Is it legal for the newly-built residential property to sign an agreement with the owner to designate the building owner to load materials?

If what you say is true, it depends on the property service contract signed when you buy a house. Buying a house is a big event in China. You need to read every clause carefully. Greedy and cheap or blind obedience, you really have to pay some tuition fees.

The solution is not without, it depends on whether there is a clear price tag on your contract. If there is no price, you can consult the property management company. If no agreement can be reached, you can appeal to the court. According to the invalid contract clauses in People's Republic of China (PRC) Contract Law.