Under counter water filtration succeeds or fails on fit. The filtration itself is settled by your water test, but whether the job takes an hour or a weekend depends on cabinet clearance, the shutoff valve you have, whether there is a spare hole in the sink deck, and whether a garbage disposal is already occupying the space the manifold wants.
Measure four things before you order
First, the clear width and depth of the cabinet with the doors open, after allowing for the sink basin, the trap and any disposal. Second, the vertical space under the manifold, because cartridges drop downward to come out and a system mounted too low cannot be serviced without unmounting it. Third, the space for a storage tank if the system is reverse osmosis, since that tank is often larger than the filter assembly and can live in an adjacent cabinet if the tubing run allows. Fourth, the sink deck: count the holes and see whether one is free, occupied by a sprayer you never use, or hidden under a cover plate. Those four numbers determine which systems are actually candidates.
Valves, tubing and the drain saddle
The cold supply needs a shutoff you can trust. A modern quarter-turn angle stop is fine; an old multi-turn stop that has not moved in twenty years is the part most likely to weep after you touch it, and replacing it while the water is off is far easier than replacing it later. Feed adapters that tee off the cold riser are standard, and they should be metal on a permanent install. If the system is reverse osmosis, it needs a drain saddle clamped onto the sink tailpiece above the trap and drilled through, positioned so the reject stream does not gurgle back into the basin. Tubing wants a gentle route with no kinks and enough slack that the cabinet contents can move.
The faucet, and drilling if you have to
Most under counter systems come with a dedicated faucet that needs a hole in the sink deck or the countertop. A free hole or a redundant side sprayer position is the easy path. Drilling is straightforward in stainless steel with a step bit, workable in composite, and a job for someone experienced in granite or quartz, where a cracked countertop costs more than the whole filter system. Some systems avoid the question with an inline cartridge feeding the existing faucet, which is simpler but means all your kitchen water passes the filter and the cartridge works far harder. Our best under counter water filtration system guide covers that trade.
Clearance for the change you will do every year
The most common regret with an under counter install is a system mounted so tightly that changing cartridges means emptying the cabinet and lying on your back. Leave the manufacturer's stated clearance beneath the cartridges, mount the manifold on the cabinet side wall rather than the back where possible, and keep the shutoff reachable without moving anything. Put a shallow drip tray under the assembly; a slow leak found by a tray is a nuisance, and one found by a swollen cabinet floor is a repair. Write the install date inside the cabinet door and the cartridge part numbers beside it.
Questions people ask about under counter water filtration
How much cabinet space does an under counter system need?
Enough for the manifold plus vertical clearance beneath it to drop cartridges out, and a separate space for a storage tank on reverse osmosis systems.
Do I have to drill a hole for a separate faucet?
Not always. A free deck hole or an unused sprayer position works, and inline systems feed the existing faucet instead, though the cartridge then works harder.
Does an under counter filter need a drain connection?
Only reverse osmosis systems do, using a drain saddle on the sink tailpiece. Cartridge-only systems run inline with no drain.
What if my cabinet is full?
A countertop system does the same job without needing cabinet space, drilling or permanent plumbing, which also suits rented homes.