How do you go from one scene, to another, then a third and back again ?

Hi there all

I am completely new to unity, and would therefore like to know where to look for tutorials or guides, on scenechanges. The reason is that I am in construction and would like to show my client the building, but the building is quite huge! so therefore I have divided my model in several scenes, and would like to be able to go back and fourth, between these scenes. I have divided like shown here:

So I tried putting in a box trigger, with a script that had the function "Application.LoadLevelAdditive" which worked, for going from one scene to another, because it loaded in another scene. But then I would like to unload the first scene, when I go to the second, because else the model vil get too big. So how do I make these transitions best? so that I can go from on scene, to another, to yet another, with only one scene loaded every time?

Anyone who can tell me how to do this, or point me in the directions of a guide or video ?

Many thanks!