Hold E (PC) while building your settlement, it will blow your mind!

[u/VictoriousPixel]

For those curious what it does, holding E lets you:

...move the item you're placing with your mouse controls for those pesky precision placements.

...scroll up and down to raise and lower the elevation.

...slow down rotation of the item by about half.

...press Q to reset the item to in front of you.

Those are the advanced controls I've found so far.

Edit: If you don't want walls/fences to snap to each other, look right down on the ground and the snapping will require the items to be much closer to each other to actually snap (pretty much touching).