How to comp an unusual property?

[Matthew Koch]

Was looking into this property here in twin falls, Idaho, for the hope that our family can move in with a FHA loan and use the underground garage to create another 3 bedroom 1 bath space within that garage. I need to get more info on the additional dwelling can or can not possibilities but, was hoping that once a down payment is scrounged up then also a construction loan was successful then it would be a 7 bedroom 4.5 bath house. The issue is the arv and what else it can be compared to for confirmation of price for a resale value after we are able to move out after 1 or 2 years. The seller would consider financing but only with a sizeable down payment. Property is 570,000. I would also think that the other space on the land would be good for another duplex to be built as well and even more valuable for the land. So. My hurdle or issue is knowing what it can be valued at after the garage is made into another whole unit fully furnished with all electrical and plumbing. Also what the remodel will cost. This place is huge inside also that could be made into other additional bedrooms and being close to the airport might be a awesome air b and b outfit or just a good sale after the additions made. Any suggestions are helpful and look forward to hearing from ya all.