Quick Answer: Adding a bathroom to a garage conversion ADU in San Jose costs $20,000-$45,000 for a full bathroom (toilet, sink, shower), including plumbing rough-in (which requires trenching through the concrete garage slab to install the drain and vent stack), tiling, fixtures, and permit. The most expensive part is the plumbing rough-in — cutting the concrete slab, trenching to the nearest drain, installing the drain, vent, and supply lines, and patching the concrete. This typically costs $8,000-$18,000 just for the plumbing rough-in. A bathroom-only conversion in a garage costs more per sq ft than adding a bathroom to a new room addition because the concrete slab must be opened.
Garage ADU Bathroom Cost Breakdown — San Jose 2025
Helpful Resources:
| Component | Cost Range |
|---|---|
| Concrete slab core drilling / trenching for drain | $2,000 – $5,000 |
| Plumbing rough-in (drain, vent, supply lines) | $6,000 – $13,000 |
| Concrete patch (after plumbing) | $500 – $2,000 |
| Framing the bathroom walls | $1,500 – $3,500 |
| Waterproofing membrane in shower area | $800 – $2,000 |
| Tile (floor + shower walls) | $3,000 – $8,000 |
| Fixtures (toilet, vanity, shower valve, faucet) | $1,500 – $5,000 |
| Exhaust fan + electrical | $500 – $1,500 |
| Permits and inspections | $1,500 – $4,000 |
| Total | $18,000 – $45,000 |
How to Reduce Bathroom Plumbing Cost in a Garage ADU
- Place the bathroom against the wall nearest the existing home’s sewer stack: The shorter the drain run, the less trenching. If the garage shares a wall with the house, placing the bathroom on that shared wall can reduce the drain run from 30 feet to 5 feet — saving $3,000-$6,000
- Consider an above-slab (macerator/Saniflo) toilet as a temporary or low-cost option: A macerator toilet (Saniflo system) grinds waste and pumps it through a small-diameter pipe — no concrete cutting needed for the toilet. However, it still requires a floor drain for the shower, so concrete cutting is still needed for a full bathroom
- Pre-plumb at rough framing if combining with a full ADU conversion: If you’re converting the full garage to an ADU, plumbing rough-in should be done early while the concrete is accessible before flooring is installed — it’s significantly more disruptive to do it after flooring is in place
Quartz Construction builds complete garage-to-ADU conversions including bathroom plumbing throughout San Jose. Learn about ADU construction. CSLB #1003436 | Call (650) 749-7436.
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