Property Conversion Cost Calculator

I know there's no simple way to answer this, but I'm working on a deal where we need to convert a double story office building into a single story warehouse. We're trying to figure out a way to calculate very basic, rough costs for this project.

Anyone know a good way to get rough estimates (on a price per square foot basis if possible)? Or if anyone has any reccomendations on how they underwrite deals that need significant capex, would love to hear it. Trying to learn this side of the business as much as possible.

Thanks.