There actually is no real estate bubble forming in China. Specifically, there are real estate bubbles forming in specific regional sectors, e.g. Shanghai, but not on the national level. More specifically, the mortgage issue in China is not much of an issue at all. Almost all U.S. homeowners borrow money to buy their homes. In contrast, the majority of Chinese homeowners do not even have a mortgage as over half of homes are bought with household savings, so no debt.
The same claim some Americans are making to deny the problem. It does not matter whether bubble concentrated on small regions. What matters is the amount of money that went into speculative investment. The bubble forms on prized real estate located at economically strategic locations, which mostly make up small fraction of geographical area of a country. However, amount of money poured into such an area could be a significant portion of a country's available money.
It is especially bad if leveraged financing is done.