Hi - My experience is that there are a lot of low-end residential design build firms that are mainly general contractors who employ a designer / cad person to draw up plans for the owner's review. Unfortunately most of these don't employ a licensed architect and may not have the design sensitivity required for a specialized mid century home.
There is another set of high-end design build contractors that can provide a lot of value to a high end / expensive renovation or addition. Unfortunately they come with a high price tag and depending on the budget for your project, may not be a good match either. How large is the addition you are planning?
I think a better option is to find a local licensed architect with a lot of residential experience. For an architect's fee, a general rule of thumb is 10% of the construction cost of the project, not the whole project cost. I am a licensed architect, and unfortunately it seems a lot of residential clients get sticker shock from our fees. BUT, there are so many architects without work right now, that you may be able to get an excellent fee due to the economic slowdown. You also may be able to limit the scope of work of the architect to help you save money.
Check the Pasadena & Foothill branch of the AIA and they can help you get in touch with a firm that is right for your project:
To be honest, many home owners don't feel like they get enough value from even a small fee (for a minor addition / renovation) to justify the expense. Unfortunately that is partly our fault as architects not being able to convince the public of the value of our professional services.
That said, right now is an excellent time to bid a project. Contractors are in general very hungry and prices should be good, at least for another few months as the economic recovery hopefully starts to speed up. Getting the help of a licensed architect that you are comfortable with is a great 1st step
Hope this helps.