Building Over or Near a Sewer: Thames Water Guide for North London Homeowners

Introduction

London's Victorian-era sewer network runs beneath many residential gardens, and building over or near a public sewer requires formal consent from Thames Water. For homeowners planning rear extensions, basement works or other groundworks, discovering that a public sewer crosses their property can have significant implications for the design and programme of their project.

This guide explains Thames Water's building-over agreement process, what it requires, how to identify sewer routes and what to do if your proposed extension lies over or near a sewer.

Identifying Public Sewers on Your Property

The first step in any project involving groundworks is to identify whether public sewers cross the property. Thames Water provides a sewer records search service that shows the location of public sewers in relation to any property. This can be accessed via Thames Water's website (a fee applies for detailed searches) or through a solicitor or conveyancer as part of a property purchase.

It is important to note that sewer records are not always accurate — particularly for older Victorian sewers that were adopted without comprehensive survey. Ground-penetrating radar or CCTV investigation may be necessary to confirm sewer locations precisely before construction starts.

Thames Water's Building-Over Agreement

Where a proposed extension or other structure is to be built over or within 3 metres of a public sewer, a Building-Over Agreement (sometimes called a Build-Over Agreement or BOA) must be obtained from Thames Water before works commence. The purpose of the agreement is to ensure that:

• The sewer remains accessible for future maintenance and repair
• The new structure does not compromise the structural integrity of the sewer
• The sewer is protected from damage during construction

Types of Thames Water Consent

Thames Water operates a tiered consent system:

Self-Certification (Simple Build-Over)

For extensions over sewers of 160mm diameter or less, meeting specific construction criteria (foundations not within 1 metre of the sewer crown, structure loading not exceeding certain limits), Thames Water operates a self-certification scheme. The structural engineer designs to Thames Water's standard criteria and certifies compliance. A site-specific agreement is not required but the works must meet all the standard requirements.

Technical Assessment (Complex Build-Over)

For extensions over larger sewers or in more complex situations (near sewers in poor condition, sewers in areas with high groundwater, proposals that do not meet standard criteria), a formal application to Thames Water is required. Thames Water's engineers review the proposals and issue specific technical requirements that must be incorporated into the extension design. They may require CCTV survey of the sewer before and after construction, additional structural protection, or in some cases will refuse consent for building over.

No-Build Zones

Thames Water will not grant consent to build over certain sewer types — including sewers over 375mm diameter (public sewers of this size are generally considered too important to build over), sewers in very poor condition, combined storm/foul sewers that require regular access, and brick-built Victorian egg-shaped sewers that are difficult to repair under a structure.

Implications for Extension Design

Where a sewer crosses beneath a proposed extension, the structural engineer must design the foundation to avoid the sewer or to pass over it safely. Common approaches include:

• Spanning the extension on a reinforced concrete ground beam that bridges across the sewer route
• Using piled foundations that pass beside (not through or over) the sewer
• Relocating the sewer — possible in some cases but expensive and requires Thames Water consent
• Modifying the extension design to avoid the sewer route entirely

New Drain Connections

Any new drainage connections from an extension to the public sewer also require Thames Water consent (in addition to building regulations approval for the drainage design). A new connection to a public sewer requires a formal connection approval and the works must be carried out by a licensed contractor. The connection must be properly designed to avoid surcharging the public sewer and to prevent surface water from entering the combined sewer system where a separate sewer is available.

Costs and Programme Implications

Building-over agreement applications typically take 6–8 weeks for Thames Water to process. Programme contingency for this process should be built into the extension project programme. Technical assessment applications may take longer and may require additional investigations (CCTV survey: £300–£600, structural calculations: included in structural engineer's fee).

Conclusion

Building over or near a public sewer is a routine but important aspect of many north London extension projects. Identifying sewer routes early — as part of the pre-design site investigation — allows the structural engineer and architect to incorporate Thames Water requirements into the design from the outset rather than having to make late-stage revisions. An architect managing your project from the start will initiate the Thames Water sewer search and building-over agreement process at the appropriate stage of the programme.