What is a build over agreement and do I need one?

Many extensions sit over or near a shared drain that became a public sewer in 2011, and owners often don't realise consent is needed. Here is when it applies and what it involves.

Why public sewers trigger it

Since the 2011 sewer transfer, many drains that were once private and shared between properties became public sewers owned by the water company. The company has a statutory right to maintain them, so it must protect its ability to access and repair the pipe. Building over or beside it without safeguards could damage the sewer, or trap it under your extension where it cannot be reached. The build-over agreement is how the company controls that risk and records the conditions under which you may build.

What the agreement involves

The process varies by company but typically includes:

• Application: you submit details of the proposal and its position relative to the sewer.
• Survey: a CCTV survey of the sewer to confirm its condition and exact line, often required before sign-off.
• Conditions: protecting the pipe, maintaining manhole access, and sometimes designing foundations to bridge the sewer or keep a set clearance.
• Fee: an application charge, plus survey costs.

Indicative triggers; exact rules vary by water company. Source: GOV.UK build-over guidance; water company self-lay/build-over pages.

What happens if you skip it

Skipping a build-over agreement is a real risk, not a technicality. The water company can later require you to alter or remove work obstructing its sewer, you may struggle to get the work signed off, and — most commonly felt — buyers' solicitors and lenders flag the missing agreement during a sale, holding things up until it is regularised (which can be harder and dearer after the fact). It is far cheaper to apply before building. The agreement is separate from your Building Regulations approval, but Building Control will expect drainage to comply with Part H and may ask whether a build-over agreement is in place.

Build over, build near, or divert: choosing the route

When a public sewer crosses your plans you usually have three options, and the water company's agreement covers whichever you choose. They are not equal in cost or disruption, so it is worth understanding them before settling the design.

• Build over: keep the sewer where it is and build above it, under the agreement, with measures to protect the pipe — for example designing the foundations so they bridge or clear the sewer, and keeping a way to access it. Often the least disruptive option, but it leaves a sewer permanently under your structure.
• Build near: where the extension sits within the agreement distance (commonly around 3m) but not directly over the sewer, a build-near consent with protective conditions may be all that is required.
• Divert: move the sewer out of the way. Cleaner long-term and keeps the pipe outside the building, but it is a bigger job — re-laying the sewer to the correct fall, providing access, and the water company's approval and inspection — so it usually costs more.

The agreement typically sets conditions such as keeping manhole access, using flexible joints near the structure to tolerate movement, designing foundations to span the sewer, and not building over a manhole. A common complication is a manhole within the proposed footprint: the water company may require it to be relocated or rebuilt with a sealed, accessible cover, which adds cost. Start the conversation with your sewerage undertaker early, ideally before the design is fixed, because their requirements can change where the extension can go and how the foundations are built. Treating the build-over agreement as a design input rather than an afterthought avoids redesigning the extension — or regularising it under pressure at sale — later on.

Self-certified vs full agreements, and the 2011 transfer

Two things help explain why so many extensions need a build-over agreement and why the process is not always the same. The first is the 2011 private sewer transfer. Before then, the shared drain running between terraced or semi-detached houses to the boundary was usually private, jointly owned by the householders. In October 2011 most of these became public sewers vested in the water company. The practical effect is that a pipe many owners still think of as "their drain" is now the company's responsibility — and building over it brings the build-over rules into play even though nothing physical changed.

The second is that not every case needs the same level of consent. Water companies increasingly offer a tiered approach:

• Self-certification / standard build-over: for straightforward cases — a smaller sewer, modest depth, building over with standard protective measures — some companies allow a simplified or self-certified route against set conditions, which is quicker and cheaper.
• Full build-over agreement: for larger or deeper sewers, a manhole in the footprint, or anything outside the standard conditions, a full application with a CCTV survey and individual assessment is required.
• Refusal or diversion: where building over would unacceptably risk the sewer or block access, the company can refuse, leaving diversion as the route.

Which tier applies depends on the sewer's size, depth, condition and position relative to your extension, which is exactly why a CCTV survey is so often part of the process. The message for any extension over or near a shared drain is the same: assume the 2011 transfer may have made it a public sewer, ask the water company early, and let their requirements shape the design before it is fixed — rather than discovering at sale that the work was never authorised.

Frequently asked questions

Sources & further reading

• GOV.UK — building over or near a public sewer
• Planning Portal — Approved Document H (drainage)
• LABC — drainage and build-over considerations

Figures on this page are typical UK ranges drawn from published sources and depend on your specific site. They are guidance, not a quotation.