Rehab Costs, where do I start?

[Daniel Pitner]

I know this topic is widely discussed, but I've finally found myself in a position where I need to know how to estimate rehab costs.

We are under contract for a home that will become our personal property.  It also has a detached garage that has plumbing and electricity that would turn into a casita to rent out. The problem is, I'm stuck on rehab costs. I have no idea where to start. The house is 1500 square feet and will need new flooring,popcorn ceiling removed, new kitchen (and the kitchen needs to be moved), and a few walls blown out. The casita would need to be completely built out (frame a bathroom, add a kitchenette, etc) 

Where do I even start?!