It depends on how you look at all this.
As stated before, I'm working from the original set blueprints (for most areas of the ship). I've already partitioned the whole starship into compartments, and each compartment is responsible for it's own rigidity. There are limited umbilical connections between compartments, but those are at very specific points. Most compartments are two decks thick (specially if a compartment deck is bisected by a turbo shaft), and there is a few inches of space under the floors on all decks... but that is not where the bulk of the "plumbing" is found.
Actually, almost all of the wiring (etc.) is found in the walls (mostly along the corridors). After all, the surface of the walls of the corridors are generally more than two feet from the surface of the walls of most of the rooms. There is enough room in the walls above the height of the doors for people to crawl inside (which is why there are alcoves with ladders every so often along the corridors).
We have some idea what the interior of the walls look like (thanks to Charlie Evans
), but here is one of those aspects that I see people doing over and over again these days... you can't make assumptions as to how much room is needed for all the things Leopardmadcat
listed unless you know how much room those things need in the 23rd century.
Just because you don't think there is enough room given how you might do it today has no baring on what people will be doing hundreds of years from now. So I've adopted a black box view on such things. There is room for something, though how it works we just don't know.
As for the general structure of the ship itself... I am constantly surprised at the thicker is better
mind set of some people. Geometry can make a structure stronger than using more material... but I'm guessing people aren't being taught this stuff in schools anymore.
But as a estimate of what I'll be using for the thickness of the walls, floors and outer hull of these plans, these images of the U.S.S. George H.W. Bush
under construction are where I'll be getting some data...
Although I'll still be referring to the hull thickness examples as seen in the show more often than not.
But remember... everyone can put this together any way they want. I mean think about it, if you told people back in the 1960s that almost every home in America would have a computer (if not several) that was many times the speed of the most advanced systems in existence at that time, what would they have been thinking our homes would be like? They would most likely guess that more than half of the internal volume of our homes would be used to house this equipment.
And that is a difference of 40 years... we're talking hundreds with this stuff.
Didn't you say that your saucer has a total of eight full-height decks?
I'd point out that even in some of my earliest sketches of possible placements of elements I used scaled versions of the original set plans. This overview gives a nice example...
And here is a close up on some corridor and wall set plans being used in an early sketch to help demonstrate scale...