I can't speak for Korea or Japan but in regards to China, the use of stools and chairs was introduced from central Asia through the silk road.

Prior to that paintings often depicting banquets and other stationary practices being seated though high status people's were always elevated on platforms.

By the Qing and Ming stools and seats where common place in formal setting while pockets of china would still use floor seating in less formal court arrangements. One example is to look at Chinese architecture from these two periods as compared to her counter parts in Korea or Japan. Main halls for domestic and official purposes begin orientating meeting hall along seated rows with Stone floors rather than an elevated straw or wooden platform for seating.

This is not a hard and fast rule as painting of offical conducting administrative work such as proctoring imperial service exams show then standing with no seats except for the most important individuals in the paintings.

In private quarters emperors like Kangxi were shone seated on the floor writing. Other more prominent examples are of Chinese wide platforms for seating one person can be found in both China and Korea for well off individuals. In painting depicting individuals of high and low ranks low ranking individuals will be shone seat on the floor with trays to eat or work.

Furthermore the reduction in use of floor or platformed seating seems to peak during the Qing. With many of the Ming interior designes for floor seating surviving in Korea traditions.

Prominent Qing famlies had meeting halls in their Manors which organized the layout with chairs for two prominent members at the of the hall towards the north and rows of seats for other guest perpendicular going from north to south.

One such example is the Qing governors office. Which I will try to link.

If you do want to look at what this would have looked like in East Asian for your self, I recommend visiting Kyoto and research long painted scrolls, many depict day to day live of people with in their kingdoms and are often commissioned by government officials for official records of events. These works of are are extremely common in all the East Asian cultures you have in mind as part of their shared artist ic tradition.

Qing administrator s office: http://www.china.org.cn/travel/2012-05/25/content_25452004.htm

Qing officials holding criminal court. https://goo.gl/images/N6AogA

In the Nara period, chairs and desks were actually in common use in Chinese-style offices and Buddhist temples. In the Heian period, chairs fell out of use in shinden-zukuri architecture, where no shoes has become the norm, but short-legged tables and desks have since been widely used to put objects, dishes, paper or books on. Traditional desks designed for reading and writing while seated on the floor are called fuzukue or bundai.

In the Heian period, national administration was conducted in the ministry offices located in the Heian-kyū Court in central Kyoto. There are plenty of written and illustrated materials about private life in residential areas and ritual events in ceremonial halls, but it is hard to find those about daily paperwork in ministry offices. This part of Nenjū Gyōji Emaki (detailed explanation here), painted in the late 12th century, shows the saburairō, the room for servants, furnished with a large red daiban table in the Higashi-Sanjō residence. This illustration from Murasaki Shikibu Nikki Emaki, painted in the early 13th century, represents Shikibu giving a lecture on Bai Juyi’s anthology to Empress Shōshi, with the book put on a black fuzukue. The Yadorigi Chapter of Genji Monogatari Emaki (detailed explanation here) shows the furniture of the Asagarei no Ma (Room for Breakfast) and the Ochōzu no Ma (Room for Washing Hands) in the Seiryō-den, the emperor’s private residence.

Kasuga Gongen Genki, painted by Takashina Takakane in 1309 (1870 replica), includes several cuts showing people writing or reading. In this cut, or this, a man is writing something without a desk, with a brush in one hand and paper in the other. In this or this, a monk is reading a handscroll put on a fuzukue. This part of Ishiyama-dera Engi Emaki, painted by the same author in 1320s, shows a monk writing sitting at one desk, with rolled books piled up on another.

Government offices of the Kamakura and Muromachi shogunates were also built in the shinden-zukuri style, but the Muromachi period saw the rise of shoin-zukuri architecture, where rooms are sometimes equipped with tsuke shoin or idashi fuzukue in front of windows, so that people can read or write something lit by the sunlight. This illustration from Tengu Zōshi, painted in 1296, represents a monk writing something on a tsuke shoin. The Dōjinsai room of Ginkaku-ji temple, built as Ashikaga Yoshimasa’s study in 1486, is the oldest surviving room with a tsuke shoin.

In the Edo period, national administration was conducted in extensive goten complexes which occupied much of Edo castle, now the Imperial Palace in Tokyo. This is a ground plan of the entire castle as of 1630s. This plan shows the most central part of the Honmaru Goten, including Goyō-beya, the room for Rōjū and the Wakadoshiyori, the two highest-ranking ministers, and the room for the Oku-yūhitsu, the most powerful working-level officials.

Domain-level administration was conducted in each goten buildings in their own castles. Today Kawagoe, Kakegawa and Kōchi castles are the only ones that keep some office rooms of original goten, though most of the rooms are now empty.

Important cities under the direct control of the shogunate were governed by bugyō-sho. This is a floor plan of Osaka Higashi Machi-bugyō-sho located in Tenmabashi, one of the two bugyō-sho that governed the city. Today no original bugyō-sho building survives, but the Hakodate, Sado and Nagasaki Bugyō-sho are partially restored.

Smaller cities and towns were governed by jinya, either controlled by the shogunate or small domains. Takayama Jinya in Gifu Prefecture is the largest jinya complex preserved, with two rooms furnished with fuzukue desks. Iijima Jinya in Nagano Prefecture is fully restored, with the Accounting Room and some others equipped with old furniture. In Kansai, you have Kaibara Jinya in Hyogo Prefecture, though the remaining part doesn’t include rooms for bureaucrats.