A foundation-plan and footingsHere's a foundation-plan example for a small home plan. The floor needs a girder in the middle to help with the span of the floor joists. The girder can be put on blocks or you can just pour a girder with cement. If you put a cement girder up the middle it will be much more expensive, but it will be stronger. I did it that way. It's really overkill though. 
You can see here that I poured a divider instead of building a wood girder up the middle of the foundation. It's not very cost effective to do it this way.I wanted a strong foundation to rest the floor joists on so I opted to do it this way. Besides the foundation on a smaller home is quite cheap any way because of the small dimensions.
Floor joists are attached to the girder crosswise to make the floor more secure. You will need to get the joist span requirements from your local building inspector before planning your floor plan. Joist span requirements differ greatly because of different types of wood, different grades of wood and different environmental factors. Let me just throw out an example of a joist span to give a little clarification. 
The chart above shows the maximum spans for Northern douglas fir. According to the grade of the lumber, the span maximum will be different. For example a 2X10 of the highest quality will be able to have a span of 19' 1" if the joists are on 16-inch centers. If the joists are on 24-inch centers, the span will only be able to be 16' 8".
This illustration shows how the foundation footings are made. The cement forms can be made from boards or plywood. The rebar is then placed into the forms before the cement is poured. The footing and foundation are usually two separate pours so that the footing can dry completely. If done correctly, you can avoid future foundation repair. Be sure to get the required frost depth from the building inspector. The footer has to be below the frost depth line to prevent frost heaves from getting under the foundation and pushing it up. Inspectors can get you the necessary info for foundation-plan requirements.
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