1. Without permission, buildings that change the appearance or function of buildings without authorization are illegal. 2. It must be approved by the relevant property department, because the reason why the balcony is not allowed to be sealed is to consider the appearance and safety of the balcony. 3. Another point is to see whether the use right is jointly owned by the owners. If it is jointly owned by the owners, whether or not the building is illegal. If it is privately owned, you can check whether there is a clause prohibiting the building according to the provisions of the relevant treaties. If it is privately owned, there is no relevant provision prohibiting the building, In this case, the construction is not illegal, but the premise is to ensure that it will not affect the living environment of others.
 Legal basis
 Article 44 of the Urban and Rural Planning Law of the People's Republic of China, temporary construction within the planned area of a city or town shall be approved by the competent urban and rural planning department of the people's government of the city or county. Temporary construction shall not be approved if it affects the implementation of short-term construction plans or regulatory detailed plans, as well as traffic, city appearance, safety, etc. Temporary buildings shall be demolished by themselves within the approved service life. Specific measures for the planning and administration of temporary construction and temporary land use shall be formulated by the people's governments of provinces, autonomous regions and municipalities directly under the Central Government.