1.When the house is being build.
2.When the house is being built.
Neither. Try: When is the house being built? JA
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Though it is in the passive. If you know that the house is planned to be built in the future, but you don't know when. Then, you should say or construct your question as follows:
When will the house be built?
PS. I hope a native speaker will contribute and answer your question and correct me if I'm wrong.
JA