You can unlock more space for your Pockie by unlocking rooms. Tap the house diagram in the bottom right corner of the game to see all of your rooms. A new room can be unlocked by tapping the blue "+" icon. Then, a menu will appear showing you what you need to accomplish before you can unlock a new room.
House value is increased by adding more decorations to your current room(s) and adding more outfits and accessories to your closet.
Master Keys are obtained from Surprise Gifts on the floor of your Pockie's house.
Coins can be earned in various ways. See this article for more help.
Once you unlock, the Master Keys and Coins will be deducted from your totals. However, your House Value will not change.
You can also unlock new rooms using Pockie Gems. To find out more about collecting Pockie Gems, click here.