Playing a FFA Aliance game, we found that you can not build next to your allied units. Such as a Scion building a jammer by the ally's Gun Tower.
I can understand not building by Enemy stuff, but Allies?
well, youactually CAN build, the ground just appears red. The wireframe is green though, and you can build there. There is a bug with building next to enemies though, if you build something 2 squares away from enemies, then you can build next to them but adjacent to the building you made.