Most tenants demand a 1 month's security deposit. This means that you pay an additional month's rent, only once. You will receive the deposit back when the tenancy agreement expires, provided that you leave the accommodation as you found it. Security payments may amount to 3 times the monthly rent. Always make sure to ask for a receipt and make solid arrangements as to the state of the room. The tenancy agreement model includes an inspection list on which defects and the like can be specified (available at the JIP).