Garage space rental

[Kevin K.]

I've looked through some of the topics on BP regarding garage rentals.  Most deal with renting them out to 3rd parties not related to housing.  I'm looking to purchase my first duplex now which comes with a detached 2 car garage.  Is it commonplace to offer tenants $50/month to rent a space in the garage? If so with two units how can you be certain another tenant isn't occupying the garage and splitting the rent?

How much should I bank on an extra $100 a month for the 2 spaces, with no driveway, and limited street parking?  Is it something you guys factor into your number when calculating income generation?