A basement suite with its own kitchen sink, bathroom, or laundry hookup draws from the same service line as the rest of the house. That changes the sizing question. You are no longer fitting a unit to one household's habits. You are fitting it to two households that share one pipe, one pressure, and often one water heater. Start with the layout before you look at any product listing.
Count the fixtures on each side of the shutoff
Walk the main house and the suite and list every fixture that uses water, including outdoor hose bibs, washing machine hookups, dishwashers, and ice makers. Note which side of the main shutoff each one sits on. If the suite has its own water heater, the hot and cold branches may split at a different point than you expect. Sketch the supply path with every branch marked. That sketch is what you will hand to anyone who quotes the job, and it is the first thing to check if the system underperforms later.
Decide what the unit actually treats
There are two common arrangements. One is a single unit on the main line that treats water for both households. The other is a whole-house unit for the main house plus a separate treatment point for the suite, or treatment only at the suite's kitchen tap for drinking water. Each has a different running cost and a different change schedule. Ask the installer which arrangement they are proposing and why. A quote that treats only the suite's kitchen tap leaves the suite's shower, washer, and water heater on untreated water, so ask about that gap directly and get the answer in writing.
Check the flow against both households, not just the larger one
Look at the unit's rated flow and compare it with the number of fixtures that could run at the same time across both households. Two kitchens and two bathrooms running on the same morning can exceed what a unit sized for one family handles comfortably. Ask the installer to show the flow figure they used and which fixtures they counted. If the answer is only 'it's big enough,' ask for the number.
Settle the money and the maintenance before work starts
Decide in writing who pays for filter cartridges, salt, or media, and who pays for each service visit. If the suite is rented, its water use may be metered or billed separately, and the owner usually carries the equipment. Name who calls for service, who keeps the warranty paperwork, and who gets the reminder when a change is due. Keep one copy of the sizing sketch and the flow figure with the equipment record so the next person can see what the unit was built to handle.
Retest when the occupancy changes
A water report describes the water at the time it was drawn, for the household that was living there. When a suite gets a new tenant or the household's water use changes, the old numbers may no longer describe the load. Compare your current treatment against your most recent test, and retest if the occupancy changes. The county report card is a useful starting point for understanding the local supply, but your own test results are what settle the question for your property.
Check local plumbing and permit rules before you change a shared line or add a second treatment point. The local building office can tell you what applies to your address, and a licensed plumber can tell you what your system needs.