I disagree.
my main objective is to maximise my space for family/entertainment purposes. how I do this, depends a lot on how much I'm allowed to build (not that I'm likely to use all the space I could legally use). if I can extend my extension 3.4m, which is the current "plan" (vision, desire, thought), then I'll do that. if I can extend 6m, I'll still extend 3.4m. if I can only extend 2m, I'll extend 2m and reasses my priorities. for example, I would really love a utility room, but not to the detriment of my entertainment space. if I can fit it in, fine, if not, fine.
all I want to understand is what options I have with regards to maintaining the "flow" of the property by not having staircased walls, weird L shape layout and other unnecessary complications. I don't think I'm doing things the other way round, I just need to know what I can realistically build in my circumstances without causing problems now and in the future (sell, remortgage etc.). once I know how much space I have, I'll be able to work out exactly what needs to happen.