At 120 sq feet, your not leaving much room for anything but a bed and and a table, but it's doable. Before you start, how sure are you on not needing a permit for this sized building? What if you have electricity? In the Bay Area, where I'm from, sheds didn't need a permit either, unless they had power. Then it didn't matter what size the building was, you needed a permit and it had to be up to code. The way we worked around it was to run an extension cord to the building. It wasn't permanent power, so it didn't need a permit.
Do you need AC in it? A window unit is the cheapest and very effective in small rooms. Concrete works as the flooring and you never have to worry about it. If you want to stain, AVOID the quikrete brand stain sold at Lowes. It's just a paint that chips off and wears quickly.
Can you have a porch on it to increase your outdoor living area without increasing the square footage? Two natural wood posts from a local hardwood tree or cedar make very nice porch posts.
A small building needs lots of lighting. A window on two sides and a door with a window will really open it up. I'd leave one wall solid for hanging stuff and storage. I'd also look into a futon for sleeping. Having a place to sit and relax will be very limited in such a small building.
For interior siding, OSB is just about the cheapest thing out there. If you don't like the look of it, paneling gives you quite a bit of options, but even sheetrock won't cost you very much in that small of a space. How will you build the ceiling? if it's flat, then sheetrock or something light in color would work. If you vault the ceiling, which is what I'd do, then sanded bead board makes a really nice ceiling. You can put a beam across the middle and create an open truss that will really make it something special. Stain the sanded beadbard, which is about $20 a sheet, and you have a wood ceiling that looks like toungue a groove for a small fraction of the cost.
There was a thread over on Pondboss from a guy who used Hardi Lap Siding on his home and stained it.
Pond Boss: Human habitat; dreaming out loud From a distance, it looks like real wood. In fact, when you look at the pictures, up close, it looks like real wood!!!!
I'm working on an experiment with regular Minwax wood stains on hardi boards to see how it lasts and looks. The pictures are from my test and are after three months outside. I don't know how the long term, year after year, results will be yet, but I'm very encouraged by what I see. The red oak is my favorite and what I plan to use when I build my cabins. I have one coat on there and the guy who did it used two coats on his home. I want to see what one does first, then I'll add a second coat to half of the already stained areas.
Have fun,
Eddie
