Porkchop

I'm looking for some help with tactics when facing infantry dug into heavy buildings. Last game I tried, I was up against a building with a single rifle squad inside. I ran three squads at it. One was eliminated crossing the ground leading up to the building, one was eliminated virtually right at the front door, and the third entered and was immediately captured. So what's the proper way to assault a building? I'd like to avoid this unnecessary slaughter next time.