The MVC application will be just a framework and backend in such a case.
You can make great 3D graphics with HTML5. Of course you need html5 enabled browsers, but I think it is an acceptable constraint these days. You can rely on WebGL also, but it has even less support - for now (check
Babylon.js[
^]). So you can find many tool to render 3d in a web application. What you will need to make on your own is the "designer" you give to your users to draw the building. I am sure, you might find out there some examples for this also (like this one:
http://www.3dtin.com/#[
^]).