A pressure reducing valve goes on the owner's side of the water meter, near where the line enters the building, ahead of the branches to the house. Manufacturer diagrams put it after the meter and after any backflow preventer, with the water heater's expansion device downstream of it.
After the meter, not before it
The meter is where the city's responsibility ends. Phoenix's customer page says "Issues on the city's side (from the street to the water meter) are the responsibility of the City and issues after the water meter are the responsibility of the homeowner." So a PRV can't sit ahead of the meter. It lives on the owner's side.
Phoenix says this directly in two places. Its service page says "The city of Phoenix requires that customers install a pressure regulator (pressure reducing valve) on the customer's side of the water meter to ensure that the water pressure does not exceed 80 psi in the home or business." The city's design manual says "a private pressure regulating or reducing valve shall be required on the customer side of the service meter" where local static pressure exceeds 80 psi. For why 80 psi is the line, see do I need a pressure reducing valve.
Manufacturers agree on the spot. Watts says PRVs "are commonly installed at the meter in residential, commercial, and industrial buildings," and explains why: "This location is desirable because it then controls the water pressure flowing to all appliances and outlets within the building." EPA WaterSense says to "INSTALL a PRV downstream of the point of connection to the water source."
Where you find the valve depends on where the line enters your building. A garage wall, a utility closet and a box near the front of the property are common spots. If yours is hard to spot, the signs a pressure reducing valve is going bad page describes what it looks like.
Before or after the backflow preventer?
Two questions people ask are really one: where does the PRV go relative to the meter, and relative to the backflow preventer? The short answer is that the manufacturers' typical installation drawings put the PRV last.
The FEBCO Series PRV-2 installation sheet, from Watts, shows the layout from supply to fixtures: shutoff valve, water meter, shutoff valve, backflow preventer, then the pressure regulator, with the branch to the sill cock (hose bib) tapping off after it. Zurn Wilkins draws the same thing for its Model 500XL: an RPZ backflow preventer on the incoming line, then the PRV after it, in the direction of flow. Zurn's Model 600XL installation sheet, which has no backflow preventer in its picture, shows the sequence water meter, shutoff valve, regulator.
Two limits are worth knowing. First, the plumbing code sections we checked do not say which of the two comes first. IPC 604.8 covers when a PRV is required and what it must be, and 607.3 covers thermal expansion, and neither one sets the order of a PRV and a backflow assembly. Second, not every line has a backflow assembly. The city design manual's service connection section says only "A backflow prevention assembly may be required." So the order matters only if you have one, and where the city or the code requires one, the local requirement controls. For the rules on what and where, see what backflow is and where your backflow preventer is.
Backflow assemblies also carry their own maintenance. Phoenix's manual says "All maintenance of the backflow preventer is the responsibility of the property owner." Testing an assembly takes a certified tester, and our page on who can test backflow assemblies in Arizona explains who that is.
Before the branches, and ahead of the hose bibs
A whole-house PRV sits on the main line ahead of the branches, so it controls the pressure to everything inside. The code makes one exception. IPC 604.8 exempts "Service lines to sill cocks and outside hydrants, and main supply risers where pressure from the mains is reduced to 80 psi (552 kPa) or less at individual fixtures." So a hose bib line can stay at street pressure without violating that section.
Watts' installation sheet for its LF123 and LFN250 valves says the same thing from the other side: "Install the reducing valve, when possible, before the sill cock line as shown in diagram." Read that as putting the PRV ahead of the hose bib line where you can, so the hose bib gets the reduced pressure too. If you want to know what your outdoor faucets are seeing, take the gauge reading on the hose bib itself.
Other placement rules that matter
Access. The code wants the valve serviceable. IPC 604.8.2 says PRVs, regulators and strainers "shall be so constructed and installed as to permit repair or removal of parts without breaking a pipeline or removing the valve and strainer from the pipeline." Zurn's Model 500XL sheet also calls for enough side clearance around the assembly for testing and maintenance. Don't box it in behind drywall.
Strainer. IPC 604.8 calls for a PRV "with strainer." Some valves have one built in, and others take a separate strainer upstream.
Water heater expansion device. A PRV closes the system, so heated water has nowhere to go. IPC 607.3 says a thermal expansion control device shall be connected to the water heater cold supply pipe "at a point that is downstream of all check valves, pressure reducing valves and backflow preventers." That puts the expansion tank after the PRV, not ahead of it. Our page on whether you need an expansion tank covers the sizing side.
Gauge. A hose bib gauge on the house side tells you the reduced pressure. FEBCO's drawing shows a pressure gauge on each side of the regulator, so you can compare supply pressure with the reduced pressure.
Moving or replacing the valve
Where the city's part ends matters here. Phoenix's design manual says "A domestic meter service connection goes up to the end of right-of-way and ends at property line." The city's connection stops at the property line, and everything past the meter is yours, including the regulator, any backflow assembly and the expansion device. That is why the city's service page also says private regulators are the owner's job to keep working. When you're deciding where a replacement goes, start from the meter and work inward.
If your PRV is in an awkward place, the right spot is the one that meets those rules: owner's side of the meter, after any backflow assembly, before the branches, reachable with clearance. Moving it changes the piping on your main line, so it's a licensed plumber's job, and it's worth asking your city whether a permit applies.
HQ Plumbing & Air can test your pressure and replace a failed PRV. We're licensed and insured, and estimates are free with the price up front before work starts. If your main line is leaking or a valve has failed and you can't stop the water, that's an emergency, and we're available 24/7.
Here's the rule of thumb. Meter first, then any backflow preventer, then the PRV, then the house. Everything on the house side sees the reduced pressure, and the expansion tank comes last of all.