Is it illegal to build a house secretly?

Illegal. The real estate administrative department of the local people's government at or above the county level shall order it to make corrections within a time limit, give it a warning and may also impose a fine.

Legal basis:

Article 349 of the Civil Code of People's Republic of China (PRC) establishes the right to use construction land, and shall apply to the registration authority for registration of the right to use construction land. The right to use construction land is established at the time of registration. The registration authority shall issue a certificate of ownership to the owner of the right to use the construction land.