Building an extension? Check public sewer locations first.

Public sewers aren’t usually one of the first considerations for homeowners when planning to build an extension, add a conservatory, or construct a detached garage or home office – but perhaps they should be.

Building over (or even near) sewer and drainage pipes can damage the pipes themselves, or sometimes lead to damage to your home. Additionally, it’s important not to carry out any building works that make it difficult for water companies to access and maintain the sewer system – not only does this push everybody’s bills up, but when access is required, you may have to withstand significant disruption to your property. To build over, or even close to, a public sewer or lateral drain, you will need permission from your local water authority.

Is there a public sewer on my plot?

Before you start your extension project, it’s best to know exactly where the drainage pipework is within your plot, especially the location of any sewers that may cross your land. Then you can plan your building works, factoring in all the necessary considerations. This not only helps you avoid delays in the build process, but also will give you a more accurate idea of the costs involved (suddenly having to add the diversion of a public sewer into your budget can be a rather nasty surprise otherwise!).

To find this out, you can check the legal documents that you have for your property. Drainage arrangements might be indicated on those. Alternatively, your water company may be able to help you identify the location of the sewers in your vicinity (there is often a fee involved) or your builder or architect (or your drainage specialist – us for example) could request an asset search be conducted by your local authority to identify whether there are any sewers or drains running under your plot. You can also request an asset search directly. Whether you make the request directly, or through a professional, a fee will be payable.

Is it possible to build over a sewer?

In general, you will not be permitted to build over a sewer greater than 225mm diameter, set deeper than 3m, or with a length greater than 6m – and you’ll never be given permission to build over access chambers. 

If you are planning to build over, or close to, any sewers or lateral drains, you will need to apply to your water authority for permission to do so before work begins. Consent will need to be given for building within 3m of a public sewer, or within 1m of a public lateral drain.

Proposals that meet certain criteria – which are roughly the same across different water companies, even though their processes may differ – will be approved; proposals which do not meet those criteria are dealt with on a case-by-case basis, and a formal build-over agreement may have to be entered into. This agreement not only makes sure that you have given the sewer the correct clearance, and that sufficient access to the sewer is maintained for cleaning or repairs if required. It also ensures that, if you sell the property, the purchaser can be assured of the facts, and that the extension was built in a way that’s compliant with the legislation.

To apply to build over or close to a public sewer or public lateral drain, you need to follow your local water authority’s process. You will need to receive consent before work can begin on site:

Sewer diversions: consent required

If there is no other option than to perform a sewer diversion to enable your building project to go ahead, these works will also need to be approved by your water company. The steps are as follows:

  1. Submit your application
    This includes the design and specification for the diversion, including the type and location of the sewer, as well as its ownership. It’s best to have a drawing of the existing sewer with the new development outlined on top, and a separate drawing showing the path of the diversion.
  2. Technical review
    The water company will make a decision as to whether the sewer is strategic.
  3. Decision
    Around 14 working days after you have applied, the water company will indicate whether they require any additional information. Once they have all they need, within 28 days they will send you their decision, and indicate what you must do next.
  4. Legal agreement
    If the work is approved, the legal documents must be signed before the sewer diversion works can start.
  5. Schedule the works
    You will need to tell the water company the date that the diversion works will start.
  6. Inspection
    A field engineer will inspect the works in progress and advise on any corrections required.
  7. Sign off
    Your contractor will provide as-built drawings and CCTV to the water company, so they can update their records.
  8. Certification
    Your water authority will issue a provisional vesting certificate.
  9. Final completion
    After a year-long maintenance period, a final inspection will take place. All being well, the water authority will issue a final completion certificate and adopt the diverted sewer.

You will need to use an approved contractor for sewer diversion work, so there’s no need to worry about keeping on top of all this admin yourself. Any good sewage and drainage contractor will be well-used to this process and will be able to take care of all the necessary drawings and application details for your sewer diversion.

At Premier Drainage Solutions, we’re experts in sewer connections and sewer diversions, with over 40 years of experience. For free advice on your works, call 01268 950050 or send us an email to [email protected].

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