A basement apartment, an in-law suite, or a converted garage unit often sits on the same water line as the main house. That is a fit problem most sizing conversations skip. The treatment may have been chosen for one household's use, and now a second kitchen, bathroom, and laundry draw through it, sometimes at the same time.
Where the treatment sits matters more than its size
Ask whether the unit will be installed before the point where the line splits to the second unit, or after it. If it sits before the split, one system treats all the water, so it has to carry the combined peak. If you put separate units on each side, each only needs to match its own load, but you now have two units to service. Ask the installer to sketch the split on paper and mark where each fixture connects.
Count the peak, not the average
Add up the fixtures that can run during a busy hour: a shower in each unit, a washing machine filling, a dishwasher, and an outdoor hose. Ask the installer what flow rate the proposed unit delivers at your actual incoming pressure, not just the rating on the brochure. A unit that is adequate for one household can drop pressure noticeably when two households draw at once. Ask specifically what the pressure looks like at the second unit's faucets during that overlap.
Write down the household numbers you give the installer
Note who lives in each unit, how many people, and whether anyone uses water heavily during the day, such as someone who works from home or runs a small business. Also note any change in occupancy you expect. Give that list to the installer and ask them to state in writing which load figures they sized against. If they sized only for the main house, the second unit is the part of the fit that has not been checked.
Questions to settle before you agree
Ask whether the unit can be bypassed for service without shutting off water to the second unit. Ask whether there is a shutoff valve on the branch to the second unit so it can be isolated during a repair. Ask who you call first if the second unit reports low pressure, and whether that call goes to the same company that installed the system. Finally, ask how the cost of any added capacity would be split if both units use the system, since that decides who has a say in upgrades later.
If the answers are vague, the sizing has not been worked out for the household you actually have. Ask for the sizing basis in writing before you sign.