Where to begin with a major home remodel/addition?

We are about 75% through an addition/remodel of our home. Here's what I'd suggest based on our experience so far -- start with a local contractor or contractor/architect duo who know the local code and requirements. Think about your space plan for the addition, whether major upgrades to the existing foundation, structural, electrical, HVAC, plumbing would be needed. Ask them for a rough estimate of what the whole project would cost - expect a number around $500 per sf. At this point, you decide whether you want to proceed... I'm assuming you'll also evaluate whether it's easier to simply sell and move. 

If you decide to go forward, you have to start drawing up the detailed plans for permitting. Secure the finances, and a loan/refi if that's required. Plan for how you'll live through the construction (stay or move out). Once construction begins, you should expect to have to be very involved (it's your home after all). You'll need a work schedule that's flexible enough to allow meetings with various contractors/tradesmen between 9-5. Expect your weekend to be completely taken over by design/purchase decisions. Expect at least one or two major 'surprises' that will require additional work/money to resolve. Expect to be at least 10% over your initial budget. Try to plan for relief (small vacations, help with childcare from family, etc) to keep the stress/anticipation from taking too much of a toll. 

I'm sure you'll find many many specific recommendations from others on this forum. Whoever you end up selecting, make sure they have local knowledge, and be sure you can trust them with your home (even if that costs 5% more). Good luck!!