Calculation Cost Basis for garage conversion to ADU

I have been reading different threads and trying to understand how to correctly determine the cost basis for my garage conversion to an ADU and enter my information in to Turbo tax.

The information has been helpful but not specific enough for my situation. We converted the garage to an to a 1 bedroom, bath kitchen ADU, my Mom is living there and paying rent to help with loan to build.

I purchased our house for 216,000 in 1997 - now valued at around 850,000. We paid $91,000 in construction costs and furnished the apartment. The Garage is 17% of the new square footage of both the house and new ADU. I'm not sure how to put this all together to get the correct cost basis. Any help would be great.