A basement apartment, in-law suite, or rental unit on the same water line changes the sizing question. You are no longer sizing for one household. You are sizing for two households that may run water at the same time, on different schedules, with different habits. Start by deciding whether one softener will serve both units or whether the second unit needs its own system, then size from the combined load, not from the household you live in.
Count both units before you accept a number
Ask the installer to size the system from a written list of everyone who will use the water in both units, plus every fixture that draws water: bathrooms, kitchen sinks, the washing machine, the dishwasher, and any outdoor hose bib on the same line. Compare that list to what the bid assumed. If the bid counts four people and only the owner's unit appears on the fixture list, the second unit is undersized before the system is ever installed.
Ask whether the peak use overlaps
A single household spreads its water use across the day. Two households sharing a line can both shower in the morning, both run a washer in the evening, and both use the dishwasher after supper. Ask the installer how they account for overlapping peaks, not just the daily total. A unit that looks adequate on paper can still fall short when two morning routines collide. Ask which flow rating they are quoting and which fixtures that rating is meant to cover, and get the answer in writing.
Check when regeneration will fall
Ask when the system regenerates and whether you can set that time. A regeneration cycle uses water and temporarily changes how water flows while it runs. If the cycle lands in the middle of a shower in the other unit, someone will call you. Ask for the regeneration time to be set for a low-use hour, and write that setting into the paperwork so the next technician does not reset it to a default.
Decide who refills salt and who pays for it
The brine tank empties on a schedule set by water use, and the person who refills it is not always the person who uses the most water. Settle this before the installation. Write down who buys salt, who refills the tank, how often it is checked, who calls for service, and how the cost of salt and repairs is shared. Keep the note with the installation papers and the warranty, and make sure both sides have a copy. A short written split prevents most of the arguments that come up when two units share one system.
Confirm each unit can be isolated
Ask the installer where the main shutoff, the bypass valve, and the drain connection sit, and whether the second unit can be isolated without shutting off the first. If one shutoff serves both units, a repair in one place will interrupt the other. Ask the installer to show you each valve and to label them so anyone working on the system can find them.
Test the water at both taps after install
Do not accept a finished install based on the owner's tap alone. Ask for a hardness check at the tap in each unit after the system has completed a full cycle. If the water is still hard at the second unit, the cause may be the layout of the lines, a valve left partly open, or a sizing gap, and you want to find it while the installer is still on the job. Ask the installer to record each reading and the time it was taken.
When one softener is not the right answer
If combined use clearly exceeds what a single unit is rated to handle, or if the two units need separate maintenance and separate cost sharing, ask the installer about two systems. Two smaller units can be easier to manage than one large system serving two households on different terms. Compare quotes side by side using the same fixture list, the same water test, and the same written split of responsibilities. A bid that skips any of these is not pricing the same job.
For the rest of the buying steps, the Missouri Water Softener Buyer's Guide walks through them in order.