My recommendation would be to build it on two 4x4 treated skids, so you don't have to have a deep hole. Just a foot or two. When it fills up, hook a rope at the base and use your truck to slide it over another small hole. With infrequent use, even a small hole wont fill up much, because it will evaporate/filter down/decompose inbetween visits. Around here people who had outhouses in use "full time" rarely had to move the house, because it broke down quickly.
Some houses around in this country, after electric came, bringing water pumps for wells, put in bathrooms inside, with the 3" pipe just running to the nearest woods. Where it came out was usually not smelly, because every rain would pack it down, and given a lot of fresh air it could breathe. During the hot summer, if it got smelly they would sprinkle a handfull of lime on top. You'd be surprised how many houses around here are still like that. It works great, and you never have to clean out the septic tank!
I know the local misgovernment would ring my neck, but why worry about a few ounces of human waste a day, when one cow gives 10 times as much? If it can "go" in the creekbed, or field, or wherever, why regulate where I can "go" ?? Answer....more govt. control = less individual freedom. After a generation of people brought up with tight govt. regulations, people nowadays can't even start thinking about how to do something without asking "what does the regulation say" where our ancestors would be mad as hornets if someone told them to "go" somewhere else!
Reminds me of what happened here a few years ago. An older man (70s) was a local farmer and had been his whole life. He was used to using the woods, and when the local "health" inspector came around noticed that he had no plumbing, or even an outhouse. The inspector told the man that country regulations required that every residence be equipped with satisfactory "facilities" and that the man would have to build a bathroom or else vacate his old house (which had been in his family a long time). The farmer told the man to leave him alone.
A few weeks later he was summoned to a hearing, and when asked why he would not comply with the regulations, this was his answer (more or less). "I grew up in that old house. When I was little we had an outhouse. After my parents died they left me the farm. Being an lone man now, with no wife or children in the house, I didnt replace the old outhouse when it finally rotted."
His closing statement was great...."I own nearly 100 acres, and on my land, when I feel the need to ***, I'll *** where I want to!" He then turned around and walked out!
Another neighbor built an outhouse a few years ago (without telling the county) as a backup for the indoor bathroom, so that when the power is out it is available. It is very nice, with a toilet seat, paper holder, screened upper window opening, and even magazine rack! They have it on skids, with the house off the ground about 4" (the height of the skids) so that air can ventilate it.
The keys to good outhouses: ventilation, good (comfortable) seat, and tight construction to prevent bugs and critters. The depth is not as important because you can move it when it fills up. Again, it'll take a LONG time to fill up a pit, even with regular use, because of the filtering into the soil and the decomposition.
