How to estimate rehab costs prior to purchase or offer

[Christopher J Woodland]

So, I am looking at doing my first rehab. Are there any tips for estimating how much the rehab will cost? Would it be frowned upon by the seller to get some contractors out to take a look at it and provide estimates before purchasing it? If this would be acceptable, what would be the most proper way to handle this?

I like the contractor estimate idea because I know when I was a commercial banker we'd commonly ask if people had estimates for the renovations or improvements that they wanted to do on a particular project or if they were eyeballing it. So, I'd feel even more prepared to take my packet into the bank if I had that estimate because people very commonly tend to underestimate the cost of the renovation. But, I am not sure how to approach that with a seller prior to either offering or closing on the property. 

Thanks in advance,

Christopher Woodland