Regardless of which type of foundation you choose, remember to tamp the ground under it and put in the pipes for the plumbing and electricity.
Think twice about using a lot of cement! Its production requires mining, transportation, and heating materials to very high temperatures twice. This entails burning fossil fuels and often creates hazardous waste. Concrete makes an enormous contribution towards ozone depletion and air pollution.
It's common to add reinforcing steel to a concrete foundation. The library will have lots of information about how to reinforce this type of foundation.
Remember to add steel anchor bolts to attach the door frame to the foundation. Be aware that wood meeting concrete forms a
potential rot spot. Wood treated with poison will fight off the rot longer than untreated wood. Some builders also put a moisture barrier between the concrete and the wood.A standard concrete mix is made of 1 part Portland cement, 2 parts sand and 3 parts gravel. You can mix it yourself or get it delivered already mixed to the site. Wet concrete does not stick well to dry concrete so ideally it should be poured all at once. Embed sharp-edged stones on the top before it cures. This will make a rough surface for the cob to hold on to.
Make sure the concrete cures slowly by keeping it wet and covered for a week or two before starting to cob.
Soil cement is otherwise known as 'poor people's concrete'. Soil cement is weaker than concrete but it is a good option for areas with limited rock and no road access, or for people who want to minimize their use of cement.
Depending on your soil (sandy is best) add up to 9 parts soil to 1 part cement plus water. Stir well. Use immediately. Make up some test batches with different amounts of cement and from different soil types until you find a suitable mix. Remember to put some stones on top of the soil cement for the cob to hold onto.
Soil cement can also be used for floors or made into large flat bricks for stepping stones or floor 'tiles'.
Any poured foundation will need forms to hold the cement mixture in place until it's cured enough to hold itself up. The library will have information on building forms and pouring concrete.
Conventional forms are made out of wood. If you'll be building a rounded house, you'll need a rounded foundation which is very difficult to form using wood. Scrap sheet metal held in place with stakes driven into the ground, or straw bales with stakes holding them in place, are simple, inexpensive forms foundation walls. Tie the tops of the stakes to each other to ensure that the concrete doesn't push the forms apart.
Line the bales with cardboard, or whatever you have handy, to keep the concrete off them. The straw bales will be used later in the cob walls.
To curve a straw bale, lean it up against something at a 45 degree angle and apply weight to the middle of the bale until you have the shape you want.