Shed rental for additional income, or included with apartment?

[Larissa Phillips]

We have a triplex with three small units (~500, 500, and 800sq ft), with a shared fenced-in yard and three sheds, one across from each unit. My husband and I use one shed to store extra supplies, the shed across from the bigger unit was purchased by our tenant before we bought the property, and one small unit (and shed) are recently vacant. 

Should we offer the shed as an optional add-on for additional income, or increase what we're asking for monthly rent and include it in the price? For either option, how much would you charge? It's an 8x10' shed, purchased last year. 

Not too far from our property is a series of small apartments/cabins similar to ours, with no additional storage option for renters. They have higher turnover and charge more for rent, but have no problem finding tenants. 

Our renters in the smaller units stay an average of 3 years, with or without a shed.