There are always some minor additions to most every job. As a contractor, I don't mind additions to the job. What can be stupid and costly are major changes in design to the job. These are usually done by people who were too busy doing, whatever, to actually put thought into the project. Decks go from 12x14 to 14x20 to 15x32. Each time new sonotubes get dug and there's a new delivery of decking material(with the associated delivery charge).
Work the project over in your head and then with the spouse and then with the contractor. By the time the project starts, 95% of what's needed to be done is known. There are always little detours along the way though.
Ron

I tried to think about what bothers me about different rooms and if a solution might be part of the remodeling. For example, we have a very cold bathroom in winter that results in most of the family taking showers in the warmer one. So, first I thought about an in-floor heating mat. Then, talking with the electrician resulted in the decision to add a toe kick heater under the vanity which can be regulated by a wall thermostat. A wall heater or towel heater wouldn't have worked for our situation.

So, my learning has been to talk to both GC and the trades people about challenges and be open to their ideas. I don't expect myself or anyone person to always have all the answers.

Put electrical outlets in the walls of staircases to facilitate vacuuming. Especially if you have pets with the run of the house. Then make sure the electricians actually put them in.

The belonings I have in the basement are not junk! It is the responsibility of each tradesman, plumber, eletrician etc. to spread a tarp over my stuff in the basement. After one tradesman moves all your stuff to one side to access a duct, then an other moves it all over to get to a pipe, then your stuff looks like a pile of junk. Someone got solter on an autographed picture of Sandy Cofax, and some other things that were brand new that I didn't get a chance to hang yet.

If you can rent a self storage place and emty out the basement, you might be better off.

My favorite add-on was to have a laundry chute installed from my DDs room to the laundry room. I also asked the electrician to change out my light switches to dimmers switches, and change out my dated fixtures (inside and out). One thing that I wish that I had asked them to add is crown moulding for the bedrooms upstairs. It would have been so easy since they were installing it on the whole main level anyway. Oh, and I would have liked to upgrade my ballusters and railings, but by the time I thought about it, the delivery time was going to be well after the project was completed.
Good luck with your remodel!

One thing we both forgot about was heating the extra space in the kitchen. He moved the staircase to the basement that went behind the kitchen, to under the stairs that go up tot he second floor. That gave me more square footage in the kitchen. The one little register that was at the end of the kitchen would not have been enough.

Ended up spending an additional $3K to run radiant heat under the floor for both the kitchen and bathroom next to it. Didn't need another room that was hard to heat.