Not to add to the unwarranted negativity here, because I do think it's a great game overall, but the settlement construction really is f'ing horrible.
Nothing snaps together easily, nor does anything snap together the way you would intuitively expect it to. Half the time I have to deconstruct 75% what I've built just to fit a single piece into place. Currently I'm wrangling with the fact that I can't put a wood wall corner down if there's a 1x1 wood floor kitty-corner from it, which makes NO sense -- and that's only the most recent issue I've had.
There's no ability to lay out interior walls and have them snap into place. You're seemingly forced to use the prefab rooms and corridors, or else go to ridiculous lengths to "game" the system by laying out the interior spaces at the same time you're laying down the floor.
Apparently there's TWO of every corner, because Bethesda had the bright idea to make certain walls and set-pieces geometrically non-superimposable -- the upshot of which is if you don't use the CORRECT corner, the geometry on the corner pieces doesn't line up correctly with the neighboring walls. A well-designed system would be smart enough to figure out which corner geometry is appropriate and select it automatically, but this arduous finiggling has seemingly been pushed onto the player. Awful programming.
Certain models don't line up no matter what you do. For example, try putting down a wooden floor. Then, right next to it, put down a Structures>Wood>Floors shack stairwell. Finally put down a Structures>Wood>Floors shack foundation off the second story landing, directly above the original floor. Come to find the poles of your lovely veranda are now hovering a foot off the wooden platform. What the hell?
On my occasions, as I've laid out a building, I get the darn thing 80% built only to realize the foundation isn't high enough and as a result, a bunch of animated weeds are sticking up through the ground floor. Maybe it would have been a good idea to NOT HAVE SIX FOOT TALL ANIMATED WEEDS in settlement locations? Or maybe the game could have been made in such a way so as not to render weeds that are indoors? Or maybe those weeds could AT LEAST have been disable-able by console commands? No such luck here, becuase they're "non AV objects," whatever the heck that means. What's worse, there's no way to just lift the construction and all of its attached parts wholesale, which means you have to manually elevate the entire structure piece by piece. Ridiculously time-consuming and not fun at all.
In fact, it seems like NONE of the existing buildings can be disabled through the console in this game, so tough luck if you don't like that eyesore of a shack Bethesda was nice enough to plop down on your settlement tract for you.
If you're constructing a tall vertical building, you're forced to lay down a wall, then a floor, then another wall, then another floor, etc. Which would be fine, except that for some reason the floors are visible from outside the structure. So no matter what you do, the side of your building ends up looking like layers of a damn cake with thin licks of frosting in-between them.
There's a depressing lack of variety in the setpieces they give us. At least they gave us two building styles, wood and metal. But seriously... two? There are so many more options they could have put into the game. What about Vault-style set pieces? Or what about Cabot House-style setpieces -- You know, something that actually looks decent? Something that doesn't look like it's out of a refugee village? Places like that exist in the Fallout universe. Why can't we build something like that if we want to?
Concrete and wooden foundations don't snap together vertically. In a word... WHY? I mean, six feet of the foundation is ALREADY being put toward raising the ground floor above the swaying weeds -- and that means if I'm building along a marginally-steep hillside, this block of concrete is bound to fall short of what I need to interface with the ground. The end result is, the concrete foundation ends up floating above the ground like a... well, like a badly-designed video game setpiece. Which wouldn't be so bad if you could just STACK THEM ON TOP OF EACH OTHER -- but no, that's too much to ask from this awful, awful settlement system.
Those are all the complaints I've managed to come up with based on my ~10 hours trying to wrap my head around this system. I don't doubt that it's possible to build some amazing stuff with this system, but holy hell, the user experience with this is just mind blowingly bad, to the point where I can't believe this even made it out of QA. This would have been so freaking amazing if Bethesda had only made it work.
As I've played, I've been continually reminded of Todd Howard at E3 when he described the system. The mantra he repeated over and over again: "It just works."
What a load of baloney.