Non-EU Student Visa Accommodation Proof

After approximately one and a half months since submitting my long-term student visa application at the embassy, I received a letter from the migration office in Bern. The letter requests proof of accommodation in the form of a rental contract. In my visa application (Visa D form), I only provided the address and details of a relative's residence where I plan to stay upon entering Switzerland in the "probable address" section. I missed out on supporting this fact with papers of some sort .

I am considering submitting my relative's rental agreement which demonstrates adequate space in the apartment, along with a letter from my relative affirming that I am welcomed at their place. Will that work ? I will contact the migration office soon, but I am open for any suggestions or tips for now. Thanks.

P.S: I did not decide if my stay at my relative's house is temporary or permanent, but it will at least give me a chance to think as finding apartments in Switzerland is very tough as I've seen on this forum.

A letter from your relatives should be sufficient. Say that it is for the duration of the study, even if you plan to find something else.