Because people's toilet space is on the floor, the ancients called it "going to the toilet". In ancient buildings, the main room is relatively high, and the kitchen is generally low. Cooking is considered as dirty work, so it is called "the kitchen".
About "going to the toilet" and "going to the kitchen", besides understanding from the perspective of the size and location of the house, some people have put forward another point of view. From the four directions of respect, China was different in ancient times. Before the Tang and Song Dynasties, the core area of Chinese civilization was located in the north, which was relatively rich and culturally developed, while there were many undeveloped areas in the south, which were often called "the land of barbarians". Therefore, the cultural mentality of worshipping the north and neglecting the south has a long history.
In addition, most of the ancient capitals of China are in the north, and cities south of the Yangtze River rarely become capitals. In ancient times when there was a strong sense of inferiority, the emperor was superior and the people were inferior, so the north was superior and the south was inferior. Therefore, people call going to the north the north and going to the south the south. In ancient buildings in China, toilets are often located in the east of the north of houses, while kitchens are located in the east of the south. So going to the toilet is "going to the toilet" and cooking in the kitchen is "going to the kitchen".