Home reno expenses, where do we start?

We have plans to build an addition on our home (3rd bedroom for 2nd child). We're a little confused as to where to start. Do we approach our mortgage company? Or do we talk to a bank to get a loan? I hear about "refinancing" mortgages, is this likely what we'd need to do to cover the cost of the renos? Do we need quotes first before approaching _anyone_? We're really horrible at this. My husband thinks it's better to talk to our original mortgage broker, but we haven't dealt with her in almost 3 years, and I'm sure she's only involved with obtaining original mortgages for clients. What do we do? Where do we start?