The first Advanced Food and Iron Storage and Advanced House become available once your Stronghold has been upgraded to level 21. Once the Stronghold has been upgraded, you can start converting your fully leveled existing Iron Storage, Food Storage and Houses to Advanced versions of themselves. As more Stronghold upgrades become available, it will be possible to convert more buildings to Advanced Buildings.
You can store more Food or Iron in an Advanced storage building. The storage capacity adds to the maximum capacity of the Base.
The Advanced House is used for housing and training Recruits. In addition to storing more Recruits than the regular House, the Advanced House also automatically produces new Recruits.