It is hard to find a flat where I live. So I went to agencies that rent for a short duration (the quickest and the only option because I have a deadline to meet). I found a flat and the landlord accepted to rent it to me. It is a short term rent (of just over 3 months) with the option of extending it. Do you think this is enough for family reunification visa in Germany or do they require a rental agreement with a longer length?
Even the law governing this (§29 AufenthG) only says:
ausreichender Wohnraum [muss] zur Verfügung stehen.
Translated: Sufficient room for living must be available.
You will have to go and ask them. Or maybe just call them. As it does not say so in the law, you should contact the exact Amt you will be contacting for the title. A different Amt may belong to a different regional authority and they may have made different rules for themselves in absence of a proper law.