This was important in the first installment of the XCOM franchise, and it's still the case with the second one - proper planning when developing your base is crucial to your success. The space to construct facilities is quite limited (but at the same time sufficient), and it Is further complicated by the fact that you must first excavate a room before you can construct anything there.
Excavating mechanic, as well as constructing new facilities is quite different than that of the first XCOM game. Now almost every action associated with it requires at least one engineer. The more engineers you assign to a given room (if it's possible - as most of them allow only a single engineer to work at), the faster they will complete their job.
When visiting a room for the first time, you will only have an option to excavate it. An engineer assigned for the job will need a set amount of days to perform it, after which the room will be ready to construct a facility in, and you will be given a small amount of resources as well. Expansion looks almost the same - you assign an engineer to a room and after a certain amount of days has passed the facility will be ready. Afterwards, it can operate on its own, but sending an engineer there will vastly increase its effectiveness (even up to 50%!).
You should remember about a couple of things:
- Excavating rooms is free - what is more, it will give you precious resources, sometimes one of the rarest ones in the game, for doing that.
- If you have the opportunity (i.e. free engineers), excavate as many rooms at the same time as possible. Base development is mostly halted by the insufficient amount of free rooms, not the resources, and the faster you excavate them, the quicker you will start constructing facilities.
- While looking at the above bullet point, sometimes it's better to just transfer all of your engineers to a single room (if there's a possibility to do so, of course), which will allow you to excavate the room faster and begin the construction of a facility.