How do I unlock more storage?

It will show you a new character that can collect materials. You have to buy the character and it will give you more storage.

You start with Apu (and Frink if you buy him) all other characters must be unlocked.
Unlocked characters in order:
Milhouse - requires you to spend 20 furniture & 20 lattes (storage increases by 10)
Quimby - requires you to spend 30 furniture & 30 lattes (storage increases by 30)
Ralph - requres you to spend 40 furniture & 40 lattes AND you must reach $5M in real estate value (storage increases by 50)
Willie - requires 50 furniture & 10 yoga mats (need to unlock LA Body Works for this) AND you must reach $8.8M in real estate

Apu, Milhouse, Quimby, Willie, Ralph, & Frink can make devices. Unlocking the non-premium characters increases your storage. To unlock the characters, you must spend a certain amount of crafted materials listed under that character.

It looks like Frink, but he costs donuts. I didn’t think the game used quests that required donuts to complete.

"beastlymm;c-1753534" wrote:
It looks like Frink, but he costs donuts. I didn’t think the game used quests that required donuts to complete.

Well they're not required, but very helpful. There are two Premium Character per Facility who earn twice the amount of the normal ones (1 with 8hr task). But they don't affect the maximum size of your storage. Max is 300 each as far as I know.

You need to increase your real estate value in the Heights. It's been a while but I think if you tap one one of the locked characters in the list it will tell you how you need to unlock it.

You do not need to buy any premium character for Springfield Heights. Unlock costs in this table:
http://i.imgur.com/Necn7kXl.jpg
http://i.imgur.com/o9qQ5fml.jpg
http://i.imgur.com/4DZBvLIl.jpg

There will be additional characters and storage to unlock when you start Springfield Heights part 2.

To put it simply, ya got to spend money to get money. The more buildings you make and upgrade the more characters you will be able to use to make more stuff to buy and upgrade to allow you to make more items to use to buy and upgrade...
It's a almost never ending circle. And to reach the max of 2 billion, you will have to spend some donuts, which is why I'm still at 1.5.