I am building with a very open floor plan. The kitchen opens to the esat in area that opens to a great room. How do I define where I can use 14/2 and where I must use 12/2?

Thanks

12-2 must be used for the kitchens small appliance circuits (Two are required).
It states: A minimum of two 20A rated branch circuits shall be provided to serve all wall and floor receptacle outlets located in the kitchen, pantry, breakfast area, dining area or similar area of a dwelling. The Kitchen contertop receptacles shall be served by a minimum of 2 20A rated branch circuits, either or both of which shall also be permitted to supply other receptacle outlets in the kitchen, pantry, breakfast and dining area including receptacle outlets for refrigeration appliances.

Note: The refrigeration can be on it's own dedicated 15A (or greater) circuit. If you use a dedicated 15A circuit to serve the refrig, then a 14-2 with ground can be used.

Based on your house plan, this will tell you where you would have to use 12-2 with ground.

If it is that open a plan, and subject to interpretation, be sure to decide what you would think is reasonable and customary, document that imaginary boundary on your plans that are file with the town.

The design is up to the owner, and to describe it for the inspectors, who will then apply the code as described by Washington. If you leave it up to them, they may not see the same logic as your design.