The court would likely consider the buyer's tardiness to be an immaterial breach resulting in no damages to you and would hold you to your deal. 

He is a good tenant, pays on-time for the most part and includes late fees if tardy.

Offer to cover the hard costs along with the tardy assessment. 

their chronic tardiness along with the unknown number of occupants along with your concerns of drug use is a recipe for disaster.  

I see late fees as a covert rent increase for consistently tardy, but paying tenants. 

They're definitely clear on our late rent policy and have been subject to fees already as a result of their tardiness. 

Remember, under the Fair Housing Act, you must use the same screening process for all applicants.Increase Communication With Late PayersIf the late payment of rent becomes habitual, take steps to increase the communication with the tardy tenant.

The new law changes are Sacramento’s tardy response to this increasingly growing problem and will only touch the surface of it.To answer the original poster’s question, as of Jan 1, 2020, no city in CA can legally stop you from building an ADU on your single family zoned lot.

The manager needs to assert themselves and make the tenants acknowledge that they are to address any correspondence about the property to them and not you.The management especially wants to correct any mistakes you may have made last time by making the tenants aware that tardiness with rent will not be tolerated and will be swiftly dealt with legally.Any maintenance problem must be reported to the manager as soon as possible to prevent further damage.If they disagree with the manager they are free to leave so you can get a better tenant.

@Jeff Wells Sorry for the very tardy reply.