I agree with xboxboy. Hire a soils engineer. Our house is a 1960’s house… our lot slopes… but the did a cut/fill of the cobble/clay when they original developed the area in the early 60’s. They did a good job. It’s all native dirt – just cut from the high spots, and used for fill in the lower spots. They did good compaction.

When we built our casita we had to do proper compaction with a geotechnical company supervising. We were happy to have another set of eyes making sure the work was being done correctly.