You have chosen a site which is higher than the rest of the terrain on your land. As we are going to build this 12x12 foot "house" on Sonotube or several stones in the colonial manner, you should not site this on a precipitous slope, cantilevered over open space etc, unless you are prepared to put in the appropriate foundation.
I do not like high posts supporting a house. It is a good idea to have access to the underside of your building, but building too high off the ground, especially in areas where there is plenty of wind, can cause severe strain on the concrete posts from twisting and buffeting. I will talk about building off the ground slightly and back filling and using a colonial style perimeter to the house site. That will give you easy access to the underside of the house while keeping you close to the ground as possible. Also, when you are close to the ground, it is wise, if you live in termite areas, to take appropriate precautions. See your county extension office.
We in the north-eastern part of the country have deep frosts. While there may be a trend for warmer winters, it is best to work with the worst case scenario. Plan on 48 inches deep for your foundation footings. Again, local county extension offices will help you adjust for your area. When in doubt, always error on the side of caution. Soil type will also alter the requirements. You may be trying to avoid local authorities, so again, always error on the side of caution and go deep, then you cannot be challenged if discovered later. Do your best to build to code or better.
Of course discovery and local authority is not the real issue. IF YOU DO NOT GO DEEP ENOUGH, YOU MAY HAVE YOUR FOOTINGS THRUST RIGHT OUT OF THE GROUND, OR UNEVENLY MOVED SO THAT AFTER A WHILE YOUR DOORS AND WINDOWS NO LONGER FIT AND INTERIOR FINISHES MAY CRACK.
You need to plan on support under all four corners, and along the walls under each side of the windows and doors. They need not be directly under the windows and doors, but spaced evenly around the perimeter of the sills so that those locations are well supported. Since the openings will be centered on each wall, I would place the posts about at every third of the distance. The center of the corner support posts should be just in from the actual corner, and the centers of the side Sonotubes should allow for bolting through as well. You should place anchors protruding from the concrete in such a way that they will extend through the sills in order that washers and nuts can be screwed on to hold the building down. If this is a building that is likely to be moved, this may not be an issue, but I am afraid that building inspectors will insist.
We will be building a 12x12 building, and it is probably heavy enough that only a tornado would move it, but better safe than sorry. If you omit the anchors because of plans to move the building, you should anchor it as soon as a permanent location is chosen, and access to the sills for this purpose should be planned on. Tall baseboards (a good idea for a nice interior look anyway) and a border of planks picture framed around the perimeter of the floor as a decorative feature will cover any work you have to do to access the sills. The floor planks would be a wonderful place to splurge on fabulous hardwoods...Birdseye maple and walnut alternating would really make the room. Plain hardwood, cork, bamboo or tile would fill in the field. Small tiles will be better than large if the possibility of movement is there. Large tiles will crack with just the slightest movement of the sub floor.
Dig down 48 inches at each post location. Use an auger, back hoe auger, post hole digger, shovel etc.. I am planning on 12 inch Sonotube for each post. If you have spare lumber, you can construct square forms to pour the concrete into. This would absolutely require you to remove the wood before burying it as the wood extending right up to the sill might encourage bugs. Sonotube can be peeled back or left to rot off in time.
You may build a form on the bottom of your very flat bottomed hole, 2x2 feet, and about 10 inches tall. Alternately, Sonotube and competitors sell forms to make a footing in sort of a squared bell shape. Once concrete is poured and just starting to firm, for the footing, immediately put your Sonotube in place and fill that as well. Work quickly. Be prepared. Make sure that the tube is vertical using a level and it is a good idea to secure the tube in place by burying it while it is held in place, or a couple of lengths of wood are nailed or stapled horizontally to the outside of the tube and extend to the surrounding ground to hold it in place with stakes. Two people working together will help. As the footings are being poured one by one, the second person might be preparing the Sonotube or forms for the next step as you progress around the building.
Note: Do not put a pier under a door. You might try putting a pier under each side of the door if you wish, for stability. If you do have some sort of foundation under the door, do not put an anchor bolt or threaded rod under the door as it will only have to be removed to put in the door sill...
