Build over or near a public sewer: technical package
A technical package to support building over or near a public sewer, including the application to the water company.
About this service
Much of London's sewer network runs behind houses rather than under the street. Many terraced and semi-detached homes have a public sewer or shared drain crossing the back garden, exactly where an extension usually goes. If you plan to build over a public sewer, or within 3 metres of one, the sewerage company must approve it first, and in most of London that is Thames Water. Our technical package provides the design and information that application needs.
This service is for homeowners, builders and architects planning rear extensions, side returns, garden rooms and small new builds with a public sewer under or close to the footprint. It is often needed alongside an extension structural package, and Building Control will also want to see that the sewer has been dealt with properly before the work is signed off.
Many drains that were once private became public sewers when shared drains transferred to the sewerage companies in 2011. A pipe serving only you and your neighbours may therefore need formal approval. Building without it can complicate Building Control sign-off and future sales, and can leave you facing difficult questions when the sewerage company needs access. Sorting it before you build is far simpler than fixing it afterwards.
When you need it
- Your extension will be over or within 3 metres of a public sewer
- Thames Water or another water company needs a build-over application
What's included
- Review of sewer records and survey
- Foundation and sewer protection details
- Application drawings and information
- Liaison with the water company
How it works
- Check the records and the pipeWe review the sewer records and, where needed, arrange a CCTV survey to confirm the pipe's route, depth, diameter, material and condition, because the records are not always accurate.
- Plot it against your designWe plot the sewer against your extension footprint and foundations, showing where the building crosses or comes close to the pipe and any manholes that must stay accessible.
- Design the protection detailsWe design foundations that keep load off the pipe, such as footings taken below the invert level, bridging lintels or beams over the sewer, and sleeved pipework where it passes through walls.
- Prepare the applicationWe prepare the drawings, sections and supporting information the sewerage company requires and help you complete the build-over or near sewer application.
- Liaise until approvalWe answer technical queries from the sewerage company and Building Control, revise details if asked, and confirm conditions such as CCTV surveys before and after construction.
What you receive
Common build-over problems and how to avoid them
Manholes are the most frequent obstacle. Sewerage companies generally do not allow a manhole on a public sewer to be enclosed inside a building, so a manhole in the extension footprint may need relocating or the design adjusting. Larger or deeper sewers, and pipes in poor condition, may be refused or need diverting. Finding this out before the planning drawings are fixed avoids an expensive redesign.
The foundation design must keep load off the pipe. Footings beside a sewer are usually taken below its invert so they do not bear on the soil around it, and where a wall crosses the pipe a lintel or reinforced concrete beam spans over it with a clear gap. The detail depends on pipe depth and ground conditions, and like all our design work it goes through our three-step quality control process, with final sign-off by our Head of Engineering.
Many sewerage companies ask for a CCTV survey before work starts and again after completion, to show the pipe has not been damaged. Allow for both in your programme and budget. Our CCTV drainage survey can provide them, and our guide to building over a public sewer explains the wider process in plain terms.
Not included
- Water company fees
Projects that used this service
Frequently asked questions
Do I need permission to build over a sewer?
If the pipe is a public sewer and you plan to build over it or within 3 metres of it, you usually need approval from the sewerage company, which in most of London is Thames Water. Building Control also considers drainage under the Building Regulations. Private drains serving only your property are treated differently, but many shared drains became public sewers in 2011, so check before assuming.
How do I find out if there is a public sewer under my garden?
Start with the sewer records, which can be obtained as a search from the sewerage company. They are useful but not always accurate, particularly for older shared drains transferred in 2011 that may not be mapped. A manhole in your garden or your neighbour's is a strong clue. A CCTV survey confirms the actual route, depth and condition of the pipe.
Can I build over a manhole?
Generally not where the manhole is on a public sewer. The sewerage company needs to reach it for maintenance, so it normally has to stay outside the building. The options are usually moving the manhole or changing the extension footprint. Any relocation needs approval and is normally carried out by an approved contractor. We look at this early so the design avoids surprises.
What is the 3 metre rule for sewers?
It refers to building within 3 metres of a public sewer. Work that close can affect the pipe through foundation loads, excavation or restricted access, so the sewerage company usually wants to review it even if the building does not sit directly over the pipe. Foundations may need taking down below the sewer level. We confirm what applies to your design and prepare the details.
How much does a build over sewer application cost?
Our build over or near sewer technical package starts from £850 + VAT. It covers the records review, design details, application drawings and liaison. The sewerage company's own application or inspection fees are separate and set by them. Every job starts with a fixed fee confirmed in writing before work begins.