How do drain lines work in a concrete slab ADU or garage conversion?

In slab-on-grade construction — which covers virtually every California garage and most detached ADUs built on a concrete pad — the under-slab drain lines must be in place before the slab serves the fixtures above it. For new construction they are laid before the pour; for a garage conversion they require cutting and trenching the existing slab. This is one of the most underestimated parts of an ADU plumbing budget.

The garage-conversion slab-cutting sequence

1. The plumber transfers the exact fixture locations from the approved plumbing plan onto the slab surface.
2. A concrete saw cuts trenches along the planned drain routes.
3. PVC or ABS drain pipe is laid at a minimum slope of 1/4 inch per foot so waste flows by gravity.
4. P-traps and vent stub-ups are set at each fixture location.
5. A rough plumbing inspection is performed — and often a water or air test on the DWV — before any concrete goes back.
6. The trench is backfilled, the slab is patched, and the concrete cures.
7. Finish fixtures are connected above the slab once the floor is restored.

For a typical two-bathroom-plus-kitchen ADU, expect roughly 40 to 80 linear feet of trenching. The drain depth at the far end is dictated by slope: every foot of horizontal run drops the pipe another quarter inch, so a long run can put the connection point surprisingly deep.

The post-tension slab problem

Many slabs poured since the 1980s are post-tension — they contain steel cables under thousands of pounds of tension. Cutting one blindly can sever a cable with violent, dangerous results and compromise the slab structurally. Before assuming a slab can be cut:

• Look for "PT" or "POST-TENSION" stamps at the slab edge or garage door.
• Look for circular grout plugs around the perimeter where cables were anchored.
• If in doubt, have the slab scanned (GPR) and get a structural engineer's sign-off before any saw touches concrete.

Post-tension work requires specialized cutting, cable de-tensioning or careful avoidance, and engineered repair — adding cost and schedule that homeowners rarely anticipate.

Crawlspace and stem-wall alternatives

When an ADU sits over a raised foundation or crawlspace, drains hang below the floor framing instead of being buried — usually easier and cheaper to route and to modify later. If you have a choice during design, a crawlspace can dramatically reduce drainage risk.

Slab-cutting permits, inspection sequencing, and post-tension review requirements vary by jurisdiction. Confirm with your local building department before scheduling concrete work.

Our garage conversion MEP plans show drain routing, slopes, and vent stub locations precisely so your plumber cuts once and passes rough inspection.