — strip foundation design near sewers