How Do I determine valuation ARV

[Ray Hayward]

Hello,

I am working on an off-market deal for a property that has a 3 family and single family. The single family has been grandfathered in by zoning. However, the current state is like a garage. There is no basement, the ground level is not habitual and is used as storage, and the 2nd level is a 1 bedroom. I would like to turn it into a 3 bedroom single family.

How can I pre-determine the value after rehab?

For simple numbers, let's say the property in its current state is $100K. After rehabbing the single family and adding sq ft that costs be $50K, how can I determine the ARV?

Tomorrow I plan to have a contractor come give me an estimate to rehab. Do I need an appraiser next to determine the current value? Then how do I determine the after rehab value?