@IrisuChan
From what I see in your pic, what you are trying to do should work even though the garden is not defined as belonging to the apartment since the only way into the garden is through the apartment since you've fenced the separate gardening areas off. The only question is if the tenants can plant anything in the ground or place a pot or planter. If they can, you should be able to have your gardens. There is only one way to find out though. Just give it a try and see if it works.
I haven't tried an apartment with your arrangement, but I have successfully tried basement gardens for my residential rental builds. One has a shared garden where all tenants can plant, harvest etc. and the other has a smaller separate garden for each apartment. The basement gardens are not connected to the apartments in either case. But so long as you set the access to the garden spaces up so tenants get to them through shared space, it will work. The only odd thing is when you go to select the apartment to play on the map view of the building, there are some extra selection icons, but you can't select the ones for the basement spaces, only the above ground apartments so it's just a visual thing. It does not affect play.
There is one odd thing about apartments that have 2 doors though. If your front door is in an enclosed foyer that has the main building door going to the outside and you have a second door exiting to the outside like you have to the garden area, the game will try to set the door to the outside as the front door. Even if you set the real apartment front door as the front door, the setting will not hold and every time you load the lot, and the garden door would become the front door again. The way around it is to make the building front entrance a breezeway with no outer door (an arch will work) so the apartment front doors will be seen as the front door by the game.
Hope this helps.
