What happens to the House card in Monopoly Deal if I lose my complete set?
This question is probably too old to be answered again, but the accepted answer sounds incorrect according to several sources.

http://monopoly.wikia.com/wiki/Monopoly_Deal


  If a property set has a House/Hotel on
  it and the property set is broken up,
  the House/Hotel is moved to the
  owner's bank, per Hasbro.


http://boardgamegeek.com/thread/375594/rules-clarification


  Hasbro wrote:
  If you break up a full
  set that has houses and hotels, those
  houses and hotels have to go into your
  bank.


And you probably had stop playing. But might be useful for new players like me :)