The problem is the plumbing, and the plumbing is solvable
Under-sink cabinets are the largest single volume of storage in most bathrooms and the least used, because a P-trap sits in the middle of the floor and there are no shelves. Everything ends up in one layer of loose bottles, and the back eighteen inches becomes a place things go to be forgotten. The fix is not more bins — it is building around the pipe in three deliberate layers.
Empty it and measure four numbers
Take everything out, wipe the base, and write down four measurements: interior width, interior depth, height from base to the underside of the sink bowl, and the distance from the cabinet floor to the bottom of the P-trap. Almost every under-sink product listing is a gamble without those four numbers, and the trap clearance is the one people always forget. Photograph the empty cabinet with a tape measure lying in it and keep the picture on your phone; it settles every in-store “will this fit” question in about five seconds.
Layer one: a two-tier expandable shelf around the pipe
An expandable under-sink shelf with adjustable legs steps over the drain pipe and instantly doubles usable surface. Choose one with independently adjustable side heights so it can straddle an off-center trap. Place the taller side over the pipe and use that upper level for lightweight backups — spare soap, cotton pads, refills — keeping the heavy things low. Coated wire shelves are worth a small premium over bare steel here, since a slow supply-line weep will rust an uncoated shelf within a season.
Layer two: pull-out drawers for the deep back
The back of the cabinet is only usable if it comes to you. One or two sliding drawers on the cabinet floor turn an eighteen-inch dark cave into a tray you can inspect at a glance. Sort by activity rather than product type: a hair-and-styling drawer, a cleaning-supply drawer, a first-aid drawer. When one drawer is one routine, you stop excavating. Two shallow drawers usually beat one deep one: things stack in a deep drawer, and anything stacked eventually gets forgotten. If the cabinet has a center stile, measure the opening rather than the interior — that narrow doorway, not the cabinet itself, decides the widest drawer you can fit.
Layer three: a tension rod for spray bottles
Wedge a short spring-tension rod across the cabinet a few inches below the sink bowl and hang every trigger-handle bottle from it. Cleaners stop tipping over, stop leaking onto the base, and free up the entire footprint they were occupying. It is the cheapest upgrade in this article and usually the most noticeable.
Protect the base and check it monthly
Line the cabinet floor with a wipeable mat or peel-and-stick vinyl. Slow supply-line drips are common, and the first sign is usually a swollen particleboard base that costs far more than the mat. Once a month, when you restock, run a hand along the back of the cabinet floor — thirty seconds of checking catches leaks while they are still minor.
Store what belongs there, and nothing else
Heat and humidity live under a bathroom sink, so keep medication, vitamins and anything with a delicate formula somewhere else — a bedroom drawer or a linen closet stays far more stable. Under the sink belongs cleaning supplies, backstock, paper goods and tools. Cap it at one bin per category and the cabinet stays organized without any maintenance at all.