Doing a DIY bathroom at the moment, luckily being an analyst I have a pedantic urge to plan projects and include contingencies etc. Not to mention I took a decent amount of time off work rather than just try to do it over a bank holiday weekend! Even so, it is usable but by no means complete. Need to remove the sink, refit the waste (it is leaking from the bottom, around where the sink's built-in waste meets it), plaster the new ceiling, fit the new lights, box in pipes, fit the new heated towel rail, do 20 sq metres of tiling, fit bath panel, fit the shower mixer to the outlets, get a bath screen, install the floor... etc. etc. It goes on!
But yes, there is a lot of work to do even if you are putting the new components exactly where the old ones went. However I have discovered the wonder that is flexible pipes.
My bath cradle was designed such that it was just not possible to fit 4 water pipes (2 for bath, 2 pumped for the shower) AND a 40mm waste pipe (sink and washing machine) between the cradle and the wall, short of running the waste up and down all over the place (which kinda defeats the object of a gravity drainage system!)
I ended up using a 30cm flexible waste pipe to scoot around between the cradle and the bath. So, my waste pipes are by the wall and the bath still fits.