What Is a Build Over Agreement and Do You Need One for Your Extension?

22, Jun, 2026

You've spent weeks planning your extension.

The drawings are ready. You've spoken to builders. You have a budget in mind. Then your architect, structural engineer, or building control officer asks a question you were not expecting: Have you checked whether there's a public sewer under the property?

Suddenly, a project that seemed straightforward becomes more complicated.

Many homeowners first discover a build over agreement during the planning or construction stage. Others find out much later when selling their property and a solicitor raises concerns about missing paperwork. In both situations, the worry is usually the same.

Will this delay the project, increase costs, or stop the extension altogether?

The good news is that most build over agreement issues can be resolved. The key is understanding whether you need one before construction starts.

What Is a Build Over Agreement?

A build over agreement is formal approval from the relevant water authority allowing construction work to take place over or near a public sewer.

Water companies such as Thames Water, Southern Water, Severn Trent, and United Utilities use build over agreements to protect access to public drainage infrastructure.

The agreement confirms that the proposed extension will not damage the sewer or prevent future maintenance if access is required.

Without approval, a water company may object to the construction or raise concerns later if issues arise.

When Do You Need a Build Over Agreement?

A build over agreement is required when building over or close to a public sewer.

Many water authorities apply guidance relating to construction within three metres of a public sewer, although specific requirements can vary depending on the location and the type of drainage infrastructure involved.

The need for approval often depends on:

• the position of the sewer

• the depth of the sewer

• the size of the extension

• foundation design

• the water authority involved

This is why drainage investigations are often carried out before detailed construction plans are finalised.

How Do You Check if a Sewer Runs Under Your Property?

The most common ways to identify a public sewer include reviewing drainage records, ordering a drainage search, or carrying out a CCTV drainage survey.

Architects and extension specialists investigate this early because discovering a sewer after plans have been approved can create avoidable delays.

If you are planning an extension, checking drainage routes early is usually one of the simplest ways to reduce risk.

How Much Does a Build Over Agreement Cost?

The cost depends on the water authority, the project's complexity, and the type of sewer involved. Typical costs include:

Additional costs may arise if drainage surveys, structural redesigns, or sewer diversions become necessary.

Will a Build Over Agreement Delay Your Extension?

This is one of the biggest concerns homeowners have.

In most cases, obtaining a build over agreement for an extension does not stop the project from moving forward. However, it can affect the timeline if drainage issues are identified late.

Approval periods vary between water authorities, and additional information may sometimes be requested before approval is granted.

This is why experienced designers and contractors often investigate drainage constraints during the early planning stages rather than waiting until construction begins.

Can You Build Without a Build Over Agreement?

Technically, some homeowners have completed extensions without obtaining approval.

The problem is not always immediate.

Issues often appear years later when:

• selling the property

• remortgaging

• dealing with drainage repairs

• responding to legal enquiries

Missing approval can create complications during conveyancing because solicitors frequently request evidence that any required build over agreement was obtained.

The short-term convenience can create long-term problems.

What Happens if an Extension Was Built Without One?

This is another situation that catches homeowners by surprise.

Often, the issue is discovered when a property is being sold and legal checks reveal that an extension was constructed over or near a public sewer.

In some cases, retrospective applications may be possible. However, approval is not guaranteed and additional investigations may be required.

The earlier the issue is identified, the easier it is usually to resolve.

Build Over Agreement vs Party Wall Agreement

Homeowners often confuse these two approvals, but they serve completely different purposes.

A Party Wall Agreement relates to work affecting neighbouring properties and shared structures.

A build over agreement relates specifically to construction affecting public drainage infrastructure.

It is possible for an extension project to require both approvals at the same time.

Understanding the difference helps avoid confusion during planning and construction.

How Long Does Approval Take?

Approval times vary depending on the water authority and the complexity of the project.

Straightforward domestic applications are often processed within a few weeks. More complex projects involving larger sewers, unusual designs, or additional engineering requirements may take longer.

The best approach is to allow sufficient time within the project programme rather than assuming approval will be immediate.

Avoiding Delays Before Construction Starts

A build over agreement is rarely the most exciting part of an extension project, but it can become one of the most important.

Understanding whether a public sewer affects your plans early helps avoid delays, redesign costs, and complications later during construction or property sales.

At Denham Crescent, we help homeowners identify potential drainage and planning issues before projects begin. From rear extensions and kitchen extensions to larger renovation projects, early investigation often prevents expensive surprises later and helps keep projects moving forward with confidence.

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