I purchased a house and built a detached ADU. I have structured the original house and the ADU as condo #1 and condo #2. I will be selling just the ADU condo, but I'm not sure how I should report the cost basis for the ADU sale?

[Critter]

Sounds like you bought a piece of land that had a home on it.  Then you built a second home on the same piece of land.  Now you are selling that built home.   So the basis of the built home is the cost to build it  and if you are selling off part of the land you need to determine the cost of that % of land sold. 

For instance ... if the property was one lot and the portion being sold is 1/2 of the lot then 1/2 the land value at purchase would be allocated to the house being sold.  So when you purchased the property there was a value for the land and for the home on it... you will need to do a bit of math. 

[DoninGA]

Depending on how the ADU was permitted, zoned, taxed, it would be recommended that you speak with a tax professional who is familiar with the tax reporting on the sale of an Accessory Dwelling Unit.