Will building an ADU make me replace old toilets and faucets in my main house?

Possibly, depending on your home's age and how your building department applies state law. California Civil Code section 1101.4 requires single-family homes built on or before January 1, 1994 to have water-conserving fixtures, and makes replacing noncompliant fixtures a condition of final permit approval for alterations or improvements. Some cities apply that trigger only to the unit you're working on; others may ask about the whole property.

What counts as noncompliant

Under Civil Code section 1101.3, a fixture is noncompliant if it is:

• A toilet designed to use more than 1.6 gallons per flush.
• A urinal designed to use more than 1 gallon per flush.
• A showerhead with a flow capacity of more than 2.5 gallons per minute.
• An interior faucet that emits more than 2.2 gallons per minute.

A fixture at or below those numbers isn't noncompliant. Replacements must meet current standards for new construction.

Two separate obligations

1. A deadline that already passed. Owners of covered single-family homes were required to replace noncompliant fixtures by January 1, 2017, whether or not they pulled a permit, and sellers must disclose the requirement and any noncompliant fixtures (1101.4(b) and (c)).
2. A permit trigger. For building alterations or improvements to covered single-family property, the permit applicant must replace noncompliant fixtures before final permit approval (1101.4(a)). The law covers property built and available for use on or before January 1, 1994 (1101.2).

How cities apply it to ADUs

Interpretations differ. Palo Alto treats a house with an attached or detached second unit as single-family property and says alterations to the house trigger upgrades in the house only, not in the second unit, and the reverse. Under that reading, a new detached ADU wouldn't by itself reach the main house's fixtures. Other departments may read the law more broadly, and several cities use an owner-signed certificate of compliance before final. Ask your city early so you're not surprised at final inspection.

The ADU's own fixtures

New ADU fixtures must meet 2025 CALGreen limits, including 1.28 gallons per flush for toilets, 1.8 gallons per minute for a single showerhead, 1.2 gallons per minute maximum for lavatory faucets, and 1.8 gallons per minute for kitchen faucets (Section 4.303). For additions and alterations, CALGreen's mandatory measures apply to the added or altered area (Section 301.1.1).

What it means for your plumbing plan

Your fixture schedule should list each ADU fixture's model and rated flow. If your city wants property-wide compliance, note the main house fixture status and plan any swaps before final. Check the stamped ratings on existing toilets and showerheads now. See CALGreen fixture requirements, CALGreen beyond flow rates, and the California ADU permit guide.

Sources: california.public.law, california.public.law, california.public.law, paloalto.gov, gocodebook.com.