Guide 03 · Cost
What drives the cost of underground work
Six variables set the price of civil plumbing: trench depth, ground conditions, existing services in the alignment, the surface being cut and restored, site access, and authority approvals. Pipe length is the least significant of them, which is why two runs of the same distance carry different prices on different sites.
Last reviewed September 2026 · No published rates — every job is quoted on site
The six variables, and what each one does to a price
Depth
Spoil volume grows faster than depth, and past 1.5 metres a trench a person enters must be supported. That adds shoring gear, a larger machine and a second person on the surface.
Ground conditions
Sandstone demands hammer time. Coastal sand demands trench support. Reactive clay demands imported bedding. A high water table demands pumps and a discharge approval.
Existing services
Live water, gas, electrical and comms crossings must be exposed by hand or vacuum. Progress drops from metres per hour to metres per day through a congested alignment.
Surface and reinstatement
Turf is inexpensive to restore. Concrete driveways, asphalt pavement and council footpaths carry saw-cutting, disposal, formwork and replacement to a specified thickness.
Access
A narrow infill block forces smaller plant, barrow or conveyor spoil handling and staged trenching. A live road reserve adds permits, traffic control and restricted hours.
Approvals and inspections
Authority fees are the visible part. The hidden part is the program: hold points, shutdown windows and inspection lead times that create standing time for crews and plant.
Reading a quote: the lines that matter
Two quotes for the same trench differ because they include different things. Check each item below before comparing totals.
| Line item | Question to ask | Cost if excluded |
|---|---|---|
| Spoil removal | Stockpiled on site or trucked away, and to which facility? | Truck hire and tipping fees land back on you |
| Reinstatement | Which surfaces, what area, what finish and thickness? | A separate concreter and a second mobilisation |
| Rock | Included to what depth, or measured at what rate? | An open-ended variation mid-job |
| Traffic control | Who supplies the plan and the crew for road reserve work? | Permit refusal and a lost day |
| Authority fees | Application, permit and inspection fees in or out? | Direct invoices you did not budget for |
| Testing and records | Pressure test, CCTV, compaction results, as-constructed detail? | An incomplete handover file at settlement |
Where variations legitimately come from
A variation is honest when it covers a condition nobody could see from the surface. It is not honest when it covers something the contractor chose not to ask about.
- Rock encountered above the assumed depth. Measured and recorded as it is excavated, priced at the rate stated in the quote.
- Unmarked services in the alignment. Plans are frequently incomplete on older streets. Exposure by hand takes time that no one can price in advance.
- Contaminated or suspect fill. Buried rubble, asbestos fragments or odorous material changes testing, handling and disposal.
- Groundwater above the pipe invert. Dewatering, sump pumps and a discharge approval become necessary to bed pipe dry.
- Design changes after set-out. A revised invert, alignment or pipe size after the trench is open is a genuine change of scope.
- Authority conditions issued late. Extra hold points, out-of-hours restrictions or altered connection details imposed after pricing.
Our quotes list the assumptions behind the price, so the trigger for each of those variations is visible from day one. Anything that moves gets priced in writing before the work proceeds.
Questions
Why not publish a per-metre rate?
A published rate would be wrong for most sites and misleading on the rest. Depth, ground and surface swamp the influence of length, so a metre of trench through Redlands sandstone under a driveway bears no relation to a metre through a sandy backyard.
Does a longer program cost more?
Standing time does. Crews and plant committed to a site during an inspection wait or a shutdown window still cost money, which is why lodging applications early reduces the final figure.
What makes a quote accurate?
A site inspection, the approved drawings, and stated assumptions on depth, rock, spoil and reinstatement. Without those three, the number is an estimate wearing a quote's clothing.
Can I reduce the cost of my job?
Yes, by sequencing it. Trenching before driveways and landscaping are poured avoids cutting them out later, and combining water, sewer and stormwater into one mobilisation removes duplicate set-up costs.
Continue