1. Because of the fact that the space to construct your facilities is quite limited, you should plan its development in advance and divide the space rationally. You should start of by checking the available rooms - some of them are more suitable for certain facilities (like the Exposed Power Coil, which is extremely useful as a base for Power Relay).
2. Expansion requires free space, which means that you have to excavate rooms to prepare them for construction. Engineers are required to perform excavations, which means that the amount of engineers you have is directly proportional to the tempo of base expansion. You should acquire them whenever you have the chance to do so.
3. If you have any doubts whether you should place a specific facility in a given room, check the "Available Facilities" and "Development order" chapters of this guide, which will definitely help you.
4. Expansion is not limited to the Avenger base alone. You can also construct Radio Relays, which will increase the monthly supply gains coming from the Council.
5. Remember that expansion comes with upkeep costs - each facility you construct requires a certain amount of Supplies each month to function, which means that each additional construction will reduce the funds coming from the Council.
6. Aside from upkeep costs, each facility requires a certain amount of power to function. It does not mean that you will suddenly run out of power and all of your facilities will stop working - it cannot happen. Not enough power will simply prevent you from constructing another facility and you will have to increase the power output of the Avenger.
7. Constructing facilities and excavating can be accelerated - engineers can be used to do that. It's definitely worth to speed them up, as it decreases the time required to construct a facility / excavate a room by 50%!
8. You can also decrease the build time by purchasing a specific upgrade from the Resistance HQ. Thanks to that, you will be able to perform a scan-like action at the mentioned HQ which will decrease the build time by 50%.