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If you are looking to extend your property close to the boundary of your property, you may need to consider the implications a Drainage Build Over Agreement can have.
Let’s take a look at Drainage Build Over Agreements and whether you need one.
When you are intending to build over or within 3m of a public sewer you will require approval from Thames Water, by way of a Build Over Agreement, prior to the commencement of building work in order to confirm that you have given the sewer the correct clearance from your proposed foundations and maintenance access.
If you are building further than 1m away from where the drain crosses the boundary and it solely serves your property, then a Build Over Agreement is not required. In the event that your proposed extension will affect the drain run, then it might be possible to divert the drain.
The definition of a public sewer is a pipe that serves more than one property and was connected to Thames Waters’ network before 1st July 2011. The exception is where the pipe only serves one property and then it is known as a drain.
A drain is privately owned and maintained to the property boundary by the homeowner and is not subject to a Build Over Agreement.
Once a drain crosses the property boundary this pipe becomes the responsibility of Thames Water and is known as a public lateral drain.
It is also likely that a Build Over Agreement will be required to obtain the final sign off for your Building Regulations Certificate – this will also give reassurance to any future buyers that the property extension has been built correctly.
In an extreme case of a serious drainage issue or when waste water flooding is affecting neighbouring properties and no agreement is in place, Thames Water has the right to take down the building to access the sewer.
When building over a public sewer, a manhole, even a double sealed manhole is not permitted due to the increased risk of internal flooding and the release of odours.
If a manhole is present where you intend to extend your property, the existing manhole will need to be relocated to outside the extension.
Applications can be made via post or online to Thames Water.
Download the Information Sheet here
The cost of applying for a Drainage Build Over Agreement varies on the size of the sewer and whether the building is residential or commercial use:
Residential: Class 1 – Up to 160mm (6”) – £343
Commercial: Class 1 – Up to 160mm (6”) – £687
Any Use: Class 2 – 160mm (6”) to 375mm (15”) inclusive – £687
Any Use: Class 3 – Over 375mm (15”) inclusive – £1,300*
*There is also a legal fee involved in entering an agreement for very large sewers which is usually in the region of £500.
For small sewers (160mm and smaller) the fee pays for:
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