In So. California just about everybody has a water pressure regulator and almost nobody has an expansion tank. Those that do have an expansion tank were relieved of approximately $800.00 by a Washington State plumbing contractor. The contractor is really Fast, if you catch my drift.
The customers are prone to asking if the expansion tank is required by the Plumbing Code. I could say, "Well sorta yes and sorta no" But I don't.
The worst is when I have to write a correction for some minor mistake and they fork over another $800.00 for what they are told is a personal preference of the inspector.
One lady was way upset about the expansion tank. She lived there for thirty years without one and knew that she had been scammed. She hadn't paid the invoice as she was waiting to pass inspection. I recommended that she instruct contractor to come and get the expansion tank....I'm quite sure that she did.
I went on a water heater inspection twenty-five years ago. A little old lady met me with all of the paperwork in hand. The invoice stated that she paid $5,800.00 for a forty gallon water heater and that didn't include an expansion tank. The previous water heater lasted almost thirty years. She said that the salesman explained the cost like this: $5,800.00 divided by 30 years = $16.00 per month for hot water...and that seems fair. I guess she was lucky that it failed when it did.
Not sure why you think the contractor was from Washington. Especially considering that California Code requires expansion tanks.
Is this a case of the Tiger Code? A section you've chosen to be unworthy of enforcement?
California Plumbing Code (2022)
608.2 Excessive Water Pressure
Where static water pressure in the water supply piping is exceeding 80 psi (552 kPa), an approved-type pressure regulator preceded by an adequate strainer shall be installed and the static pressure reduced to 80 psi (552 kPa) or less. Pressure regulators for potable water distribution systems shall comply with ASSE 1003. Pressure regulator(s) equal to or exceeding 11/2 inches (40 mm) shall not require a strainer. Such regulator(s) shall control the pressure to water outlets in the building unless otherwise approved by the Authority Having Jurisdiction. Each such regulator and strainer shall be accessibly located aboveground or in a vault equipped with a properly sized and sloped boresighted drain to daylight, shall be protected from freezing, and shall have the strainer readily accessible for cleaning without removing the regulator or strainer body or disconnecting the supply piping.
Pipe size determinations shall be based on 80 percent of the reduced pressure where using Table 610.4.
An approved expansion tank shall be installed in the cold water distribution piping downstream of each such regulator to prevent excessive pressure from developing due to thermal expansion and to maintain the pressure setting of the regulator. Expansion tanks used in potable water systems intended to supply drinking water shall comply with NSF 61. The expansion tank shall be properly sized and installed in accordance with the manufacturer's installation instructions and listing. Systems designed by registered design professionals shall be permitted to use approved pressure relief valves in lieu of expansion tanks provided such relief valves have a maximum pressure relief setting of 100 psi (689 kPa) or less.
608.3 Expansion Tanks, and Combination Temperature and Pressure-Relief Valves
A water system provided with a check valve, backflow preventer, or other normally closed device that prevents dissipation of building pressure back into the water main, independent of the type of water heater used, shall be provided with an approved, listed, and adequately sized expansion tank or other approved device having a similar function to control thermal expansion. Prepressurized water expansion tanks shall comply with IAPMO Z1088. Such expansion tank or other approved device shall be installed on the building side of the check valve, backflow preventer, or other device and shall be sized and installed in accordance with the manufacturer's installation instructions.
A water system containing storage water heating equipment shall be provided with an approved, listed, adequately sized combination temperature and pressure-relief valve, except for listed nonstorage instantaneous heaters having an inside diameter of not more than 3 inches (80 mm). Each such approved combination temperature and pressure-relief valve shall be installed on the water-heating device in an approved location based on its listing requirements and the manufacturer's installation instructions. Each such combination temperature and pressure-relief valve shall be provided with a drain in accordance with Section 608.5.