A sink can be the right width and still be the wrong sink. If your parent washes, shaves, or brushes teeth while seated, what matters is the space underneath, the height of the basin, and whether the faucet is within reach from the seat. Measure the person and the room, not the catalog.
Local option: New Plan Care gives families in the St. Louis area in-home help so a parent can stay home: companionship, help with bathing and dressing, meals, and overnight care. Call (314) 405-0887.
Start with who uses the sink
Write down how your parent uses the sink now. Do they sit in a wheelchair, a shower chair rolled to the sink, or a stool? Do they need to pull close to the basin, or do they only lean in? Is anyone else sharing the bathroom, and does that person need the sink in the same position? These answers decide how much room to plan for.
Measure under the basin
Measure the height from the floor to the underside of the basin, at the front edge. Then measure the depth from the front edge of the basin to the back wall of the cabinet or wall. Measure the width of the open space between the cabinet sides or the wall edges. Write each measurement down with a note about where you took it.
Check what is under the sink
Look at the pipes. The drain trap, the supply lines, and any shutoff valves can take up the knee space that a seated person needs. A plumber can often offset the trap or move the supply lines, which may open up the space without removing the sink. Ask what a trap change would involve and whether it changes the wall or the floor.
Any hot water pipe within reach of a seated leg should be fully covered with an insulated cover. Ask the installer to confirm the cover fits the pipe size you have, since covers are sized by diameter.
Measure the seated person
Have your parent sit at the sink as they normally would, with their feet on the footrests or the floor. Measure the height from the floor to the top of the knee, and the distance from the front of the sink to the front of the knee. Then check where the knees land under the basin. If the knees hit the pipes or the cabinet back before the person is close enough to reach the faucet, the sink is too tight for that seat.
Check the faucet reach
From the seated position, check whether your parent can reach the faucet handle without stretching or leaning forward out of balance. A lever handle is usually easier to operate than a knob. Check that the water controls are placed where your parent can turn them without lifting an arm above the shoulder.
Check the mirror and the basin rim
Place a hand at the sitting eye level and see whether the mirror can be seen clearly from the seat. A mirror mounted for a standing person may be out of view. Also check the basin rim height, because a rim that is too high makes reaching into the basin harder.
Decide among the options
There are a few ways to get more knee room: offset the drain trap, replace the cabinet with an open-leg vanity, or install a wall-hung sink with the pipes behind the wall. Each option has its own installer questions, its own cost range you should ask for in writing, and its own effect on storage and counter space. Compare them against the numbers you measured, not against a picture.
Bring the numbers to the installer
Give the installer or plumber your written measurements, the seated height, and a short description of how your parent uses the sink. Ask them to confirm the trap type, the pipe cover fit, and whether the layout works before any work starts.