The AP has to be open, notorious and hostile...those terms have a legal meaning and essentially it is that you as the owner have been fully deprived of your right to the property.Absent those items above, it sounds like at best they have another legal right which is a "prescriptive easement".

You also mentioned meters, do you have to separate meters, get approval from the city and pull permits?

The permits and architecture was done prior to start.).For most people the finances are the limitation to scale.  

Insurance- coverage for new engineering, permitting, etc8.  

Just to add to what has been said here - if the seller described this as a triplex on the sale documents, they might have a legal liability.  

So, just give it your best self-help effort to find them, and then let the GAL do his or her job.As far as an ejectment lawsuit, the real question is, was the property legally abandoned?

Tahoe isn't ideal right now as the local community is vehemently anti-Airbnb and the permitting system is challenging to navigate.

Consider Hiring a Legal Expert- German property law can be complex.

Also know that building a home seldom goes as expected - I have relatives that just built a nicer house… they thought it would take 9 months… it literally took 18-24 months between delays in permitting and the builder just not being able to get it done in a timely manner - often due to sub-contractor issues.  

.: I haven't passed this past legal review so for what its worth: Part of my standard requirements for everyone include income is at least 3X rent, minimum credit score, say 640, Clean record and no evictions as well as deposit is paid in full at signing of lease and 1st month rent is paid in full prior to handing over the keys.