How do I create storage in a smalll bathroom?

I keep a basket on the floor between the wall and toilet for my extra rolls.

A cardboard box decorated with contact paper or painted, next to the ceiling, backwards like a fold down shelf using colorful duct tape as a hinge. Out of the way and pretty, and you can change it when you get tired of the pattern or for seasons. One extra roll is handy. We gave up on toilet rolls and just put ours sideways on a tri corner shelf stuck nearby with the basket directly underneath. I cleaned stores before retiring so am a neat freak. I clean the bathroom and disinfect each time I use it. That takes care of most of the problem even if others forget. We buy the LARGEST packages and store extra ones in a closet or the garage on a shelf, our own private FREE store because it is already paid for. We are retired and the kids and grandkids just moved out, wow, we are downsizing, feel for you. Much love your way.

Post a pic of the bathroom and you may get more suggestions.

I use a small white coated shelf under my sink. This doubles the amount of space I have. If you don't have a medicine chest consider replacing the mirror with a medicine chest. Consider making shelving between the studs. Go to a Habitat For Humanity resale store for a cabinet to put above your toilet. You can finish the cabinet however you want. Edit what you have in your bathroom. I bought a great, but somewhat pricey Command shelf, white with a silver colored holder. It fits my toothbrush and toothpaste. The shelf is handy and keeps some things out of sight. You could put several on your wall.

You could find a cute basket to put extra toilet paper into! That way it looks like decor but is also functional!

I use an oversized "hatbox" that I found in Michaels store. It is very decorative, comes in different patterns, and can be hung on the wall if no other space is available. Depending on the roll size, it will hold four or five rolls.