Remember, you are trying to prevent having a lawsuit because on of the two of you gets really pissed off.I hope that helps.  

The recent foundation inspection report provided by the seller shows that several preventative measures are needed to protect the foundation from further damage; including new and added gutters, french drains, and a curb poured alongside the occupied side of the property. 

Dirty, overpriced, piss on your lawn, no parking, horrible traffic 6 months a year (when the zonies come by the 100s of thousands clogging Garnet and Grand (the 2 major streets into PB).

If your neighbor files an action to obtain title to the land, or defends an action from your to remove him, and he/she can establish the land has been used exclusively by him/her, continuously, and openly for ten years, then the fact that you hold title is no defense.RCW 7.28.320Possession no defense.The fact that any person or corporation against whom such action may be brought is in the possession of such property, or evidence of title to such property, shall not prevent the maintenance of such suit.So I recommend first stopping the adverse possession clock by claiming your land. 

I feel as this should be prevented as COVID-19 was no one's fault. 

They take all of the 3am calls for clogged toilets and the like. 

The goal is usually to prevent a single unit from becoming a multi-family in an SFR district.

I'm not a fan of flushing agents, but 410a reverts to an acid at the slightest sign of moisture or contamination and makes a substance that clogs all of your components of the system and it's a royal pita to get that contamination out.

I am looking for ideas on how to prevent sound from traveling so freely down to the first floor.

Tenants lost jobs so 80% of building vacant.If so, are there any guidelines to follow to prevent issues with IRS?