If you build over or within 3m of a public sewer, you need the water authority's agreement before the foundations go in. In most of London that is Thames Water, and the application needs a site plan showing the sewer run, the manholes and your proposed foundations.
Last checked against current guidance on 2026-08-09. Rules differ by borough — the local pages below say how.
Whenever an extension sits over or within three metres of a public sewer. The agreement is with the water authority, not the council, and it is separate from both planning permission and building regulations approval — all three can be needed for the same extension.
| Consent | Who grants it | When it bites |
|---|---|---|
| Build-over agreement | the sewerage undertaker | over or within 3m of a public sewer |
| Planning permission | the council | beyond permitted development limits |
| Lawful development certificate | the council | to prove permitted development |
| Building regulations approval | building control | always, on structural work |
| Party wall award | appointed surveyors | shared wall or excavation within 3m |
| Householder application fee | — | £548 |
Sources: the Water Industry Act 1991 and the sewerage undertakers’ published build-over procedures; the Building Regulations 2010 (as amended), Part H; the Town and Country Planning (General Permitted Development) (England) Order 2015; the Party Wall etc. Act 1996; fees from the Fees for Applications Regulations 2012 as amended. Read at 1 September 2026. Public sewers transferred to the undertakers in 2011, which is why many older London properties have one under the garden.
Public sewers and lateral drains — including pipes that were private until the 2011 transfer and are now the water authority's responsibility.
That transfer is why this catches people out. A shared drain running down the back of a Victorian terrace, which the deeds treat as private, is very often a public sewer today. Assuming otherwise is how foundations get stopped mid-dig.
A drain serving only your own property, upstream of the point where it joins anyone else's, generally remains private and outside the regime.
Order a drainage and water enquiry, then check the manhole positions on site against it.
Water authorities publish criteria under which minor, low-risk cases can be self-certified through building control instead of a full agreement.
The published criteria typically cover shallower pipes of small diameter, extensions that do not build directly over a manhole, and cases where the sewer is not a strategic main. Anything outside them — building over a manhole, deep or large-diameter pipes, or a structure that would prevent future access — needs the full application.
Confirm the current criteria with the authority for your area before relying on self-certification; they are revised periodically.
Build cost moves with your builder, your specification and the month you tender. What you can pin down early is the design and compliance scope — how many drawing sets the council and building control will want, and whether structural input is in play. The estimator below works that out from the project type and floor area.
Move the controls for an indicative figure. It anchors your budget — it is not a quote. Every project is priced after a free survey.
Build costs are separate and quoted by your contractor. See our published fees.
Diverting a sewer is a separate and slower application, so a design that avoids the diversion is usually worth the layout compromise.
Apply once the foundation design is fixed and well before groundworks — this is a common cause of a stopped dig.
The agreement is separate from planning permission and from building regulations approval, though Part H4 covers the same subject and building control will ask about it.
Run it alongside the building regulations application rather than after it: both need the same drainage drawings, so the marginal effort is small and the schedule risk removed is large.
Sometimes, with the authority's agreement and a design that keeps the chamber accessible — often a double-sealed cover inside the extension. Many authorities prefer a diversion instead.
No. It is a separate consent from the water authority. Building control still assesses the drainage design under Part H.
Unmapped public sewers exist, particularly post-2011 transfers. If a manhole on your land serves other properties, treat it as public and ask the authority.
Allow several weeks for a standard application, longer where a diversion or a strategic main is involved. Apply before you book the groundworker.
Either can, but the application needs the foundation drawings, so it is usually made by whoever prepared the building regulations package.
A build-over agreement is not a planning matter at all — it is consent from the sewerage undertaker, triggered by proximity to a public sewer rather than by the size of what you are building. Part H of the Building Regulations is the technical standard behind it, and the undertaker applies its own build-over policy on top:
