I made a closet under our staircase. The stairs are in two flights, anti-parallel to each other, with a large landing in the middle where you make a U-turn. There was originally a small coat closet under the upper flight, adjacent to the lower flight, but the closet only went back about 4 feet and I knew there was a lot of wasted space under the landing. So one year I bashed out the back wall of the closet, pulled one stud, re-framed it with additional headers and jack studs, then went to work finishing the inside space.
First order of business was to remove the construction debris, styrofoam cup, mcdonalds wrapper, and other garbage from this interior space. Yep, the builders really do leave their garbage inside your walls.
I finished the entire space including the rectangular space under the landing, plus the additional, low area under the first flight, with sloping ceiling under the stairs. Tons of space! Drywalled it all, which was a PITA because many of the pieces had to be cut in weird shapes and angles, and in the meantime I had to be mindful of the maximum dimensions of pieces that I could cut and still bring them through the closet and through two 90 degree turns into an increasingly confined space. Working for hours in this cave that was only high enough to kneel down in, it was a relief when the last piece of drywall went in, and I flicked on the switch to light up the little LED can light that I had installed in the ceiling, and beheld my new storage space.
I decided that screwing the drywall onto the walls (and adding corner beads where needed) was enough, so I vacuumed and put all the things in storage that we had meant to put in there, with drywalled though unfinished walls.
A year later, I decided that it was time to mud the walls and paint them, put in a floor and finish the space. So I hauled all the items back out, and got to work mudding and taping. Lessons learned from mudding:
1. You don't need as much mud as you think. I used up way too much mud on the back part, and when I had used up my second small bucket I had only one inside corner left to do. Not wanting to go back to the store and buy more mud, I looked around my garage and...:
2. Spackle is not an equivalent substitute for mud. Although I eventually got the tape to stick and coated it to look right, and it all sanded down and painted to look ok, I will never use spackle again as a substitute for mud.
3. A respirator mask and goggles are very helpful when sanding drywall mud in a tiny confined space. There is nowhere for the dust to disperse, so it just billows around in there and irritates you.
4. The more complicated your job is with the more edges and corners, the more PITA the entire process is. To include measuring and cutting various weird size and shape pieces; screwing them in; doing too many corner beads and inside corners; etc. Far better to take a look at the framing and re-frame as necessary to achieve more flat continuous walls.
5. A normal sized room has got to be WAY easier than a cramped confined space. Either my knees hurt from kneeling, or my knee joints from squatting, or my back from hunching over. Meanwhile my kids would walk right in and ask if this was the new secret room I was making for them....
6. All that said, it did NOT take nearly as long as I had feared. And it did not cost very much at all, the materials are cheap.