Japanese culture was heavily influenced by the Chinese. Historically, the capital of Japan in the eponymous Nara period, Nara, was based off of the Chinese Sui Dynasty capital, Chang'an (modern Xi'an). Furthermore, the Chinese writing system is still used today in Japan, mixed, of course, with hiragana (which developed from Chinese characters) and katakana. Buddhism complements the native Shinto faith in Japan, and reached the latter through China. Japanese art and architecture also display striking similarities, although despite the Japanese attempts to emulate Chinese government system, they ultimately failed in creating a centralized state.
I don't intend this to be a debate, but to justify my points:
China has developed its own architecture and culture. Yes, the Chinese were advanced than the surrounding countries, and in most cases, the surrounding countries adopted some of Chinese architecture. For example, you have mentioned Chang'an and Heijokyo. The Heijokyo was voluntarily adopted by the administration of Emperor Monmu. This is just one of the examples of Japanese architectures you have mentioned.
Japan has adopted and changed many outside cultures and architectures. It has also created its own culture. Chinese influence has only been strong in Japan until the end of the Heian Period.
Even in the Heian period, Japan has distinctly developed, or evolved from its previous architecture. For example, take a look at the Gingaku-ji.
This building is part of the Higashiyama Bunka, one of the two major cultures during the Muromachi Shogunate period. I am not an architecture expert, but none of this can be seen in China. Japan has developed a whole new architecture and culture over time.
In the Edo period, Japan was able to develop a very independent, internal culture because of the Sakoku. (Not trading with any nations except China, Korea, and the Netherlands). Leading to Japonism, Japan has created an independent, culture. One famous example is this Ukiyo-e.
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Japan has also adopted many of the Western (European) elements in the Meiji Period. To catch up with European Industrialization and Imperialism, Japan has put more effort into adopting the European architectural techniques. Take a look at the painting of Ginza, Tokyo in the Meiji period. You can see many European buildings in the background, along with people dressed in European styles and European horse carts.
After World War II, Japanese architecture is primarily modern.
Over all, Japanese architecture is a blend of many different cultures with its own developments. China, on the other hand has developed primarily on themselves and did not take in much culture from the outside, with the except of Buddhism, and after the Shino-Japanese war. I wouldn't say that Japanese and Chinese architectures are the same.
