Empty the cabinet completely before you plan anything. Every bottle, the sponge multipack, the wad of grocery bags, the paint can that migrated in from the garage. Wipe the base while it is bare, then put a flashlight on the plumbing. A white crust on a joint, a swollen seam in the particleboard, or a musty smell means a slow leak, and organizing over a leak only hides it until it becomes a floor repair. An empty cabinet also shows you the real space, which is lumpier, and more useful, than the full one suggested.
Quick answer: Sort ruthlessly, then measure the clear blocks of space around the P-trap and supply lines instead of trusting the cabinet’s listed size. Fill the floor with sliding two-tier drawers and a U-shaped riser that straddles the pipes, hang light gear on the door and side walls, and give any blind corner a turntable. Only items used at that sink go back in, with daily ones in front.
Step 1: sort what came out, and be strict about it
Under-sink cabinets fail as storage because they succeed as dumping grounds. The door hides everything, so everything ends up there. Reverse that with one admission rule: an item goes back only if you use it at this sink. Dish soap, dishwasher pods, trash bags, a scrub brush, and the everyday cleaning spray belong under a kitchen sink. The turkey roasting pan and the vase collection do not, and under a bathroom sink the equivalent freeloaders are the hotel toiletries and the hair tools nobody has plugged in for a year.
Work the pile in passes. Consolidate duplicates first; three half-empty glass cleaners become one full bottle. Discard anything that failed at its job, since a product you avoid is clutter with a scent. Set relocations in a box by the door so they actually leave the room.
Then rank what remains by frequency, because the cabinet has good seats and bad ones. Daily items earn the front six inches. Weekly items take the middle. Anything monthly or rarer goes to the back corners or leaves entirely. If small children or pets live in the house, drain openers and dishwasher pods move to a high cabinet or get a latch on this one before anything else happens.
Step 2: measure the space the plumbing actually leaves
The cabinet may be listed as 36 inches wide, but you cannot use 36 inches. You can use the blocks the plumbing leaves behind, so measure those. Record the clear width from the left wall to the trap, and from the trap to the right wall. Record the depth from the door opening back to the first pipe, and the height from the cabinet floor up to the trap arm or the garbage disposal body, whichever hangs lower. A typical kitchen setup yields two usable columns beside the trap, a shallow strip across the front, and odd air above the shelf line.
Measure the door opening as well as the interior. On face-frame cabinets, the frame overhangs the box on both sides, so the doorway is narrower than the space behind it. Every organizer has to pass through that opening, sometimes at an angle.
Write the numbers in your phone and treat them as law in the store. Buy at least an inch under each dimension, because supply valves bulge, drawers need side clearance to slide, and a unit that fits with zero margin fits only in theory.
Field note: The face frame is the classic trap. An interior can run 34 inches wall to wall while the opening between the frame stiles measures 30, and a wide organizer that would sit happily inside will never pass through the doorway it has to enter. Check the opening first; it is the number that disqualifies purchases fastest.
Step 3: fit the floor with sliders and risers
Fill the measured zones with three workhorses. First, two-tier sliding drawer units for the clear columns beside the trap. Most run 5 to 12 inches wide and 16 to 18 inches deep, close to the footprint of those side blocks, and the slide matters more than the tiers. A drawer brings the back of the cabinet to you, which turns dead depth into usable depth and ends the crouch-and-excavate routine.
Second, an expandable under-sink riser for the middle. These are shelving units with adjustable legs and slotted or removable shelf panels that notch around the trap, and they convert one lumpy volume into two flat surfaces. Expand it to your measured width, drop the shelves in around the pipes, and the awkward center of the cabinet starts holding things.
Third, stackable clear bins for the back and the corners. Clear beats opaque here because the door already hides everything once; opaque bins hide it twice. Group by task, one bin per category, so a whole category comes out in one pull.
Two material rules for a space that lives next to water. Choose plastic or coated wire and skip cardboard and bare wood, which swell and smell after one drip. And lay a waterproof mat or a low-sided tray on the cabinet floor first. It is cheap insurance that catches small leaks where you will see them on a normal reach-in, instead of letting the subfloor absorb the evidence.
Step 4: put the door and the walls to work
The back of the door is prime space doing nothing. A door-mounted rack, either a wire basket that screws on or an over-door caddy that hooks the top edge, holds the spray bottles, brushes, and gloves that otherwise clutter the floor zones. Screws into a solid door beat adhesive every time; if the door is thermofoil or melamine and you must use adhesive mounts, degrease the surface first and load them with nothing heavier than gloves and sponges.
Before mounting anything, do the clearance check. Hold the loaded rack against the door, swing it slowly shut, and watch where it passes the trap, the riser, and the new drawers. Mount where it swings through clean air. A rack that collides with the shelf on closing will be torn off within a month, usually by someone shutting the door with a foot.
Side walls take hooks. Two or three small screw-in hooks hold a dustpan, a spare towel, or the rubber gloves that never have a home. Keep the daily cleaning kit, one spray and one cloth, in a small caddy at the very front of a slider so it comes out in one motion and returns in one. The lower the effort to put a thing back, the longer the system lasts.
Step 5: solve the corners, then keep it working
Blind corner cabinets swallow whatever enters the dark side, so stop storing loose items there. A 10 to 14 inch turntable converts the corner from an archive into a menu; spin it and every bottle faces you. In a tall corner cabinet, stack two. For deep straight cabinets, use long shallow bins as pull-out drawers, gripping the bin instead of fishing for items, and load only the front half of any deep shelf you cannot slide. The front-half rule and the rest of the small-home playbook are covered in storage systems that actually work in a small apartment. Tall narrow cabinets take a wire file divider, standing sheet pans and cutting boards on edge so no stack has to be unstacked.
Maintenance is smaller than the setup. When the trash goes out, return strays to their bins. Every few months, pull the front items, wipe the mat, and shine the flashlight on the trap again, since the best time to find a leak is during a wipe-down rather than during a smell investigation. If one category keeps ending up on the counter, its home costs too many motions, so move it forward or give it a hook. That is the same effort-per-use rule that keeps an organized closet standing, and it holds twice as firmly in a cabinet built around a drainpipe. The space will never be pretty. It only has to hand you the dish soap without a search party.