I am hoping to purchase this home but want to make sure I can utilize this addition before I do. 

For those counting in California (Riverside County at least), we would not be responsible for any utility bills that are unpaid prior to our assuming ownership unless there was a lien placed by the utilities on the title, in which case that would have to be cleared.

Figure out your goals, how you want to utilize REI to accomplish them and utilize your strengths to select which method as there are tons of ways to make money in REI.   

When utilities (maybe just water) gets turned on, city requires an inspection.

We manage a lot of properties for investors that utilize Realtors very successfully

If they aren't at market rate,  yes raise the rent (QUICKLY before WA passes some form of rent control)  and if the current leases don't bill back for utilities,  consider putting that in place.  

In our case, we had an assessment of thousands of dollars for each condo once because utility bills weren't paid for six months and we were threatened with a building wide shutoff. 

Closing costs to start and then monthly costs of the mortgage, utilities, insurance, and advertising/tenant placement.

In the center of the park, there is a 30’x30’ of ground area that is not officially utilized.

It is basically a bedroom (has a window and closet), but it also has a second closet that serves as the utility room.