Ah I see what you mean Nicolai - however I've got more buildings which I'm fading in and out and I'm not sure I've got time to remodel. Also even with interior faces deleted, you'd still see for example the back wall through the front one. I'm trying to avoid that, like in photoshop when you have a final rendered imaged and bring the trasparency from 0 to 100, you don't see through the object to rear walls etc, you just see a transparent version of the fully opaque render. See what I mean?
I think what I'll have to do, although I was trying to avoid it, is render the animation twice, once with the building hidden and again with the building fully visible and go from one to the other in Premiere. Bit of a pain as it means twice the render time so if anyone's got any other alternatives I'd love to hear them.
Cheers.