Have you ever dealt with a Party Wall Agreement?

[Steve S.]

I'm acquiring a townhome that has under the same roof, a second town home adjoined by a wall that is owned by someone else. 

Halfway through closing, I was informed I have to sign a Party Wall Agreement and the other owner must sign as well.  

I've never heard of this.  I own other properties in a similar configuration.  Anyone had this experience?

Any gotchas I should consider?  Do I have to sign it?  Supposedly under Fannie Mae, it's required per my lender.