First, check to make sure you can get permits to expand. some areas have zoning restrictions on height and number of bedrooms, etc.

Then sit down and brainstorm traffic flow, overall look and feel ... concentrate on the functionality you want and why it's important.

Architect next ... to make the plans and get the details completed.

then you take the plans to several builders (general contractors) to get prices.

After you recover from the pricing shock (moving to a larger house may be cheaper than expanding the current one)

the general contractor usually takes care of hiring all the subcontractors.

Figure out your finances first. That's the usual limitation to the plans is the money that can actually be spent. It's the ultra rare project that plans for everything they want and can actually do all of that with cash. Spending the whole price of the home all over again just to renovate it doesn't usually happen unless the home was acquired as a real bargain like a foreclosure. It's just not economically feasible for most.

Also plan for the temporary housing to move to while all of this is being done. The home won't be habitable. And yes, it would be cheaper to buy an already renovated home with the space you already need than to do all of that to your current house.