The Design Law was radically revised in 2019, and as of April 2020, exterior designs of buildings can be protected by a design right.
An interior design can be protected by the design right if it creates a unified aesthetic impression as a whole, as a single design.
The Ministry of Economy, Trade and Industry (METI) has disclosed four cases in which architectural and interior designs were registered for the first time in Japan.
The ministry plans to release graphic designs in the future as well.
Ministry of Economy, Trade and Industry
https://www.meti.go.jp/press/2020/11/20201102003/20201102003.html