A duplex, a house with a basement apartment, or a home with an accessory unit in the back often shares one water service, one meter, and sometimes one water heater. Most sizing advice assumes one household. Here, the number that matters is the load both units can place on the equipment at the same time, and that number is often higher than either unit's average use suggests.
Find out who owns the meter and who pays the bill
Start with the plumbing and the paperwork before you look at any equipment. Ask whether each unit has its own meter or whether a single meter feeds both. With separate meters, each unit's draw shows up on its own bill, so you can size to that unit alone. With a shared meter, the only bill you have is the combined one, and it does not tell you the peak. Ask the owner or the utility for the last twelve months of readings if you can get them, and note whether the usage was steady or spiked in certain seasons.
Also confirm who pays the water bill and who is responsible for the water heater. Those answers decide who benefits from treatment and who carries its running costs, such as cartridges, salt, or backwash water. Put the answers in writing before you buy anything.
Decide where the equipment goes
There are two basic layouts. A whole-house unit at the point where the service enters treats every tap in both units. It is one installation, but it has to be sized for both households at once, and it sits in a shared space where both parties need access to change cartridges or check the gauge.
The other option is treatment at the tap that matters. If only one unit wants filtered drinking water, a point-of-use filter at that unit's kitchen sink is often simpler. It keeps the other unit's load out of the sizing calculation and avoids a shared maintenance schedule. Ask yourself which of these two layouts you can get agreement on, because the layout you choose decides who has to be involved later.
Count the peak, not the total
For each unit, list the fixtures and appliances that draw water, then ask how many would run at the same time on a busy morning. Add the peak for both units together, not the sum of every fixture in the building. A dishwasher in one unit and a shower in the other is a different load from two showers and a washing machine running together. Check that the equipment you are considering lists a rated flow at a given inlet pressure, and compare that rating with the peak you counted.
Look at the service line where it splits, too. A line that is adequate for one unit may be tight for two, and a pressure-reducing valve that serves the whole building affects every fixture downstream. If you do not know the pipe size or the pressure at the split, have a plumber measure it before the sizing is final. You can check the inlet pressure yourself with a gauge, but the pipe size and any valve work belong to the plumber.
If there is a softener or a water heater involved
A softener regenerates on a schedule tied to use, so ask whether the controller counts both units together or whether a shared system can be split. A shared water heater has a fixed recovery rate, and a treatment system does not change that rate, but it can affect how often the heater runs. Ask the installer how the backwash volume and any drain connection will be shared, because a single drain line may not handle the discharge from two households on the same day.
What to ask the installer
Bring these questions to any provider you are comparing. What flow rate did you size to, and was it based on a measurement or an estimate? Will both units have adequate flow at the same time at the peak you described? Who is responsible for the cartridges, salt, or backwash water in each unit? Where will the control valve and the filter housings be located, and can both parties reach them without going through the other unit? What happens to the equipment, and to the cost of replacing it, if one unit is sold or the tenancy ends?
Ask for the answers in writing. A provider that cannot describe the peak load it sized to, or that treats the two units as one household without asking how many people live in each, has not finished the job of sizing.
Set the agreement before the equipment arrives
Shared equipment works best when the owner and the other occupants agree on the basics before installation. Write down who has access for maintenance, who orders and pays for replacement parts, how filter changes are scheduled, and what happens to the equipment if the arrangement changes. A short written note signed by both parties prevents most disputes later, and it gives the installer a clear picture of who to call.
For a broader view of how to compare providers and what each part of an estimate should cover, see our review methodology and editorial criteria.