Building Over or Near a Sewer: The Check That Kills Projects
Why sewer alignment is the highest-value pre-purchase search in South East Queensland, what build-over approval involves, and when a main has to be relocated.
The cheapest search with the largest consequence
Finding out where the sewer main runs across a block costs almost nothing and takes very little time. Discovering it after you have bought the land and paid for a design can cost more than everything else combined.
This is the single highest-value pre-purchase check available in South East Queensland, and it is the one most consistently skipped.
It matters most on duplex and dual occupancy projects, where the building footprint is large relative to the block and there is far less room to move the house away from the main.
Why it matters
A sewer main is an asset owned by the water authority, and they need to be able to access, maintain and replace it. A building sitting on top of it interferes with that.
There is generally a defined zone of influence around the main, related to its depth. Build inside that zone and the loads from your footings can transfer to the pipe, and any future excavation to reach the pipe could undermine your building.
So the authority controls what can be built near it, and their consent is required before the building approval can proceed.
Build-over approval
Where the proposed building encroaches on the zone around the main, you apply for build-over or build-near approval from the water authority — Urban Utilities in the Brisbane, Ipswich, Lockyer, Scenic Rim and Somerset areas, Unitywater across Moreton Bay and the Sunshine Coast, and the council in the Gold Coast and Logan areas.
Approval usually comes with conditions. Common ones are piered or deepened footings designed so the load bypasses the pipe, a CCTV inspection of the main before and after construction, restrictions on the type of structure permitted, and maintaining access to any manhole.
These conditions cost money — engineered footings in that zone are meaningfully more expensive than standard ones — but they are manageable and predictable once known.
When the main has to move
Sometimes build-over is not achievable. If the main runs directly through the only viable building envelope, or the depth and condition make building over unacceptable, the alternative is relocating it.
Relocation is expensive. It involves design, authority approval, excavation, new pipework, connection works and reinstatement, and the cost is borne by you, not the authority.
On a duplex project a relocation can consume the entire development margin. This is why we treat it as a feasibility question rather than a construction detail — it can be the difference between a project and a block you now own for no reason.
How to check
Request a sewer or property services plan from the relevant water authority for the address. These are available on request and show the alignment and depth of mains on and near the property.
Read it before you make an offer. If a main crosses the middle of the block, that is not necessarily fatal, but it changes what can be built and it should change what you are willing to pay.
Also confirm your stormwater discharge point while you are at it, because a site with no legal discharge path has its own expensive problem.
We run these searches before design on every project, and before purchase for clients who ask. Our dual occupancy feasibility checklist sets out the full list.