Not every extension or outbuilding sits near a public sewer, but where one does, the water authority responsible for that sewer has a formal say in how the foundations, drainage and access around it are designed. This process runs alongside planning and building control, not instead of it, and it has its own timescale.
Understanding roughly how it works — survey, application, design response, approval — explains why a build-over agreement needs starting early rather than treated as paperwork to sort once digging has begun.
Finding out what's actually below ground
The starting point is establishing exactly where sewers and drains run in relation to the proposed building, which usually means requesting sewer records from the relevant water authority and comparing them against a site survey. Records show the approximate line, but a physical trial hole or CCTV survey is often needed to confirm depth and condition before a design can respond to it sensibly.
Older properties in particular can have drainage that doesn't match the records at all — a run that was diverted decades ago and never formally recorded is a genuine possibility, which is one reason a site check matters as much as the paperwork.
Applying to the water authority
Where a proposal comes within a set distance of a public sewer, or proposes building directly over one, a formal application goes to the water authority describing the works, the foundation design and how the drain will be protected or diverted. This is a separate process from planning permission and building control approval, and it can run at a different pace to either.
The application typically needs drawings showing the sewer's position relative to the new foundations, along with a description of the construction method being proposed to protect the run during and after building work.
Designing foundations around the agreement
Which of these applies depends on the depth and size of the sewer, the ground conditions and how close the new structure needs to sit. An engineer usually proposes the foundation solution, and the water authority either accepts it, asks for changes, or in some cases requires a different approach entirely before agreement is given.
- Piled or specially spanned foundations that bridge over a sewer rather than bearing directly on it
- A defined clearance zone kept clear of new structure
- Access left to any manhole or inspection chamber affected by the works
- A diversion of the drain itself, where bridging isn't practical
What happens on site once agreement is in place
With a build-over agreement granted, the foundation design gets built exactly as approved, and any inspection chamber or access point identified in the agreement stays reachable throughout and after the works. Deviating from the agreed design on site — even a minor adjustment that seems sensible at the time — can invalidate the agreement.
Building control will usually want to see evidence the agreement is in place before signing off the relevant stage of work, so it needs to exist before, not after, the foundations go in.
Where diversions fit into the picture
Sometimes bridging a drain isn't practical and the sensible answer is diverting it around the new building instead. A diversion is its own piece of work, usually carried out or approved by the water authority, and it has to be complete and recorded before the foundation it was protecting against can go ahead.
Diversions add time and coordination to a programme, which is exactly why establishing early whether one is likely matters far more than discovering the need for one once excavation has started.
Common questions
Do I need a build-over agreement for every drain near my extension?+
Not necessarily — it depends on distance, depth and whether the drain is public or private. A sewer records check and, where needed, a site survey establishes whether a formal agreement is required.
How long does a build-over application typically take?+
It varies by water authority and by how complete the supporting information is. Submitting clear drawings and a proposed foundation solution from the outset avoids repeated rounds of queries that add time.
What happens if a sewer is found that wasn't on the records?+
The design has to respond to what's actually there rather than what was recorded. This can mean revisiting the foundation solution or the application itself before work continues.
Ask us to check your drainage position before you design around it
We'll establish what's below ground and what a build-over agreement is likely to involve before your foundations are drawn.