Due to Wyatt (the youngest cat) being scared of Simon when my mom first got him, he spent all of his time downstairs to avoid him. However, the litter was upstairs. To try to salvage the situation, my mom put a litter box in the downstairs bathroom specifically for Wyatt. Unsurprisingly, he was not the only one to use it.
Even now that Simon has been moved out to the game room over the garage, the cats still like using the litter box downstairs. But my parents have gotten a bit tired of having to move the bathroom around their tiny bathroom, so my mom decided they'd put it in the laundry room off the kitchen.
They didn't want to keep the laundry room door open all the time, so my dad decided to put a cat door in it. It was just a square hole in the door until my dad went to my mom, "Hey, I've got an idea ..."

My mom still has to paint whiskers on it.