1. The drinks are on the house.
2. Beer is on the house.
3. You meal is on the house.
4. Dinner is on the house.
5. The pie is on the house.
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Are they all grammatical?
Why do you use 'house'? Don't you use 'restaurant'?
All the sentences are fine.
It's an idiom that's been around a very long time! The "house," in this case, refers to the owners of the restaurant or pub or whatever.
http://dictionary.cambridge.org/us/dictionary/english/on-the-house
Yes, all of the sentences you provided are grammatically correct.
In the context of these sentences, the phrase "on the house" is an idiom that means something is provided or given for free, usually by a business or establishment. The use of the word "house" in this idiom is a metaphorical reference to a place where people are hosted or entertained, such as a restaurant or a bar.
While it is common to use the word "restaurant" when talking about a complimentary meal, the phrase "on the house" is a more general expression and can be used in various settings, not just restaurants. It can also be used in hotels, bars, or other hospitality-related establishments to indicate that the cost of something is covered by the establishment itself as a courtesy to the customer.