I personally framed in, hung this door and trimmed it out a few years ago. This was one of Mom's long time wishes - a door on the hot water heater closet. Before the opening was covered by a piece of wall paneling, backed by wallboard, and screwed in to the wall. It was a giant pain in the butt to access the hot water heater, not to mention cumbersome and heavy. Okay - just plain stupid! Mom found the narrow door and enlisted my construction skills to make it happen.
I had to add a few 2 x 4's and some lathe to get the opening to size - which suddenly became an issue now that we needed to get the heater out of the space. Well, it turned out that since I was the one to build it, I was the perfect person to be here when the plumber arrived.
I removed the door and the trim work. The plumber grabbed his Sawzall and cut out one of the 2 x 4's framing the space. It was non-load bearing, so all was well. That gave him just enough width to wrestle the old unit out. Getting the new heater in became a two man job (not me!), as it had to be lifted up and wrangled around the existing pipes, before being lowered into position.
The 2 x 4 was replaced, the door rehung and the trim work nailed back into place and we are good to go. And I now know exactly what needs to be done for the next time, which hopefully won't be for many years!