Fig.: drinks, food or other services are free of charge because they are paid by the host or by the establishment

For the opening of the new bar, free beer for everyone: it's on the house!

All'inaugurazione del nuovo bar birra gratis per tutti: offre la ditta

When Mr. Brown is on a business trip he spends money like there's no tomorrow: it's on the house

L'impiegato Rossi quando va in trasferta spende e spande, tanto paga la ditta

This expression uses the word house in the sense of a bar, a restaurant, a hotel or any other establishment serving the public