Article 31(1)(a) of the Vienna convention on diplomatic relations allows for the receiving state to execute real actions on any private immovable property located on the land thereof, so where lies the reportedly insurmountable difficulties involved in evicting diplomats for even very egregious rental arrears?
Diplomats are immune from local law
Unless waived, a person with diplomatic immunity is not answerable to local law. That means, a court or tribunal has no jurisdiction to issue or enforce an eviction notice if diplomatic immunity is raised as a defence. Even if one is granted, the police (or whoever) have no ability to enforce it.
So, the landlord issues the notice, when they try to enforce it, the court says “sorry, can’t help you.”