Typically 15cm projectiles contain six to eight kilos of explosive.  A twenty cm round might have fifteen kilos.

Either exploding well inside a fifteen by fifteen meter buliding has the potiential to collapse it, if the structures resistance to several types of stress is below average.  Usually either 15 or 20cm rounds will not collapse a building that size when expoding on the roof or exterior wall surfacce.  Of course there are always exceptions.  Thin masory walls with few interior bracing walls can collapse in a stiff wind.

I'm a bit more familar with the results of  natural gas explosions from broken pipes or furnaces.  The dynamics of such a explosion are different from a artillery round, but it does not take much to collapse a typical house. 

I see you point, just saying a 2 storie house is a bit too vague, the subject came from an idea I had a while ago about using a purpose built urban assault tank, something similar to the Sturmpanzer IV Brummbar, which was armed with a modified sIG 33 150mm howitzer. If I remember rightly we came up with a 120mm mortar as the main armament, like you said the rounds will have a better effect if they explode inside the building, so using the mortar the shells could be used to penetrate the roof and then explode.

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Seems the Canadians had a similar idea.

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Of course if you really want to flatten a house, a thermobaric round could be used

Yeah, that's what the Russians do.

The USMC infantry fighting to clear Fallujah (part Deux), used a little trick called "the Houseguest". They would take a standard sized propane tank, and tape a small amount of C-4, or perhaps a grenade to it, an toss it into a building strongpoint. They seemed muuuuuuch pleased with the results. I'm not sure if it was enough umph to collapse a whole concrete house, but I'm sure Hank Hill would have been proud.

Cato