Since 2011 many pipes that look like private garden drains are legally public sewers, and building over or near one without your water company’s agreement can force costly alterations later.

This article covers building over or near a public sewer in England and Wales. It is general information, not a substitute for checking with your specific water and sewerage company before you build.

Why this catches more people out than it used to

On 1 October 2011, ownership of a large number of previously private drains and sewers in England and Wales transferred to water and sewerage companies, as part of a nationwide transfer of private sewers. The practical effect is that a pipe running through or near your garden that you might reasonably assume is a private drain – something you’re free to build over without asking anyone – may now legally be a public sewer, owned and maintained by your water company. This significantly increased the number of homeowners who need to check before planning a conservatory or extension near what looks like an ordinary garden drain.

What Building Regulations require

Building Regulations Part H covers drainage and waste disposal, and includes specific requirements about building over or near an existing sewer, precisely because a structure sitting on top of a sewer makes it much harder – sometimes impossible without demolition – for the water company to access it for maintenance, repair or replacement. Before building over or near a public sewer, you’re generally expected to check with your water and sewerage company, and in many cases will need their formal agreement before work goes ahead, not just a building control sign-off.

A common point of confusion: two very different “section 106”s

The formal agreement covering building over or near a public sewer is sometimes referred to using a section number from the Water Industry Act 1991. This causes genuine confusion because property buyers and self-builders more often encounter “section 106” in a completely different context – section 106 agreements under the Town and Country Planning Act 1990, which relate to developer contributions on larger planning applications, and have nothing to do with drainage. If you see “section 106” mentioned in relation to your conservatory project, check which piece of legislation is actually being referred to, since the two are entirely unrelated despite sharing a section number.

What the process actually involves

You contact your local water and sewerage company (which company covers you depends on where you live, and is separate from whoever bills you for water use in some areas) and provide details of the proposed structure and its position relative to the sewer. The water company assesses the risk and may:

• Grant a build-over or build-near agreement, often with conditions – for example, a specific foundation design that spans the pipe rather than bearing directly on it, or a requirement to preserve access points such as manholes or rodding eyes;
• Refuse permission to build directly over the sewer and instead require the sewer to be diverted at the owner’s cost before the structure can proceed;
• Ask for further information, such as a CCTV survey of the pipe, before making a decision.

Even if your conservatory is positioned near rather than directly over a sewer, it’s worth checking with the water company rather than assuming distance alone makes it exempt – how close is “close enough to matter” depends on factors like the pipe’s depth and diameter, which is exactly why it’s a case-by-case assessment rather than a fixed rule you can apply yourself.

What happens if you skip this step

Building over a public sewer without the necessary agreement can leave you facing a requirement to alter or remove the structure at your own cost later, if the water company needs access and finds the unauthorised structure in the way. It’s also a real practical problem when selling: a standard drainage and water search carried out during conveyancing (commonly a CON29DW search) will typically reveal the presence of a public sewer and flag any unresolved build-over issue, which can make a property difficult to sell or mortgage until it’s sorted out. It’s considerably cheaper and less stressful to check and get agreement before you build than to resolve a problem after the fact.

Practical first steps

• Check whether there’s a public sewer on or near your property – your water company can usually provide sewer location plans on request;
• Contact your water and sewerage company before finalising your conservatory’s design and position, not after;
• Factor in the possibility of needing a specific foundation design, or in rarer cases a sewer diversion, when budgeting and planning your timeline;
• Keep any written agreement from the water company with your other property paperwork – you may need to produce it if you sell.

FAQs

How do I find out if there’s a public sewer near where I want to build my conservatory?

Contact your water and sewerage company – they can usually provide sewer location plans on request – before finalising your conservatory’s design and position.

What could happen if I build over a sewer without the water company’s agreement?

You could later be required to alter or remove the structure at your own cost if the water company needs access, and a standard drainage and water search during conveyancing will typically flag any unresolved build-over issue, which can complicate a future sale or mortgage.

Sources

This guide draws on the following primary sources, current as of 15 September 2026:

• Planning Portal, Building Regulations Part H (drainage and waste disposal) – planningportal.co.uk
• GOV.UK, Building Regulations approved documents – gov.uk/approved-documents
• Legislation.gov.uk, Water Industry Act 1991 – legislation.gov.uk
• Water UK, industry information on sewers and drainage – water.org.uk

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