Originally posted by Meewec:

you have to aim at the edge you want to connect to. there's also a build limit from the ground, you can see it displayed in the color of the parts you're putting down. blue is touching the ground and then green through red is how far it is. anything red won't be able to have more parts stuck on it unless they're not getting further from the ground in part count.

The limit is specifically for "Weight", if you build wood beams that touch the ground near the squares that are turning red it'll let you build further/higher, just to add to that.