Water Connections: Usually the Easy One

Good news lesson: water is usually the utility that behaves. Here's the standard setup and the three situations that complicate it.

The standard connection. A plumber tees off your existing water service line and runs a new branch to the ADU, with a dedicated shutoff valve so either home's water can be turned off independently (do not skip this valve — future-you, mid-repair, will be grateful). Typical cost: $1K–$5K depending on distance and what the trench crosses. Often the water line shares a trench with electrical, saving money.

Complication 1: Undersized service. Older homes sometimes have ½" or ¾" service lines that struggle to feed two dwellings' simultaneous demand — think both showers running. The fix is upsizing the service from the meter, which adds cost. Your plumber can assess pressure and flow early.

Complication 2: Separate water meter. Like electrical, a separate meter is optional — most ADUs share. A second meter means an application to your water utility, meter and connection fees (varies hugely by district — from around $1K to well over $10K in some districts), and their timeline. Usually only worth it for fully independent rentals or where required locally for new detached construction — check your water district's ADU policy early.

Complication 3: Fire sprinklers. If your ADU requires sprinklers (generally only when the main house has them), the water service must support the sprinkler demand, which can drive service upsizing. Confirm sprinkler requirements during design — the answer changes pipe sizes.

One design note while the trench is open: running a hose bib, irrigation stub, or future-proofing conduit alongside costs almost nothing. Trenches are expensive; what goes in them is cheap. Fill them wisely.