It's easiest to decide the floor height(s) early on in the design and construction processes. That way you can do the rough leveling of the floor area before you start to build. It's also best to decide what kind of floor you are going to make before you start building, so you can dig down to, or fill in to, the appropriate height. If you leave it until after the builders have tramped back and forth across the ground, it will be a lot harder to dig the dirt out to level it.
Scrape off the topsoil in the floor area and put it where your garden will be. Any subsoil that you dig out of the floor area can be used for making cob. The building process and all those feet will help do a lot of the tamping work for your future floor. If you are building up the floor level, you can do that before you build or anytime during the building process to make the most of the tamping feet of the builders.
Go over the ground and each layer of floor base with your tamper. You can't tamp too much. You may even want to rent or borrow an electric powered vibrating compacting machine to save you work.
To help you estimate a level floor surface, use a long level, string level, or water level. If you use a long level, sit it on top of a straight 2x4, like you used for getting the slope for the drainage. (See the illustration on page 31.)
Attach a 3/4 inch clear plastic hose to a container, a bucket, or an old kettle. Put a clamp or a tap on the hose so you can open and close the flow from the container to the hose. Set the container in a secure spot where it's above the highest point on the floor area. Fill the container with colored water. Hold the free end of the hose above the container and open the clamp so the colored water can flow into the hose. The water in the hose will always seek the same level as in the container, so you can establish even floor heights by measuring down from the water level in the hose all around the floor area. There are other types of water levels. Any sort will do the job.
Knowing the floor height will help you measure the heights of the counters, seats, windows, etc. as you build. Mark the future floor height on stakes pounded into the ground inside the floor area. If you plan more than one floor height, mark each one with stakes. Put the stakes somewhere out of the way because they can be perfect toe stubbers! Once the foundation is made, the
floor height can be marked along the inside of the foundation with a chalk line or pencil. Give yourself plenty of space on either side of a door before raising or lowering the floor level.