How do I bathroom / kitchen / extension? etc.

Fees? What is fees?
I did the drawings myself, no party wall involved, I kept a little bit of wall on the neighbours side and used that as the side of one of the built-ins. The only professional was the structural engineer to do the maths for the steels, they way over specced them though. We also kept costs down by getting the builders to salvage the roof tiles from one of their previous jobs. The floor was odd boxes/ends from I think 6 different places. On the kitchen cupboards we used plinths instead of end panels where they wouldn’t be seen. We used one deep long panel cut below the counter top on the tall cupboard and used the left over for the fridge end. We reused the windows and doors. Got a ‘broken’ range off marketplace and had it refurbished, just needed a fan and a service. If there was a way to trim the cost we did it.
I think it turned out pretty well.