When I was a young man, we bought our first home. It was being built and I didn't know a lot about much of anything. The stairs leading upstairs were at the back of the living room. Where there would usually be an under the stairs access door, the builder planned to put in the kitchen breakfast bar. So, the left side of the COULD be under the stair storage was in the living room. Couldn't do anything there. The front, directly under the top stop, had a breakfast bar attached. The right side was a wall in the guest bedroom and there was a door leading off of the kitchen to get into it.
One day after all of the crew had left and before the drywallers came in, I went to that side of the stairs and put in a horizontal stud. A few months after we moved in, I took out a drywall saw and cut out the hole. It was the perfect size for one of those short (about 4') furnace doors. It looked just like the other doors in the house, only shorter. I mounted it, put in a deadbolt, and ran some electric so I could have a light. Basically, I had a built in gun locker that even the builder didn't know about. It was easily concealed because if you opened up the bedroom door, the new short door was behind it. A casual, or hopefully rushed, intruder would open the door, look in the room and see nothing of value. I put in shelves, racks, and a few other little hidden details as well.
When I showed it to the person who bought, he got a look on his face that showed me he totally understood the value of the room. I just went back and read over this. I hope it makes some sense!