I am still a beginner in creating 3D games, so I would be very grateful for any hints and tips for how to design fully accesible buildings.
Try a free program tried Blender. It’s great for making full 3D models of your desire! You can then import them into UE4.
You can also use Unreal Engines built in BSP’s to get squares and other shapes you can then form them into a building. Then use the geometry editor to make more dynamic shapes.