I'm not so sure a storage business would work so well. You rent out a 3x3 meter cubicle with a metal shutter and charge a couple thousand a month? After that you'll also need a round the clock security guard or workers because if you have a dozen cubicles storing stuff suspected as valuable like a person's house furnishings and such that would make it a prime target robbing 10 houses for the price of one. Value isn't really there and multi level cubicles don't go over well.

However the bigger issue I see here is what to do when someone doesn't pay? If they are 1 month, 3 months, 6 months late you'd think you could open it up and sell off whatever is inside to recover your loss but if the person shows up to try and reclaim w/o paying or taking you to court because their stuff came up gone after months of not paying I don't see the courts siding with the business owner even if their is a written agreement / contract.

In the end the space you would consume to do a storage facility could be rented out for the same price or more where you have no liability and no need to hire and pay security guards. Here in the Philippines its cheap and easy to find some crappy 3-5k hole in the wall apartment that you could reinforce the door and any window (which there usually isn't) and have far more space and better security as its not known what is stored there if anything. However in western countries there lacks other options other then storing with a friend/family and space for such a business is cheap.