Installing an under the sink filter means tapping the cold water line inside the cabinet, mounting the filter, and deciding where the filtered water comes out: through your existing faucet or a small dedicated one. Simple cartridge systems are a careful afternoon's work for a homeowner; reverse osmosis adds a drain connection and raises the stakes a notch.
The connection work at the cold line
Everything starts under the sink at the cold supply. Close the angle stop valve, relieve pressure at the faucet, and insert the feed adapter, usually a tee that threads between the valve and the faucet's supply line, or a quick-connect fitting supplied with the system. Modern kits use push-fit tubing that seats by hand. The skills involved are turning a wrench without overtightening, cutting tubing square, and checking for weeps afterward: put a dry paper towel under every joint and look again in an hour, then again the next day.
The faucet question
Inline systems feed your existing faucet, so every drop from the cold side passes the filter: zero counter changes, but the cartridge also treats dishwater and exhausts faster. Dedicated-faucet systems reserve filtered water for drinking and cooking, stretching cartridge life, and need a deck hole: an unused sprayer or soap dispenser opening handles it without drilling, while granite or quartz drilling calls for a diamond bit and nerve, or a pro. Reverse osmosis always takes the dedicated faucet route and adds a drain saddle for its reject line, which is the single most code-sensitive step of the job.
Cabinet planning before you buy
Measure the cabinet with its real contents in mind: garbage disposal, trash pull-out, cleaning supplies. Mount the filter head where a cartridge can actually be swapped, remembering the cartridge needs clearance below or in front to drop free, plus room for your hands and a small bucket, since changes always spill a little. Keep tubing runs tidy and away from the disposal's heat and vibration. Ten minutes with a tape measure prevents the classic outcome: a system installed so tight that every filter change means half-uninstalling it.
DIY or hire it, and choosing what to install
A straightforward cartridge system with an existing faucet hole is honest DIY territory. Hire it out when the counter needs drilling, the drain saddle worries you, the angle stop is ancient and seized, or local code requires specific backflow arrangements; our under sink water filter installation guide goes deeper on each step. As for what to install: read the evidence first. Every community water system publishes an annual Consumer Confidence Report and the EPA maintains a finder, so match the cartridge to what your supply actually contains rather than to a box's promises.
Questions people ask about under the sink filter
Can I install an under the sink filter without a plumber?
Usually, yes: modern systems use push-fit connections designed for homeowners, and the job is an afternoon if a faucet hole already exists. Stone drilling, drain saddles and corroded shutoff valves are the fair reasons to hire out.
Do I have to drill a hole in my countertop?
Only for a dedicated faucet with no spare deck hole. Many sinks have a covered sprayer or soap dispenser opening that fits a filter faucet perfectly, and inline systems that feed your existing faucet need no hole at all.
How long does installation take?
A simple inline or cartridge system takes an hour or two for a careful first-timer. Reverse osmosis with a faucet, tank and drain connection is more like half a day, plus the initial tank fill and flush the manual specifies.
What maintenance follows the install?
Cartridge changes on the maker's months-or-gallons schedule, a leak check under the cabinet every so often, and replacing the filter promptly when taste returns or flow slows. The install is once; the change-outs are the relationship.