Can I use PEX, copper, or CPVC for ADU water lines in California?

Yes. PEX and copper are both used for ADU water piping under the 2025 California Plumbing Code, and CPVC is allowed inside homes when your local building official permits it under specific conditions. Whatever you choose must comply with the standards in Table 604.1, and pipe, fittings, solder, flux, and solvent cement in drinking water systems must meet NSF/ANSI/CAN 61.

Rules for every material

• Drinking water safety. Section 604.1 applies NSF/ANSI/CAN 61 to pipe, tube, fittings, solvent cement, thread sealants, solder, and flux.
• Like materials. Materials in the water supply system, other than valves and similar devices, should be like materials unless the building official approves otherwise.
• Brass with plastic. Copper-alloy fittings and valves with more than 15 percent zinc used with plastic pipe must resist dezincification and stress corrosion cracking (NSF/ANSI 14).

PEX

PEX is flexible, needs few fittings, and suits manifold layouts. California adds a rule for new residential construction: when PEX is the initial plumbing, the system must be flushed for at least 10 minutes, filled and left to stand at least one week, then flushed again, with a tag at each fixture, and the contractor must certify compliance (CPC 604.1.2). That rule doesn't apply where PEX replaces existing pipe. Follow the manufacturer's limits on sunlight exposure and on connections near the water heater.

Copper

Copper is rigid and has a long track record. Use lead-free solder and flux that meet the drinking water standard, and protect buried copper from corrosive soil and damage.

CPVC

Under California's residential amendment (CPC 604.1.1), a local building official may permit CPVC for hot and cold water inside residential buildings if:

• Only listed CPVC is used, installed per the code and the manufacturer's instructions.
• The contractor certifies compliance with flushing and worker-safety requirements, and the company's injury and illness prevention plan covers CPVC hazards.
• Installers in enclosed spaces, such as crawl spaces under 3 feet high, enclosed attics, small rooms, and trenches deeper than 24 inches, use mechanical ventilation or organic-vapor respirators, and wear nitrile gloves.

Because approval rests with the building official, ask before you specify CPVC.

Choosing for your ADU

• For an underground service to a detached ADU, use material approved for buried water service and follow the trench rules.
• In a slab-on-grade garage conversion, routing water lines overhead or in walls avoids extra slab work.
• All domestic hot water piping must be insulated per CPC Section 609.12, whatever the material (Energy Code Section 150.0(j)).

What it means for your plumbing plan

Your plans should name the pipe material for the building supply and distribution, the sizes, and the insulation, plus any CPVC approval or PEX flushing note your jurisdiction expects. See what plumbing plans include, garage conversion plumbing challenges, and the ADU plumbing guide.

Sources: gocodebook.com, energy.ca.gov.