We did an addition over the garage this year. The actual addition part is very fast -- maybe 3 weeks? But before that is drawings, permits, any foundation work that may be needed. And after that is the tie in to the original house, which has been a PITA. Plan on 4 months of active work, plus drawings/permits which could maybe be 6 weeks if you're extremely motivated, plus however long it takes for a builder to slot you into his schedule. That assumes you're willing to choose supplies and finishes that are available now, rather than waiting for something specific.

I'm not made of money, so it took us 6 months of active construction, and about 16 months from the first meeting with an architect.

Fast and done well. How fast can it be done and how much would you overpay?

Fast, inexpensive and done well. Pick two. Or one.

Would you recommend going with a Design/build? What's the shortest timeline that you can imagine starting from conceptual conversations and surveys to a finished addition?