It seems to me that any addendum signed later after the initial lease may create a messy situation. 

All it takes is one non-payment or messy eviction to set you back months and even years in cash flow.

This will create a huge mess. I

Hello fellow landlords,The properties in question are in a market that although it is not cheap, it draws lots of money-cautious tenants. I am foreseeing a bit of a push back or even turned-off potentially decent tena...

There is more messiness and risk.

Legal violations and penalities(fines) even if you have accounting software to track the difference. 

Will they base their calculations solely on the income from the two legal units and disregard the non-conforming basement?

Quote from @Stylianos Kalamaras: Yes they are, they cost me $800 knowing they would not close my deal and waste my time, if I had any possible legal recourse I would take it they shouldn’t be allowed to operate it’s really sad.  

“Broker” is a legal term that often requires standardized coursework and a state license. 

Dealing with people is messy, its not uncommon to have difficult situations and conflicts to deal with. 9.