(Example: Pink wall tile all around the bathroom and a certain floor pattern = original bathroom and lead pipes underneath have not been replaced).

That was until I pulled up a corner of the carpet to discover hardwood floors underneath.

Usually this is a General Contractor with a specialty in foundations.I myself have rebuilt a raised foundation from underneath a house.

That said, I have broken that rule a few times in my career, but always with what I believe is a foolproof strategy in place to protect my interest and position in the deal.For starters, record a memorandum of option with the county so the property can't be sold from underneath you.My agreement with the homeowner allows me to make the upgrades and they are stated in advance. 

Then I backfill with gravel to help with drainage top with soils and plant seed.   

Assuming there are no other issues like soil, etc that would prevent this type of addition, does anyone have any experience in this, and any ballpark estimates for cost?

If you establish your LLC in Minnesota and put the Colorado property underneath it then your Minnesota LLC will be considered a foreign entity in Colorado. 

You might be sitting on land for a year waiting for permits, soil reports, inspections, etc. etc. before being cleared to build.

I have read much about Radon, and it really doesn't end on just the soil content, as it could also be in the water..

We have zero weather, great soil and everything is “newer”.