Unlocking Expansions gives you more farmland to build and gather resources. Each expansion needs a certain level to be unlocked. Tap on an expansion that is surrounded by a white dotted border to view the level requirement. Once you reach the required level to unlock a particular expansion, a lock icon will appear over it and it will start to glow. 
You’ll see the number of coins you will need to spend to unlock the expansion.
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FarmVille 3 – Animals Game GUIDE