Let's see:
Heavy Air/Artillery Strike U/T 3 Range LoS Damage 2xD6+2H Features Blast/4“, One-Shot
Building Longest Side 6" x 6" Height = 36/2 = 18, check!
Let's presume it is a 1930s stone Building, So Critical 10+ Hit 8+ Provides Hard Cover
Please check P.41 12.5: "All enemy units may shoot at a visible unit instead of the structure itself. The unit inside the structure counts as in Cover. The type of cover is determined by the type of structure. Infantry Cover Saves apply, roll Damage Dice as usual."
So you can attack the unit inside.
Infantry in Hard Cover= Crit 8+, Hit 5+, Save 4+
Let's again presume that all of the infantry squad are within the 4" Blast radius (should be easy since the building is only 6 long).
So you roll 2xD6+2H against every mini in the unit!
You will most certainly hit all of them, considering the cover modifier, you will most likely end up killing half of the unit.
Shooting at the building is another thing.
Indeed more difficult, you may get lucky and hit one time, scratching off 1 SP.
The idea behind it is pretty much the same as explained in the design notes under ““Un-dominating” Vehicles“.