Is it illegal to seal the balcony privately?

This is illegal, and this is not allowed. According to Article 22 of the State Council Order No.379 of the People's Republic of China "Property Management Regulations", the construction unit shall formulate a temporary owners' convention before selling the property, and make an agreement on the use, maintenance and management of the property, the interests of the owners, the obligations that the owners should perform, and the responsibilities that they should bear in violation of the convention.