Most budgets for underground detention start with a manufacturer’s quote. That quote is a real number, but it is a materials number. It covers the chambers and some of what goes around them. It does not cover the hole they go in, the crew that sets them, or the structures that make the system work. Owners who carry the quote into the pro forma as the installed price find the gap later, usually when the civil contractor’s bid comes in.
Within the last few years, a small chamber system on the Front Range, roughly 1,000 cubic feet of storage, priced out at about $23 per cubic foot for the chambers and stone alone. Excavation, installation, the outlet control structure, and pretreatment were all on top of that.
On a small system, the materials alone can run past $20 per cubic foot. The installed price is higher.
Why Small Systems Cost More per Cubic Foot
A chamber system has parts that scale with storage and parts that don’t. The chambers themselves scale. Each one holds a set volume, and adding storage means adding chambers. The rest of the system is closer to a fixed cost. Every system needs end caps on each row, an inlet manifold, an isolation row wrapped in woven geotextile to catch sediment, inspection ports, and an outlet control structure that meters the release. A 1,000 cubic foot system needs most of the same pieces as a 20,000 cubic foot system, and it spreads their cost over a twentieth of the storage.
The stone works the same way. Chambers sit in an envelope of clean crushed stone, typically 9 inches below, 12 inches above, and 6 inches between and around the rows. The stone between and around the rows is a bigger share of a small, narrow system than a large, square one. On the common arch chamber sizes, the stone holds roughly a third of the system’s storage in its voids, so it isn’t optional fill. It is part of the volume you are paying for.
Larger systems, in the tens of thousands of cubic feet, spread the fixed costs out. Published comparisons for systems that size put the complete installed cost closer to $10 to $20 per cubic foot. Most commercial pads, a quick-service restaurant, a bank branch, a small retail building, need systems in the low thousands of cubic feet. That puts them at the expensive end.
What a Manufacturer’s Quote Usually Covers
Quotes vary by supplier and by how the project is bid, so read yours line by line. This is the typical split.
| Item | In the materials quote? | Notes |
|---|---|---|
| Chambers and end caps | Yes | The core of the quote. |
| Clean crushed stone | Sometimes | Often supplied by the site contractor instead. Roughly 9 to 10 tons per large arch chamber. |
| Non-woven geotextile wrap | Usually | Separates the stone from the native soil. |
| Isolation row fabric | Usually | The sediment-capture row that gets cleaned out. |
| Inlet manifold pipe | Sometimes | Depends on how the inlet is configured. |
| Impermeable liner | Often separate | Most Front Range systems are lined. Expansive soils and water law both push that way. |
| Outlet control structure | No | A precast or cast-in-place structure with the orifice plates and weir. |
| Pretreatment device | No | Many jurisdictions require it upstream of the chambers. |
| Excavation and haul-off | No | Roughly 12 cubic yards per large arch chamber. Hauling the spoil off site is the swing cost. |
| Installation labor | No | Setting chambers, placing and compacting stone, wrapping fabric. |
| Pavement over the system | No | You’d build the parking lot either way, but the cover depth has to meet the chamber’s load rating. |
Stone and excavation quantities are from the manufacturer’s published installation tables for a large arch chamber with a 9-inch stone foundation.
What Pushes the Installed Price Up
Depth. The system has to drain by gravity to the storm sewer or outfall, and it needs enough cover to carry traffic. A deep outfall lets you stack storage in a compact footprint. A deep system with a lot of cover means more excavation, more haul-off, and sometimes shoring.
Groundwater. A high water table means dewatering during construction and buoyancy checks on the finished system. Near a creek or irrigation ditch, plan for both.
The liner. An impermeable liner adds material and labor, and the seams and pipe penetrations have to be done right. It’s a real line item, and it’s common here.
Hauling the dirt. Every chamber displaces about 12 cubic yards of soil. If the site can’t use it, it leaves on trucks. Haul distance and tipping fees vary a lot across the metro.
The structures. The outlet control structure and pretreatment device cost about the same whether the system is large or small. On a small system they can add several dollars per cubic foot on their own.
How to Budget It
Get the storage volume from the drainage design first, not from a rule of thumb. The required volume depends on the site’s imperviousness, the jurisdiction’s criteria, and whether part of the water quality volume can be handled elsewhere on the site. Then get the materials quote, and ask the site contractor for an installed price that includes excavation, haul-off, liner, structures, and pretreatment. Compare that installed number, not the quote, against a surface pond, and count the land the pond would take. The land side of that comparison is often what decides it.
Before any of that, confirm the jurisdiction will approve underground storage on your site at all. Some Front Range cities only allow it by variance. See what jurisdictions require for underground detention.
Common Questions
How much does underground detention cost per cubic foot?
It depends heavily on system size. A small chamber system around 1,000 cubic feet has recently priced at over $20 per cubic foot for chambers and stone alone, before installation and structures. Large systems in the tens of thousands of cubic feet commonly come in around $10 to $20 per cubic foot installed.
Does the manufacturer’s quote include installation?
Usually not. A materials quote typically covers chambers, end caps, geotextile, and sometimes stone. Excavation, haul-off, installation labor, the outlet control structure, the liner, and pretreatment are usually priced separately by the site contractor.
Why is a small underground system so expensive?
End caps, the inlet manifold, the isolation row, inspection ports, the outlet control structure, and pretreatment cost about the same for a small system as a large one, so they are spread over far less storage.