If people are going from outside to inside but they have a transition window like a door, you could make an opaque material with a fake cubemap interior that’s created from a reflection capture of the inside of the apartment. That way, the material would technically be opaque but would look translucent, so the shadows would just work. If you need a smooth transition, your options are much more limited. Should I assume that your shading model is ‘surface-forward shading’?