Just in Annapolis , the difference between Clay street and Wardour would be hundreds

In Chicago we have clay sewer lines and commonly there are tree roots that cause problems.  

In michigan we have a ton of clay, check with health department and see if you can do a cut system. 

It is all but confirmed that Amazon is building a massive distribution center in Upstate NY / Liverpool / Clay, NY / area. 

Old clay tile lines can break up and partially collapse which provides the same outcome as roots.

When conditions are heavy clay, we frequently tie drains for each well into an interior drain-tile system (though this is when we're finishing the entire basement). 

Thanks for the advice @Michael Clay I will make sure to keep that in mind when evaluating properties

@Caleb Clay.

But we have sandy soil , clay starts at about 12 feet .  

I excavate the whole site down further with a backhoe, (clay, water and all) export all that out of there, bring the whole pad down another six inches,  have the foundation guy do his form work, and then backfill the whole foundation area with six inches of self compacting 3/4 inch stone.