An undersink RO system is four connections and a lot of cabinet geometry. It needs a cold water feed, a drain connection, somewhere to mount the manifold and the storage tank, and a hole in the countertop or sink deck for a dedicated faucet. Most failed installations are failures of space or of the sink deck, not of the system, so measure before you order.
The four connections
The feed comes off the cold angle stop under the sink, usually through a supplied adapter tee that lets the faucet keep working. The drain connection is a saddle clamped onto the sink tailpiece, drilled and positioned above the trap and on the side away from a garbage disposal so that waste never backfeeds into the line. The tank connects to the manifold through its own valve, so you can isolate it during filter changes. The fourth is the dedicated faucet on the sink deck, which exists because RO water is produced slowly and stored, and cannot simply be fed to the main tap at household flow.
Space is usually the binding constraint
A standard system needs the filter manifold hung on the cabinet wall with enough clearance below it to swing the housings off, plus floor space for the storage tank standing upright. Under a sink already holding a disposal, a trap, a shutoff pair and cleaning supplies, that is a real squeeze. Measure the clear internal height and the depth behind the bowl before ordering, and check the housing removal clearance rather than the system's overall dimensions, since that is the figure that decides whether filter changes are pleasant or miserable. The tank can be laid on its side or relocated to an adjacent cabinet if the run is short.
The faucet hole problem
Most kitchens have a spare hole in the sink deck under a blanking plug or a soap dispenser, and that is the easy path. If yours does not, you are drilling: straightforward through stainless steel or a composite countertop with the right bit, and a job for a fabricator with a diamond bit and water cooling through granite or quartz. Do not attempt an unsupported hole through stone yourself. Check the clearance beneath the chosen position too, because the faucet shank and its tubing need to drop past the sink rim and the cabinet framing without kinking.
Living with it day to day
The faucet delivers at a deliberately gentle rate, so filling a large pot takes patience compared with the main tap. The system refills quietly on its own afterward and you rarely notice the drain flow. Filter changes are the routine chore, and a system with a sensible manifold turns that into a few minutes twice a year. Plan for two extras while the cabinet is open: an inline shutoff you can reach without contortion, and a small leak detector on the cabinet floor, because a slow drip under a sink is the most expensive failure mode this equipment has. Our undersink RO comparison covers which layouts fit tight cabinets.
Questions people ask about undersink ro
Can I fit an undersink RO myself?
Most people can, if the sink deck already has a spare hole. The one job worth handing over is drilling through granite or quartz, where a fabricator with a diamond bit avoids an expensive mistake.
How much cabinet space does one need?
Enough for the manifold on the wall with clearance to unscrew the housings, plus floor space for the upright tank. Measure the housing removal clearance, not just the overall dimensions.
Do I have to drill a hole for the faucet?
Only if the sink deck has no spare hole. Many kitchens have one under a blanking plug or an unused soap dispenser that the RO faucet can take over.
Can the system share the disposal drain?
The drain saddle goes on the tailpiece above the trap and on the opposite side from a garbage disposal, so nothing from the disposal can push back into the line.