The physical first stage of any remodel is demolition. This simple process can be full of surprises, as in the following case. I am remodeling the first floor of a very nice home. I was surprised to discover significant problems with the HVAC ducts below the floor. It is clear that the installed didn’t care about his work. See photos below:

Here you can see that the plumber stepped on the duct to make room for his pipe resulting rendering the duct useless.
Here you can see that this duct goes nowhere. It is dumping air under the floor below a shower base.