Personally, once "the goals, the why, the impact, the external limitations" have been defined, we start to do mockups, a programmers "differential", to zero in

Personally, once "the goals, the why, the impact, the external limitations" have been defined, we start to do mockups, a programmers "differential", to zero in on a solution that satisfies the demand. With that on the board/paper/code, we can start to test our assumptions and iterate on the solution. I do not know if that classifies as planning, but it works very well.