If you rent, the question is not what the ideal system for the house would be. The question is what you can install, use, and take with you when the lease ends. That changes the sizing answer, because the form factor you are allowed to use sets the capacity you can realistically live with.

Get the Landlord's Yes in Writing First

Read the lease for wording on fixtures, drilling, plumbing changes, and what must be left in place at move-out. Then ask the landlord four things in writing: whether you may add a unit at the kitchen tap, whether you may replace or modify a faucet, who pays for a leak that starts at the unit, and whether they will accept the unit removed at move-out. Keep the reply with your copy of the lease. A verbal yes is not enough when a dispute comes up later.

Measure the Tap, Not Just the Cabinet

Note the faucet type (single lever, two-handle, or pull-down), the spout height, the aerator thread if there is one, and whether the sink base has room for a unit plus a drain connection. Photograph the existing supply lines and fittings before you touch anything. Those photos are your record of the starting condition.

Pick the Form Factor by What Can Leave With You

• Pitcher or countertop unit: no plumbing changes, moves with you, and usually the easiest to approve. The tradeoff is manual refilling and lower output.
• Faucet-mount unit on a standard aerator thread: clamps to the tap and comes off without tools. Confirm the spout and thread match, and check whether the faucet can carry the added weight.
• Under-sink unit: the most capacity and the cleanest look, but it needs a cold supply connection, a drain route, and cabinet room. This is the option most likely to need written approval and the one most likely to leave a mark on the cabinet or plumbing.

Decide Which Taps the Unit Must Serve

Many renters only need one tap treated, usually the kitchen cold tap used for drinking and cooking. Write down which taps you actually draw from for drinking and cooking, and whether the bathroom or laundry matters to you. A unit for one tap is a different purchase from a system meant to cover the whole home, and buying the second when you only need the first is a common way renters end up with equipment they cannot move.

Test the Shutoff Before You Connect Anything

Close the cold shutoff under the sink and confirm it stops the flow completely. Then open it again and check for drips at the valve. A valve that will not close fully, or that leaks, is a landlord repair, not a reason to work around it. Report it in writing before you install, so the record shows the condition was there first.

Plan the Move-Out Now

Keep the original faucet parts, adapters, fittings, and any supplied hardware together in one labeled bag. Photograph the sink base before the install and again after removal. Ask whether the landlord wants the original faucet restored and whether any holes may be patched, and only patch if the lease allows it and the landlord approves the method.

Questions to Settle Before You Buy

• Does the lease allow an added fixture or a change to the plumbing?
• Who pays if the unit or its connection leaks?
• Can the unit be removed without leaving damage, and what does removal require?
• Which tap connection does the unit need, and do you have the matching fitting?
• Where does the drain water go, and is that route allowed?
• Who do you call if the shutoff fails or the supply line leaks after install?

For the county baseline on the water itself, the St. Charles Water Ratings report card gives the local picture, and the grading method page explains how those grades are assigned, so you can judge what the local water needs before you pick a unit that fits your lease.