Building an extension might seem like a simple project but it’s important to find out the position of any public drains before finalizing your plans.

According to the Architects’ Journal, one of the most frequent problems with small build projects comes from, ‘the failure to check whether or not there are drains running beneath the footprint of a proposed building or extension.’ 

Why might this prove important? Thames Water says, ‘Without our permission, your local authority might not provide a Building Regulations Completion Certificate. This could cause problems when you try to sell your property, as a buyer’s solicitor could advise them that the building was extended over or near a sewer without our permission.’

How can I find out the location of underground drains and sewers?

Maps of public sewers can usually be obtained via your local water company or council. If you obtain plans for the underground drain layout for your home from the council, be aware that they may not be up to date, possibly dating from when the house was originally built, therefore things may have changed. 

If in doubt, it’s advisable to enlist the help of a sewer specialist who can help you locate any sewer pipes running underneath your property. It’s best to tackle this question during the ‘feasibility study’ part of your project, so that you can be sure that your plans won’t need to be altered at a later stage. 

If you are unsure about the location of your drains or how to locate them, you can check with us. Premier Drainage Solutions offers a CCTV survey that will indicate the condition of the drains as well as their location and depth. 

Why do I need to think about underground drainage?

Building over an existing drain or sewer could potentially damage the pipes, leading to blockages or leakages, bad smells, or other environmental damage. It can also make it more difficult, time consuming and costly for water companies to carry out maintenance on or replace faulty drains. Therefore, if there is an existing drain below, or close to, your proposed extension, it may need to be moved or protected.

Planning and Regulations

Once you have located the drainage system, it is advisable to seek professional advice about your individual project. 

If you want to build within 3m of a public sewer, or within 1m of a public lateral drain, you will need to get consent for your works from the water company that collects your wastewater, following Part H4 of the Building Regulations. You don’t need water company approval if you’re building outside these distances.

According to Southern Water, ‘If the affected sewer requires a diversion (relocation not replacement of existing line) you will be required to apply for a sewer diversion. For minor sewers (if the pipe size is <225mm and the depth of the sewer is <3.0m) you have the option of requesting a small sewer diversion through a build-over application.’

There are 2 types of water company approval available, dependant on your specific circumstance:

1) A Self-Certification Declaration – This only applies to building works at a single domestic property involving pipes of 150mm diameter or less. 

2) A Formal Build Over Agreement – If you don’t meet the criteria to complete a self-certification declaration, you will need to apply for a Build Over Agreement.

Why is it so important to get build-over consent?

Consent from your water authority means your works will not affect the structural integrity of the sewer pipe, and vice versa. It also demonstrates they are happy that they will have suitable access for maintaining their sewer after the build has completed. If the necessary approval is not in place, the water authority has the right to remove any structures blocking their access, and they will not be liable for any damage they may cause in doing so. You can read our full blog about build-over agreements here. 

If in doubt, get the drainage experts in

Modifying drainage systems is not always straightforward and something that appears to be a simple job can quickly become complex if the right factors haven’t been investigated during the feasibility stage. Often the time it takes to obtain permissions isn’t factored into jobs and this can prove costly at later stages when work is stopped, waiting on their arrival. 

If in doubt, get some advice from qualified drainage experts like us. Premier Drainage Solutions can help you with CCTV surveys, build over and diversion work and connection. Get in touch and let us know how we can help: call 01268 950050 or email [email protected].