What Happens If an Extension Was Built Over a Public Sewer Without a Build Over Agreement?
Blueprint UK
8/4/20263 min read
Discovering that an existing extension has been built over a public sewer without an obvious Build Over Agreement can be concerning — particularly if you are buying the property or planning further work.
However, it does not automatically mean that the extension has to be removed.
The first step is to establish exactly what sewer or drain is involved, who is responsible for it, when the extension was constructed and whether the necessary approvals or agreements were obtained.
What Is a Build Over Agreement?
A Build Over Agreement is an agreement with the relevant water or sewerage company that permits building work to take place over, or close to, a public sewer.
The purpose is to protect the sewer and ensure that the water company can retain appropriate access for inspection, maintenance and repairs.
The requirements can vary depending on the water company, the location and the nature of the proposed building work. Therefore, it is important to check the requirements that apply specifically to your property.
What If the Extension Has Already Been Built?
If you discover that an existing extension appears to have been constructed over a public sewer and there is no obvious Build Over Agreement, don't assume that the extension is automatically unlawful or needs to be demolished.
The first step is to establish the facts.
You should check:
Whether the drain is private or a public sewer.
The exact location and route of the sewer.
When the extension was constructed.
Whether planning permission was required and obtained.
Whether Building Regulations approval was obtained.
Whether a Build Over Agreement already exists.
Whether the relevant water company has any record of the works.
If the location or condition of the sewer is unclear, a drainage survey may also be appropriate.
What If You Are Buying the Property?
If you discover the issue during a property purchase, raise it with your solicitor or conveyancer before completing the purchase.
They can investigate the property's legal documentation and determine whether there are existing agreements, restrictions, indemnities or other matters that need to be addressed.
You may also need to contact the relevant water company to establish whether the sewer is public and what options are available.
Importantly, the fact that an extension has existed for many years does not necessarily mean that all relevant approvals or agreements were obtained.
Can the Situation Be Resolved?
There is no single solution that applies to every property.
Depending on the circumstances, the water company may require additional information, inspections or technical documentation. In some cases, remedial works may also be necessary.
The appropriate route will depend on factors such as:
the type and size of the sewer;
its location beneath the extension;
the structural arrangement of the building;
access requirements for maintenance;
the age of the extension; and
the documentation available for the original works.
If you are planning a new extension, dealing with the sewer before construction begins is considerably easier.
Your architect and structural engineer can take the sewer into account when developing the design, while the relevant water company can confirm whether a Build Over Agreement or other approval is required.
Does an Extension Over a Sewer Have to Be Removed?
Not necessarily.
The existence of a sewer underneath an extension does not, by itself, mean that the extension must be demolished.
The key considerations are whether the sewer is adequately protected, whether the relevant water company can maintain and access it where necessary, and whether the appropriate requirements have been satisfied.
Every property is different, so the situation should be assessed based on the specific circumstances and available documentation.
What Should You Do?
If you discover that an extension has been built over a public sewer and there is no obvious Build Over Agreement, consider taking the following steps:
Establish the location of the sewer.
Confirm whether it is a public sewer or private drain.
Check the property's planning and Building Regulations documentation.
Speak to your solicitor or conveyancer if you are purchasing the property.
Contact the relevant water company to establish its position.
Obtain professional technical advice where necessary.
Consider a drainage survey if the sewer's location or condition is uncertain.
Addressing the issue early can help prevent delays, unexpected costs and complications if you later sell the property or carry out further alterations.
Final Thoughts
An extension built over a public sewer without an obvious Build Over Agreement is something that should be investigated, but it is not automatically a reason to assume the extension must be removed.
The important questions are:
What type of sewer is it? Who is responsible for it? When was the extension built? What approvals were obtained? And does the existing structure affect the sewer company's ability to access and maintain it?
Getting these questions answered early can give you a much clearer picture of the situation and help you determine the appropriate next step.
If you're planning a new extension, identifying drainage and sewer constraints at the design stage can also help avoid costly complications later.
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