I have a 16ft by 20ft (exterior) "stick built" storage building in the back yard. I think the siding is called T111 wood siding?, ONE window at about 3 inches above the floor. I have my AC UNDER the bench in the center of the "narrow" wall.
The layout is a "duck under" entry. Since you have to step UP to get in, just "hunch your shoulders and stay ducked as you take one long step into the building. When I built the building, I went to the City building inspectors who informed me that I could cover a specific portion of my lot (city code for rain absorption) so that limited my size. I also had to position the building based on "set backs" and existing underground plumbing or electric and natural gas lines.
I built the building in the very early 90s and 220vac for large window AC was common, so I built in the capability to have 220vac which included a breaker panel just for the building rather than simply a circuit from the house breaker. I also took advantage of the 220 and installed baseboard electric heat (8ft long baseboard heater that heats a fluid.) I have a conventional roof with shingles and the building is painted to match the house trim.
A friend from church helped me finish the mud and tape on the sheetrock walls. Then he stuck around and helped me sand the seams so it looked nice. Next, I carpeted the floor with USED carpet. $100 to a contractor and I had the same friend from church install it. (Sheet rock done and painted walls and ceiling before the carpet was laid. I elected to use a sky blue for the wall paint, plain white for the ceiling.)
YES, the building is on the tax roll for my property. Every state does property taxes differently so I was prepared for the change...and honestly I don't even feel it these days.
The interior is 15ft wide by 19ft long. Back when I built the building, I was modeling in HO so I designed for that. I have the exact same bench work set up today as when I first started. The long wall shelves are 30 inches wide. The baseboard heaters are on the two long walls about 4 inches above the carpeted floor. The entry door is along one of the long walls and the layout narrows to 24 inches over the entrance. By the way, I ran sheetrock over the top of the door to hide the door. That was a very good choice.
The narrow shelf on the wall with the AC is only 18 inches deep. The opposite short wall has a 30 inch deep shelf, but it also has a 4ft by 10ft peninsula that attaches to that short end shelf. That same short wall plays host to the staging. There will be a façade of structure flats to hide the two stub end storage tracks. There is also a disguised continuous run connection on that same short wall.
The entire layout is a flat table. My plan was always to build scenery above the table top (Homasote on top of plywood sheets). Because I changed track plans several times in HO and then changed scale, I am VERY glad I did not opt to build open grid.
The top of the bench work is 53 inches above the floor. Why, I measured my armpit and subtracted 2 inches. I can stand flat footed and still reach the back tracks.
I would point out that my layout theme is industrial switching district. I like to run reasonably close to prototypically. I am not real sure I will follow every Rule...I don't know most of them that well anyway. But every freight car will go to a specific door on the various industries. There is NO engine terminal. The concept is the operator is working an 8 hour trick (or up to 12 hours) on a switching district for the Santa Fe in 1963. (I like ATSF zebra stripes from my childhood.) So, I am freelancing my version of a real Santa Fe industrial switch district in Oklahoma City. The branch (OKC Stockyards District) had lots of other customers besides the stockyards and the packing plants. While I could accommodate a second operator, I did not specifically plan for that. Rather than have trains from the switching district originate from the main OKC yard, I am changing reality so that union rules dictate that stockyard crews stay on their district or get paid extra. So cars are dropped on the connection track to the ATSF mainline by the "south local switcher" (GHA) which is not modeled.
Regrets? I am unhappy with the length of my runaround tracks. On the long shelves/long walls, I can position "about" six cars and caboose inside the switch clearances. DOES NOT INCLUDE LOCOMOTIVE. I am trying to restrict cars to 40ft with the exception of gondolas and flat cars.
Beyond that, I am OK with what I have. Would I like 24ft. OH YEAH! Would I be happy with 12ft wide? Not so much. I would be happier with 24ft by 18ft. That would let me use O-72 around the end of the peninsula. That would free me up for secondary main line with a couple of small towns and hidden stage. Maybe some distance between the towns with space for a small limestone crusher (no coal in my part of the state). Sigh....(sob...what could have been...sob.)
Would I do it again? You Betcha! I would change little things. As I noted above, more space would be nice...and I did not need that much HEAT and I need a larger AC because the 220 unit died (compressor) and the 110 model is not quite enough for my room.
Go Thunder.