I was watching a let's play of the new update (swimmingbird391) when he said that you need to at least have the second floor to unlock the secret storeroom, and from what I've been seeing from others online, that seems about right. If my memory serves me right, it costs 148,000 Bells and you get to use the touch pad to move furniture after you unlock it, so it's definitely a good buy for 360 more storage spaces!