a good rule of thumb is $100/square foot.

Another good rule of thumb is take your best estimate, double it, add 20% and that puts you in the ballpark.

Renovations can be tricky because you really don't know what's behind that wall or under that floor.

Anyway, plumbing and electrical (plumbing esp.) are where all the expenses lie, try to avoid touching those two areas and the costs can go down dramatically. Adding additional square footage can many times overtax the A/C/heat unit so you end up redoing that also (big bucks, not foreseen)

You'll probably need an engineer too to sign off on some of the foundation and extension beams work.