We're doing something similar (building up to add a master and renovating some existing spaces.) I started with design-build firms, which was useful. I met with several; they came out, looked at the space, discussed needs/options, came back with preliminary estimates and explained their particular business approaches.

This gave me some sense of what was feasible for us and I did like one of the firms, but ultimately I didn't feel confident in making a commitment for the project from soup-to-nuts. (My husband is very hands-off except for the $$$, so all the decisions are on me!) The process with the firms that I spoke to seemed to be give them an initial tranche of money based on a relatively quick and broad estimate of the costs, then they would design, then you'd get the final estimate for them to do the work. Obviously you have some capacity to shape that outcome, but I heard stories of people who began the full design process only to get a final figure that was vastly different than the preliminary estimate.

Instead, we opted to work with an architect who designed a neighbor's major addition. We're just now starting to shop the plan for bids. I'm told it's usually more cost-effective with an architect+builder as compared to design-build, but there are no guarantees. We'll at least have the advantage of talking to multiple contractors with a specific, defined plan scoped out, and we can compare apples-to-apples in making a decision about which firm to hire.

I don't think there's any single answer to this question BTW. I have friends who were very happy with design-build firms.

At the risk of stating the obvious, remodeling is very different from new construction.

If you have architects and design-build firms to choose from in your area, speak to both and make sure to select a firm with plenty of remodeling design experience. If you go the architect route, I encourage you to select your remodeler early on and have them participate in the design phase--it's the best way to avoid unpleasant "surprises" later in the project.

Talk to your real estate agent and see if doubling your dollar investment in your house has a hope of paying off in 10 years time. Talk to a bank about what it’s going to take to buy your house all over again and how creative refinancing might need to happen. Talk to your spouse to see if you really want that year long stress and giant expense when you could just go buy s different house.