Can I build over a public sewer at all?

Frequently yes, subject to agreement and conditions on foundation design and access. Building over a manhole is much more likely to be resisted.

What if the drain only serves my house?

A drain serving only your property is private and is dealt with through building control rather than a build over agreement, though it still needs protecting and detailing.

Who applies for the agreement?

The property owner or their agent applies, and we normally prepare the drawings and information that accompany it.

What does it do to cost?

The agreement itself is modest. The engineering response, deeper foundations or bridging, is the part with real cost, which is why identifying the pipe early matters.

Utilities · Cornwall

Build over agreements, the drain under your garden that changes the foundation design

Since private drains serving more than one property transferred to the water authority, a great many Cornish gardens contain a public sewer that the owner is unaware of. If your extension sits over or close to it, you need the water authority's agreement before building, and the requirement affects both the programme and the foundation design. It is one of the most common late surprises in domestic extension projects.

Book a free consultation See built work

Free · No obligation

Ask us about your project

A free first conversation — no obligation, no sales calls.

How to find out whether it affects you

A drainage and water search shows recorded public sewers, and lifting the manhole covers in and around the garden tells you where the runs actually go and which way they fall. Recorded plans are approximate, so on a tight site a survey of the actual invert levels is worth the small cost, because the difference between a pipe at half a metre and one at a metre and a half changes the foundation solution entirely.

Order a drainage and water search early, not at the tender stage
Lift covers and record invert levels, cover levels and flow direction
Establish the pipe material, diameter and condition where possible
Note any manhole that would end up inside the extension, which is normally resisted

What the agreement involves

The water authority reviews the proposal and either agrees the build over, agrees it subject to conditions, or requires the sewer to be diverted. Conditions commonly cover foundation type and depth, maintaining access for maintenance, protecting the pipe during construction and avoiding load transfer onto it. Some straightforward cases are handled under a self certification route, while anything close to a manhole or over a larger main takes longer.

How it changes the structure

The usual engineering answer is to carry the new load past the pipe rather than onto it. Deeper foundations either side, a lintel or bridging slab spanning over the run, or a piled solution where depth is impractical are all standard. Each has a different cost, and each needs to appear on the drawings rather than being resolved by a builder standing in a trench with the inspector.

Programme

Applications take time, and building control will want to see the agreement or evidence that it is in hand. Starting it alongside the building regulations submission rather than after planning approval keeps it off the critical path. Diversions, where required, take considerably longer and should be identified before the design is fixed.

Frequently asked

Straight answers.

Can I build over a public sewer at all?: Frequently yes, subject to agreement and conditions on foundation design and access. Building over a manhole is much more likely to be resisted.
What if the drain only serves my house?: A drain serving only your property is private and is dealt with through building control rather than a build over agreement, though it still needs protecting and detailing.
Who applies for the agreement?: The property owner or their agent applies, and we normally prepare the drawings and information that accompany it.
What does it do to cost?: The agreement itself is modest. The engineering response, deeper foundations or bridging, is the part with real cost, which is why identifying the pipe early matters.

Extension near a drain run?

Send us the address and photographs of your manhole covers. We will tell you whether an agreement is likely to be needed.

Free · No obligation

Book your free consultation

Tell us about your project — we usually reply within 2 hours (working day).