I have done this type of project 3 different ways (what can I say, I am a glutton for punishment):
1) drew my own plans, pulled my own permits and did almost all the work myself (w/ husband)
2) hired a big architectural firm and hired a general contractor (LEAST FAVORITE RESULT)
3) hired a small architectural firm, acted as my own general contractor, hired really good subcontractors to do most of the work, did some work myself (with husband) (THIS WAS MY FAVORITE OPTION)
Are you working with an architect or not? That will determine a lot of my answers. Are you going to be your own general contractor or not? If you are hiring an architect, go with a small firm for a project like this as you will get better quality and better personal service. The architect can give you some good recommendations for builders to interview.
If you are hiring a general contractor, then that person will be responsible for contacting the city for approval of plans and to get permits for all the work. If not hiring a general contractor then you will be responsible for some of that. I have done it both ways. Being your own general is hard work but very rewarding. But I don't recommend doing it if you have a full time job.
Start by making a list of what you MUST have in the project. Then make a list of WOULD BE NICE to have. Think about whether you want to emphasize energy efficiency, matching the existing house style or changing the style (and if so what style do you want), and any other specialty concerns. Then look for a small architectural firm that specializes in remodels and any special needs you have. For example, for our latest project we chose an architect that specialized in super energy efficient building. Interview a couple firms. They can then do a preliminary reality check on general price estimates for you, but the final estimates will be when you take bids on the job from general contractors.
I used Angie's List, Service Magic, and my architect's referrals as a good starting point to find good contractors.
Try to have as many details worked out as possible before taking the bids, including plumbing fixtures, lighting, landscaping, siding, roofing, etc.
Even if you are hiring a general contractor, learn as much as you can about the steps they will undertake. I have seen a lot of decent contractors still cut a lot of corners especially in areas where they don't think anyone will look (insulation, window installs, etc.)
The project will go over budget, but that doesn't mean you have to let it get out of control. The general rule is to plan for 10% over budget, but it can go higher if you make changes or additions later in the process. Make sure you get an itemized budget from any contractor including the bids from all the subcontractors. Read them carefully and/or have your architect look over them to make sure they have including everything that the architect specified for the job. (The spec document from the architect was my best friend).
PM if you want more advice.