[Lord Malachi]

I agree with Lord Creampuff. When I play with buildings using the regular BBB rules we basically count them as bunkers and allow the standard 6" horizontal, 6" vertical movement. As far as measuring distance to target we measure from a birdseye view (i.e. from where the model's base would be on the tabletop) and LOS based on terrain level (i.e. 1st floor level 1, 2nd floor level 2, etc.). Whether this is Tourney legal I don't know, but that's how we do it. We also discuss shooting/movement rules for buildings before the game starts so there is no confusion midgame.
Using Cityfight rules is also an option, particularly if your board will have many buildings of differing heights. For ease of gameplay we usually just spell out the level heights of any buildings beforehand and don't use the actual physical height of the terrain piece since our selection is limited.
An example - we have a few towers made of plastic tubes that are about 8" high in reality. We count them as level 2 terrain that is 6" high so that models moving into them (embarking into a vehicle style) are allowed to go to the top in the turn they enter it.

 

[Lord Malachi]

Care to give page refs for any of these? Just curious.

p. 17 BBB - There is a photo of a model moving up a building. It doesn't mention coherency though. There is a passage entitled "Moving Up & Down" also.

p.21 BBB - the last paragraph on the page addresses LOS from higher terrain.

p.25 BBB - There is a "Cover Summary Table" which addresses area terrain cover saves and height ranges.

p.193 BBB - The rules for bunkers are here.

There is one correction to what Lord creampuff said - inside of a bunker you can have a model shoot out for each vision slit or 1" of continuous slit. Potentially an entire squad could shoot out of the bunker if there are enough slits facing the target unit.