Hi, we bought in December (not moved yet, have rental to pay for till March so figured we'd do the building work first!) and are planning a rear extension along the whole width of the house.
So far, we've found an architect (contacted three, and took recommendation from one), found a builder who is going to give us a quote now that we know more or less what we want built, and we've asked a specialist surveyor to give us his advice re party wall issues (foundations will be within a metre of boundary wall on both sides).
Its been quite confusing knowing where to start, but tbh I think it hasn't mattered that much - we just had to start somewhere, iyswim, and it gets clearer as things move along.
We're meeting with the architect, and my db who is an interior designer and a great project managed (for support/advice) this week and will submit plans the week after.
I think you have to have plans to submit before you can go for planning consent - not sure, because we're doing it under permitted development, but I think so.
And the builder has had a look round, but can't actually quote until he knows pretty much exactly what it is he's building.
Oh, one other important person: the structural engineer. Architects usually have one they work with, and their input is needed for the foundation/support side of things - no use drawing up plans if they're structually unfeasible.
I'm a total amateur beginner, be glad to share the process with all others who are doing this!