faniej,
Start with a good geotechnical investigation on the site to define what the soil conditions are. The soil and site characteristics will play a major role in what an economical and serviceable mat foundation will be. The soil and site conditions will be site specific, so there are no good general guidelines for an economical foundation without first looking at the soils. Be sure to take some of the borings deep enough since the raft foundations are large and can influence soils at deep levels.
Mark