Short Answer Letting your partner stay continuously in your apartment beyond roughly six to eight weeks turns them — in the eyes of courts and landlords — from a guest into a co-occupant who must be reported. That threshold is a guideline from case law, not a statute, and for a partner the landlord generally has to approve the move-in once you request permission — what he can punish is never asking. What Most Expats Don't Realize You let your partner stay over every single night in your flatshare room, assuming it was a private matter that did not affect anyone else. Your roommates noticed the sudden spike in the household water and electricity consumption and reported the unregistered occupancy directly to the main property owner. The landlord issued a formal written warning ("Abmahnung") for housing an unreported occupant, the annual utility statement landed €450 higher with the extra person factored in, and the trust inside your flatshare never recovered. What To Do * Call a meeting with your roommates to discuss your partner's stay before their visit extends past a few consecutive weeks. * Send your landlord a written request for permission once the stay becomes permanent — for a spouse or partner, approval can generally not be refused without a serious reason. * "Ich bitte um Ihre Erlaubnis zur Aufnahme meines Partners in die Wohnung." (I ask for your permission to take my partner into the apartment.) — send this sentence in writing; in most cases the landlord is legally obliged to say yes. The Truth Germany's tenancy courts draw the guest line by habit and case law rather than by any statute, and they side with tenants who ask. The system punishes silence, not love — an unreported permanent occupant invites warnings and utility recalculations, while a simple written request usually has to be granted.