If an asteroid large enough to incinerate any and all living organisms were to hit Earth?

It's really a crap shoot about man made structures surviving. I immediately thought of NORAD's Cheyenne Mountain command center which was built to withstand a 30 megaton blast within a 1 mile radius. If this is hit directly, it would still be gone. If it wasn't hit directly, it would probably survive, although it would be hard to find if you were looking for evidence of our existence. However, depending on the severity of the impact, regular buildings may survive the blast if they are far enough away, even if all life is gone. Life around the globe would die from lack of drinkable water, the sun light would be blocked out disrupting the food chain, that kind of stuff, but that wouldn't effect buildings. As for life to restart, no one knows. We don't know how common life is, whether it is random chance, or an inevitability given the right conditions. That is one of the great mysteries of the universe.