Very difficult to give you a site-specific solution with general information. With proper compaction, you could build immediately. It is just very, very difficult to achieve that level of compaction without testing and close monitoring and some experience. You have to have the right amount of clay and a very specific moisture content, and use lots of compactive energy to do so.

If your trees are within the footprint of the building, but not the foundation, that required compaction would (could) be much less. Concrete floor? You could put extra rebar in the excavated and recompacted area.

If the compaction hits smack where a pole would be located, it could be made longer to reach down to the undisturbed soil, or you could build a concrete pier, or ...

To an engineer, most all problems are tractable. The only limitations are the other guy's money.