I think the easiest way to do it would be to have someone text you in-game to say "we've got a sick new extension to the garage bruh, check it out bae!". Then when you get there there's an extra option next to your current set of cars to head into the new extension where there are a few more slots (3-5, maybe?). It would require modelling a whole new garage but since it won't appear in cut-scenes (they can say it's through a door that's out of view or something) it won't affect the storyline or any of the work done already; it would just need the new bit adding.
Any other options are going to require new animations for driving in and out, new menus for switching up cars or deciding which garage to use, additions to the map and so on. This way they can use most of the existing assets.