Building Over or Near a Sewer: Thames Water Build-Over Agreements

Why London extensions so often meet a sewer

On 1 October 2011, shared private drains and sewers that connected to the public network were transferred to the water companies. In a London terrace, the drain that runs behind several houses and collects from each of them is usually one of those, so it is now a public sewer owned by Thames Water, even though it sits in your garden. A drain that serves only your house and stays inside your boundary is still yours. Many of the transferred pipes have never been mapped, so a clear sewer map does not prove there is nothing there.

When you need an agreement

Thames Water must approve any building work within 3 metres of a public sewer or within 1 metre of a public lateral drain. Building regulations add their own rules through requirement H4, explained in Approved Document H, for work within 3 metres of a drain or sewer.

To find out what is there, lift the inspection covers and trace the runs, order an asset location search, and, before the design is fixed, commission a CCTV drain survey to confirm the position, size, depth and condition of the pipe.

How Thames Water decides

Class	Pipe size (internal diameter)	What is needed	Published fee
Class 1	Up to 175mm	Approval of your drawings, then work can start	£130
Class 2	Larger, up to 350mm or 375mm	CCTV survey before and after the work	£490
Class 3	Larger again	Engineer's assessment, a legal deed and further surveys	£585 plus legal costs from £1,000

The fees are those on Thames Water's homeowner page on 3 November 2026; it says they are updated on 1 April each year, and its own pages give both 350mm and 375mm as the upper limit for Class 2, so check the current figures before you apply. Thames Water reviews applications within 21 days and usually issues Class 1 and 2 agreements in that time; Class 3 can take three to nine months or longer.

An application normally needs a location plan, the existing and proposed drainage layout, a section showing the foundations in relation to the sewer with any bridging and invert levels, the sewer's internal diameter and the age of the house.

When Thames Water says no

• Building over or enclosing a manhole or inspection chamber, including sealed covers
• A new structure less than 500mm from a public manhole
• More than 6 metres of sewer under the building, unless an external manhole is available
• Foundations closer than 1.2 metres to a sewer running alongside them
• A sewer in poor structural condition
• New houses over a sewer, rising mains and strategic sewers

Options when it cannot be built over

• Redesign the footprint so the extension stops short of the pipe or the manhole.
• Bridge the sewer: the foundation spans over the pipe on a reinforced concrete lintel, with flexible fill around the pipe; Thames Water publishes an example detail. Where the sewer is more than 2 metres deep, Approved Document H says the foundation should be designed as a lintel spanning at least 1.5 metres either side.
• Move the manhole outside the extension, with Thames Water's approval.
• Divert the sewer: Thames Water's 2026/27 charges list a minor diversion application at £965 plus a refundable £2,000 security deposit, with larger diversions costing much more.

A CCTV survey is usually a few hundred pounds; protection works and diversions are priced on the design, so they belong in the build price from the start.

What if it is missed?

Thames Water has a legal right to access its pipes to inspect, maintain and repair them. If an extension was built over a sewer without approval, a retrospective agreement may be refused if the pipe was not protected, and Thames Water warns that you may have problems selling later: conveyancing drainage searches ask whether a public sewer runs within the boundary, and building control expects drainage approvals to be in place before signing the work off.

Frequently Asked Questions

Sources

• Thames Water: building near pipes (homeowners)
• Thames Water: developing over or near sewers
• Thames Water: charging arrangements for new connection services 2026/27
• Thames Water: build-over technical specifications
• GOV.UK: water customers no longer liable for sewerage repairs (2011)
• GOV.UK: Approved Document H, drainage and waste disposal
• legislation.gov.uk: Water Industry Act 1991, section 159
• Water UK: build-over guide for homeowners

This guide is general information, checked against the sources above on 3 November 2026. Every house is different, so take advice on your own property before starting work.