It is vital that you read the following chapter on floors (starting page 57) to help you design your foundation and threshold.
The positioning of the door(s) must be decided when you are designing your foundation. Underneath the door, the height of the foundation must be lower than the height of the rest of the foundation. The depth of the foundation under the door may need to be increased so that there will be enough material to make sure the foundation is continuous. If you live where the ground is stable, you may opt to have no foundation under the doors or a much less substantial foundation under the doors and under any sunny, glass-filled, lightweight walls.
Decide what material you will use for the top surface of the threshold, and build it accordingly. If you are using the foundation stone itself, make sure it is as flat as possible so the bottom of the door seals well.
Make sure that the threshold will be a dry place.
Consider which way the door will swing open. It invites you to go the
way it swings. (If you live where it snows a lot, make the door open inward
so you don't get trapped inside by snow piling up against the door.) However,
doors opening in take up precious indoor space.