How do I bathroom / kitchen / extension? etc.

Seems worth it, especially for something which is not currently an essential part of your living situation (right?).

If you have time, you can also save on materials, because understandably trades tend to work in the fastest, most efficient way possible.

Something worth considering though is the dynamics with your partner. To give the most extreme eg the adult sitting room friends I mentioned, the husband did pretty much everything on a major renovation. He is very handy. But that plus a full time job meant he did no childcare and spent effectively no time with the wife.

My OH wouldn't be happy with that. But other couples are much more chilled about time apart, or sole parenting.

The key thing is having the ability to carve out decent size chunks/blocks of time. You can't just do an hour an evening for 2yrs.

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.