I am speaking about an improvement I noticed in UFO Extender : the possibility to build new facilities, without having to wait for previous one to be built.
I mean, in original game, it is not possible to build a facility if not connected to an existing one. In Ufo Extender, it is possible to put first a facility on a connected square (also with immediate effect of "building in progress"), and put a second facility next to it, which will be pending until first is built.
It is clearly not a really major improvment, but I find it user friendly, and useful in terms of budget management (In a custom version where money is scarce, this would be a way to reserv money for building purposes).