Tenant wants to put Utility Trailer in backyard?

[Jeff White]

Hi all,

My tenant has asked me about putting a utility trailer in the backyard.  My property is a single family house hack with a huge backyard where these tenants live in the upstairs part of the house and I live in the bottom part with my girlfriend.   There is a RV rock driveway that has a wood fence gate that opens up to the backyard where he could easily get in and out of. 

I don't really want any eyesores, and "junk" in the backyard. He works in sheet metal fabrication, so I know he will use it a lot, but it is just a strange request.   I guess I could charge an extra "parking fee" for leaving it there, thoughts?

Here's what it looks like: