The option of obtaining unique properties (save garages) unlocks after you have completed the eighteenth mission. Depending on the individual property, some missions will become available right after you obtain that property, or they will become available at random moments throughout the game (you will usually learn about the possibility to take up a mission by receiving a message from the manager of that property). It is worth noting here that each of the properties is connected with the protagonists that purchased it. For example, if you have purchased a hangar in the airfield while playing as Trevor, he will be the only character entitled to perform missions connected with this property.
Note- in case of profitable properties (e.g. all three movie theaters in Los Santos) it is not recommended to ignore the messages, from the managers, of the possibility to perform a mission. Otherwise, you may lose some, or the whole, of the weekly income. In some cases, this may be as much as several tens thousands of dollars.