As you finish pouring the first post, a mark should be made on the outside of the tube to indicate the level of the concrete. As soon as the form for the second is prepared, your home made TUBE LEVEL or very long level should be held between the two Sonotubes to determine the correct filling height for the second post, and so on. You could of course, fill the tube to the very top, and make each form exactly level with the others before you pour. Always check back to the first post as well. Tiny errors get worse and worse if you only check against the last one poured. If you are using a long level, check level twice. After the first try, turn the level over (end to end) so you discover if your level is off a bit. This is quite possible, especially if your level has had a hard life. Also the shorter the level, the less accurate.
Starting with the footing, you may stand rebar into the wet concrete in such a way that it extends up into the future location of the Sonotube. Perhaps three or four would be good. Do not place these in such a way that they are exposed to the air or soil. This could start rust on the rebar. Rust expands metal, and the expanding rebar could crack your foundation in time. You might also bend a piece of rebar into a square, and submerge it into the footing horizontally while still wet.
Notice the void and seams from pouring improperly
When the pour is complete, place anchor bolts into the concrete, 5 1/2 inches from the outside edge of the pier, just as it starts to firm up a bit, or suspend the bolt into the concrete by screwing it at a right angle, through a drilled hole in a piece of scrap wood that will rest on the top of your form as the concrete hardens. Then, just unscrew the piece of wood to free it when ready to continue.
Cover the exposed part of the concrete piers with plastic sheeting squares or garbage bags upside down over the tops of the forms to allow it to cure slowly rather than drying quickly, and to avoid heavy rain or freezing. A little dampness is a good thing as it cures, but not a beating rain while it is still early in the process..
Be patient. The inside of the Hoover Dam is still curing all these years after the pour. It takes time, and will become stronger if you give it that time. Perhaps this is a good time to finish your work for one weekend, and do the next stage the following week. Alternatively, since we know the exact dimensions of the wall panels, you could construct them on the ground over a few days while you give the concrete a chance to cure without weight on it.
If you have a yen for the colonial way, but want the stability of deep footings, fill your Sonotubes to just above ground level, plus or minus a bit for the varying thickness of the stones you plan to use under the sills, then lay the stones on top before constructing the sills. If the stones are large, you need them off the ground slightly so that frost will not lift them off your posts. Grass and low plantings will cover the gap, and it will look like the building is resting just on the stones. Be prepared with a cold chisel to chip off layers of stone to lower it to level. You will not have the option of using anchors in the concrete unless you are into drilling through the stones. You can purchase ground anchors as a substitute. These are a bit like the anchors that you use to tether a dog on the lawn etc. This will absolutely be inferior. If you are doing a larger building and want the stones to be visible, you may have to rig something up yourself that will do both jobs. Email and we can discuss it.
When your foundation is in, strip off the upper ends of the Sonotube paper, or remove the board forms for the cast piers. After back filling the holes, fill the entire area under the building to the top of the piers with coarse gravel. The closer to the bottom of your sill and floor joists, the less likely an animal is likely to try to get in. You cannot make this fool proof. A mouse will get into a hole the size of a dime, and if it is not provided with a hole it will make one. But raccoons might say: "This is just too much trouble for me." I have often thought that a layer of flashing on the under side of the floor joists, sealed with silicone, or sheets of galvanized metal or even copper if I was rich, might just keep most pests out of the insulation under the house. Spray foam insulation might work out well too. Allow the gravel to spill out onto the surrounding earth on a slope. When construction is complete, flat stones laid on this slope will allow water to drip from the eaves, then bounce the droplets outward instead of back up onto your sills and wooden trim. This is an old colonial trick, and it works too.
Another colonial trick is to build a series of X shaped crosses of wood that will straddle the ridgepole of your roof. A "V" troth made of lumber is laid into the resulting cradles to form a sort of gutter. The ends of the troth are blocked with wood as well. Fill this troth along the ridgepole with lime. The rain will over flow the troth on a regular basis, dissolving a bit of lime and allowing it to soak the wooden roof shingles as it trickles down. The roof will last much longer, look fresher and lichen and moss will hate it.