What's "Building Defense"? Buildings have no defense stat, and specific defense buildings are designed to add extra defense to the player's defense stat only to the buildings in their perimeter, but they are so outdated that the best non-gold one adds 75 defense points. As you should know, the reward for the last FLTQ masters event was a unit with attack over 6,000,000, so those are exactly useless.
Also, when you raid someone's building (attack a building), the "Raid Results" screen shows the player's defense stat and all his/her defense units. Nothing anywhere in the game suggests that buildings have inherent defense stat values.
Modern War is not like other games where buildings have their own hit point values. The player's defense stat defends every building on his/her base with the math being something like if your attack stat is at least 1/3 - 1/2 of the defender's defense stat, the raid has x% chance of being successful, with the % being closer to 100% the closer your atk stat is to their defense stat. And with the higher stats we see these days, that % is marginalized to ineffectiveness.