Do you need a Thames Water Build Over Agreement for a manhole cover in your back garden, and can you build over a manhole?

What do you do when a Thames Water manhole in your garden is in the way of your extension, and can you build over a manhole? My last two projects were afflicted with the problem of Thames Water public sewers running across the property with the manhole cover being right in the way of the project.

How to get a Thames Water Build Over Agreement

The good news is, yes, you can build over a manhole in your garden.

The answer is simple. You need a Thames Water Build Over Agreement with building permission granted by Thames Water. If your proposed development comes within three metres of the pipe, you need permission to build near it. It will be apparent if you check the paperwork and title deeds from the sale.

Thames Water owns and is required to maintain and have access to the pipes as long as they are connected to its network of sewers. To build over or near their sewers on your property, you require Thames Water’s written agreement, which must be passed on when the property is sold. This build over agreement assures Thames Water that the foundations of the new development do not impact the sewer system. Building Surveyors will ask to see the build over agreement before they sign off on the Completion Certificate. Extra weight from the foundations can cause the sewer to collapse and cause wastewater flooding, which may cause structural damage to the nearby building. The new building may have to be knocked down to prevent further damage to sewers and structures and reduce disruption to wastewater services. Thames Water will not allow you to build a new detached property over a sewer.

What is the cost of moving drains for an extension?

In this case, a sewer diversion (at your cost) is required, but it must be built to Thames Water specifications. These public sewers are in three (3) classes and three (3) sizes, requiring different application processes and fees. Class I is 160mm in diameter or smaller, Class II is greater than 160mm but less than 375mm, and Class III is greater than 375mm in diameter. Classes II and III require a CCTV camera survey inside the pipe before work begins. This survey ensures no repair work is needed. It is repeated after confirming that the results have not damaged the pipe.

What if my extension was built over a sewer without permission?

Building an extension over a sewer without permission can lead to legal, financial, and safety issues. It violates building codes and regulations in many areas, potentially resulting in fines, penalties, or legal action. Your property’s value may decrease, and you may be liable for costly sewer repairs or relocation. Additionally, it can compromise sewer access for maintenance and create health hazards. To address this situation, contact local authorities, seek legal advice, and be prepared to work with them to bring your extension into compliance, which may involve alterations or relocation. Acting proactively is essential to mitigate potential consequences.

Build Over Agreement Example

Share This Post


At Greenway Associates, we have completed over 2,000 architectural design projects