However, I am somewhat of an expert at outhouses, having built one, and used it religously for, well some time.
1. The best information can come from your county agricultural agent. You may discover that the county does not allow outhouses.
The depth is related to two things, the usage, and the soil conditions. I, for instance, welded two open ended 55 gallon drums together, sunk them in the ground, and deposited away.
2. Pea Gravel. Pee gravel?
( I just could not resist.) Again, it depends on the soil conditions.
3. For company. Or, to speed up the long lines.
4. This is, actually, a good question. It is variable, Sometimes the heat, and the flys are a problem, and a white powder dusting each deposit may be called for. Other times, or seasons, perhaps it may not be needed at all, or only occasionally.
A few thoughts:
There were tens of thousands of WPA toilets built during the depression, and some of the cast concrete bases are still sitting around, often with no stucture on top. They make a great start. Plop on a toilet seat, build your favorite building on it, put a crescent on the door, you're ready to go!
The TP can get damp in some climates. A large coffee can, with a plastic top works well.
A skylite is a welcome addition. I donated one to an uncle for his outhouse, and it was great.
Make sure you vent the hole. PVC or something straight out the roof. It makes a lot of difference. And, close the lid EVERYTIME. You will thank yourself the next time you enter.
I think that an outhouse should be built without a level or a square. The door needs to swing open by itself, or maybe close by itself. It is just part of the experience.
Come to think of it, some folks put on a self closing door. Pulleys, rope, a window weight or two and you get that reassuring slamming door sound when your business is done.
And do not, DO NOT buy new hinges for the door. They hafta squeek. Shop around if you must, but an outhouse door that doesn't squeek is hardly worth the trouble.
A two story outhouse can be problematic. There is one that I know of that actually works. Still, I would rather be on the second floor.
Books have been written on this subject. One of them that I skimmed was quite entertaining, and could be useful as well. The title escapes me.
By now, you must have guessed that I am somewhat passionate about my outhouse experience. You would be correct. There is nothing quite like sitting there with the door open, gazing contentedly across the back 40, and having the family dog stick his cold nose on your bare legs. Life is good!
Dale