A reverse osmosis under counter system is the standard way to get membrane-filtered water at a kitchen tap. Everything lives in the cabinet: the filter stages, the membrane housing, a storage tank on tank models, and connections to the cold supply, the drain and a dedicated faucet on the countertop.
What the cabinet has to accommodate
Space is the first constraint and the one people misjudge. A traditional system needs room for a bank of vertical filter housings that must be pulled downward to change cartridges, plus a pressure tank roughly the size of a small propane cylinder, standing upright. Add the waste disposal unit and the trap and many cabinets are tight. Measure the clear height under the sink, the clear floor area beside the trap, and the vertical drop needed to remove a cartridge. If the tank will not fit, it can often be sited in an adjacent cabinet or a basement below with a longer tube run.
Tank systems versus tankless
A conventional under counter unit produces water slowly and stores it in a small pressurized tank so a glass fills at a normal rate. The tank takes space, holds an air charge that needs checking occasionally, and adds a surface where water sits between uses. Tankless designs use a pump to push a higher membrane output on demand, so they are far more compact and give fresher water, but they need a power outlet under the sink and are audible when running. Both approaches send reject water to the drain; the tankless units generally send less per gallon produced because they are not repeatedly refilling a tank.
The three connections
Installation comes down to three taps into existing services. The feed is taken from the cold supply, ideally with a proper quarter-turn angle stop adapter rather than a self-piercing saddle valve. The drain connection is a saddle clamp or a fitting on the sink tailpiece, positioned above the trap so nothing can siphon back. The faucet needs a hole in the countertop or sink deck: many sinks have a spare knockout used by a soap dispenser or sprayer, and if not, drilling stainless is straightforward while drilling granite or quartz is a job for someone with the right bit and nerve. That hole is usually what decides between doing it yourself and hiring an installer.
Living with the system
Pre-filters are changed on a schedule measured in months, and they exist to protect the membrane, so skipping them is expensive rather than economical. The membrane lasts far longer and is judged on performance: compare a dissolved solids reading of the untreated feed against the treated output on the same day, and replace when the treated figure starts climbing toward the feed. Sanitize the tank and lines when you change the membrane. If the flow at the faucet weakens gradually, suspect exhausted pre-filters or a tank that has lost its air charge before assuming the membrane has failed.
Questions people ask about reverse osmosis under counter
Will an under counter system fit with a garbage disposal?
Often yes, but measure first. The disposal, trap and filter housings compete for the same space, and cartridges need clear vertical room to be pulled out.
Do I need a new hole in the countertop?
You need a hole for the dedicated faucet. Many sinks have a spare knockout for a sprayer or soap dispenser that can be used instead of drilling.
Is a tankless system better?
It is more compact, gives fresher water and typically wastes less per gallon, but it needs a power outlet under the sink and makes noise while running.
How do I know when the membrane needs replacing?
Compare dissolved solids readings before and after the unit on the same day. When the treated figure creeps toward the feed figure, rejection is falling.