Building apartments

I'm afraid that building your own apartments is not a feature of this pack (I'd have liked to build midrises and rowhouses, personally). You can customize the interior of any apartment, but not the exterior or public areas. When you have an apartment lot loaded, you do have access to the central neighbourhood area (outdoors) and to any festivals happening there without a loading screen. You can encourage Sims who live in the building to gather in your Sim's apartment (whenever they want) by befriending them and giving them a housekey. You could also use Get Together's Club feature to create clubs of building residents that could gather on another lot or in a resident's home.