Call us today on
020 7935 2502
Property Surveying Blog
Written by Beau Davies

A Thames Water build over agreement is required for any excavation works within 3 metres of a public sewer or drain. Given the density of construction in London and other cities, this will likely mean that any work you intend to undertake to your property, that includes excavations, will likely require Thames Water’s review and consent.

With the risk of ignoring or overlooking a Thames Water build over agreement being an injunction or non building regulations sign off, and with the costs of obtaining the agreement ranging from £299 to £1,300. We thought it would be helpful to take a look into this topic in a little more detail to make sure that when it comes to your proposed construction, this is not something that is missed.Blocked drains? Always check them before and after #patywall work!

The first point to note is that a build over agreement only applies to public sewers and drains.

So what is a public sewer or drain, put simply, a sewer that serves more than one property falls into the scope of a Thames Water build over agreement. As does a drain and in particular the point at which the drain pipe crosses the boundary from one property to another.

As Thames Water is legally responsible for the maintenance of these sewers and drains, build over agreements are in place to ensure that these elements have been considered within the design of your proposed foundations.

The agreement not only ensures that Thames Water can have sufficient future maintenance access to the sewer or drain, it also ensures that the proposed foundations are designed in a manner to reduce risk to the sewer or drain.  Alternatively, Thames Water may require that the proposed foundations are constructed at a safe distance from the sewer or drain, thereby avoiding the risk of damage.

Thames Water build over agreement

A Thames Water build over agreement is a seal of approval for the proposed works and generally speaking will be required for any excavation work within 3 metres of a public sewer or drain.

When considering the impact of new foundations and the current building regulations requirements which specify that new foundations must be a minimum of 1 metre deep. The depth of these foundations and the added weight of the new structure can lead to ruptured drains or pipes, which in turn could lead to wastewater flooding.

It is for this reason that that Thames Water build over agreements are in place and are generally required for the proposed structure to obtain a building regulations completion certificate, also know as building regulations ‘sign off’.

Thames Water build over agreements require various different information to be submitted to a Thames Water engineer prior to construction.

Although each application will require different information based of the location, lay of the land and the proposed works. Generally speaking, Thames Water will always require:

  • A completed application form
  • A drawing illustrating the site location plan
  • A drawing illustrating the existing drainage layout
  • A drawing illustrating the proposed drainage layout
  • A drawing confirming the cross section of the proposed foundations and existing drain or sewer

In circumstances where the Thames Water engineers feels it is appropriate, they may require a site visit to be undertaken to inspect the sewer or drain, or they may even request a Closed Circuit Television (CCTV) Survey to be undertaken.

This involves the drain or sewer being rodded with a small camera, thereby confirming the run of the sewer or drain, the condition of the sewer or drain and finally if they are currently sufficient when taking into account the changes the proposed structure will have upon them.

Overlooking or ignoring a Thames Water build over agreement could cause significant delay to a project as legally speaking construction works cannot commence until it is in place.    Therefore, it is key to ensure that this is factored both into the timings and costings of the project.Lintel over a drain pipe

There is also the risk of Thames Water requesting the structure is removed in the event that it is deemed to cause a risk to the public sewerage network, which is pretty much the last thing a homeowner undertaking construction works would want to deal with.

Causing damage to a public drain or sewer not only carries the risk of the homeowner being legally responsible for the repair costs to the public drain or sewer, but also the making good cost to the surrounding land or homes. Therefore, in the event of damage being caused to neighbour’s property, the costs could quickly run into the tens of thousands.

Here at Berry Lodge Surveyors, we are involved in hundreds of construction projects each year and would recommend speaking to Thames Water prior to commencing the proposed works, thereby ensuring your proposals meet all of the statutory requirements.

Build Over Section DrawingWe have seen a case whereby the homeowner assumed the contractor had taken care of the Thames Water build over agreement, while the contractor assumed the homeowner had. This confusion lead to a 4 week delay to the works while the appropriate site investigations were undertaken, plans were drawn up and Thames Water engineer visited site.

As you can imagine this not only added significant delay and cost to the homeowner, but also risked them losing their contractor, not to mention the added stress of the mistake.

If in doubt, a quick call to Thames Water could save a significant amount of future cost and delay!