Building additional storage in unfinished basement

[Jon Coleman]

I am in the process of purchasing a 4 unit property that currently has secure basement storage for tenants.  The storage spaces are no more than 5x5 but the basement itself is fairly spacious.  This has resulted in some tenants storing their items outside of their lockers and cluttering up the general area of the basement.  I can imagine the amount of items will only continue to grow unless something is done.  Their current rental agreements stipulate storage is only allowed in their designated storage units but that obviously hasn't been enforced.  My initial inclination was to just give notice that anything in the general area as of a certain day would be tossed/donated but I'm now thinking there might be a better way to address the issue.

I'm considering building additional and/or larger storage spaces and giving the tenants the opportunity to rent them for a monthly fee.  Given that they don't need to be pretty and can seemingly be built out of 2x4s I wouldn't expect the cost to be too bad but want to get a ballpark and use that to help determine if it is worth pursuing.  There is a Public Storage location near the property that charges $26/month for a 5x5 unit and $33/month for 5x10 so I at least have an idea of the maximum I'd consider charging if I built similarly sized secure storage spaces.  Building a couple larger storage spaces and renting them for just $20/month would generate nearly $500/year and so depending on cost to build and tenant usage they could seemingly pay for themselves in a couple years.

Have you ever built additional storage for your tenants?  If so, how much did you spend and how much did you charge?  Thanks!