When you write a novel, there is bound to be lots of extra material that you have edited out. Would you use this in a next novel?

In my experience, which isn't extensive, I'd say no. Most of what I write and discard ends up in the bin. The next novel has a different story, theme, message and I don't find it worth hanging onto the unused material.

That said, my first novel was cracking along at 100k words and only half done. I realised I actually had 2 books on the go. I split around 40% off for the second book and used around 70% of that. I think that's an outlier and don't expect that situation again now that I've learned the lesson.

A final thought. I'm not attached to hanging on to writing just because it's written. If it doesn't work for what I'm after then it has done its job in making that obvious to me.

Hope that helps!