LilShaay From the way you've described your issue, I'm not certain what it is really. I realize you're on Xbox so taking a couple of pictures may be difficult for you, but they would certainly help me, and others understand what your problem actually is. I have built and played a residential rental building with 6 apartments and have had no problem with accessing any of the apartments. Four are on one level and two are 2 level. And I have no idea what you mean by "home unit". I have never seen that term used in relation to residential rentals or anywhere else in the game. Are you talking about using one of the apartments for the building owner?
The only thing I can think of as to why you can't access the single-story apartment is that you haven't defined your apartments correctly. You comment about settings also indicates that. If you can take a pic of your settings, that would be helpful. If you have set your apartments up correctly, you should be able to simply move the residents of any apartment in and out just like you move households in and out of residential houses.
And if you are willing, I am willing to take a look at your residential rental lot. All you need to do is upload the lot to the Gallery and reply here with the lot name and your userid if different than what you use here (LilShaay). I will download it and see if I can figure out what is going on with it.
Hope this helps.