An under counter reverse osmosis system hides the treatment in the sink cabinet and gives you a dedicated faucet on the deck for drinking and cooking water. Most of the decision is not about the membrane at all: it is about what physically fits around the trap and the disposal, and about three connections you have to make cleanly.
What is actually in the cabinet
A conventional under counter system is a bracket of cartridges plus a storage tank. Water passes a sediment prefilter, then a carbon prefilter that protects the membrane from chlorine, then the reverse osmosis membrane itself, which pushes treated water into the tank and sends the concentrate to the drain. When you open the dedicated faucet, tank water passes a final polishing carbon cartridge on the way out, which is what stops stored water tasting flat. The tank is the bulky part, and it is the part people forget to measure for.
Measuring the cabinet before you buy
Open the doors and look at what is already there: the P trap, the disposal body, the shutoff valves, a soap dispenser pump, sometimes a water heater expansion tank. The filter bracket needs clearance below it to swing cartridges out, not just space to hang. The storage tank can stand elsewhere in the cabinet or in an adjacent one if the tubing run allows. If the cabinet is genuinely full, a tankless unit or a countertop system solves the problem better than forcing an installation you cannot service later.
The three connections that matter
First, the feed: an adapter tee at the cold water angle stop, with its own shutoff so you can isolate the system. Second, the drain: either a saddle clamped to the sink tailpiece above the trap, or a purpose made standpipe fitting, with the connection made high and angled so it drains and does not siphon. Third, the faucet: a hole in the deck or sink, which on a granite or quartz counter means a diamond core bit and a real decision about placement, because you cannot undo it. Our under sink water filter installation guide walks the same three connections in more detail.
Living with a tank system
A tank system trades speed for storage. It fills slowly in the background and then delivers a glass quickly, so household experience depends on how much you draw at once rather than on the membrane rating. The tank holds an air charge behind a bladder, and that charge is the single most common cause of a system that seems to have lost capacity: an empty tank should be checked with a tire gauge at the valve when it is drained, not when it is full. Beyond that, upkeep is cartridge changes on a schedule and an occasional sanitize.
Questions people ask about under counter reverse osmosis
Do I need to drill the counter for the extra faucet?
Usually yes, unless the sink has a spare hole from a sprayer or soap dispenser. Stainless is straightforward with a step bit, while granite, quartz and composite need a diamond core bit and a steady hand.
How much cabinet space does an under counter reverse osmosis system need?
Enough for the cartridge bracket with swing room underneath, plus a standing storage tank. Tankless units remove the tank entirely and are the usual answer for a crowded cabinet.
Can the system feed my refrigerator ice maker?
It can, with a tee off the treated line and a long tubing run, but a tank system may struggle to keep up with both the ice maker and the drinking faucet, so check the production rating first.
Is a drain saddle allowed everywhere?
Not always. Some jurisdictions require an air gap fitting or a standpipe connection rather than a saddle clamp, so check local plumbing rules before you cut into the tailpiece.