How do I bathroom / kitchen / extension? etc.

Also worth considering how long you're likely to stay in your current home.
If you take 2 years to finish the work yourself in odd evenings and weekends, and move shortly after, you wouldn't have actually been able to use the space that you've created and have been living on effectively a building site for 2 years.

If you're getting the trades to do everything up to where the scaffolding can come down, and windows/veluxes installed, stairs etc, then you should be able to DIY most of the rest (plumbing, heating, flooring, decorating)

I had planned to decorate our extension myself, but managed to find a decorator for about £1000 to do the woodwork, doors, skirting boards, walls.
With lack of practice, and large rollers, dust sheets, high platform, and all the annoying prep around kitchen units, I estimated around 60 hours of my time.

That's probably about 3 weeks of every spare hour in evenings between kids and other commitments.
My wife and I thought that it was an acceptable spend to avoid having the room pretty much unusable for 3 weeks.