Nobody renovates houses in Japan. You're supposed to destroy it and build anew. For 15k you're buying the lot and the right to spend a lot more than that.
He mentions renovation companies as well as builders who will undertake renovations. He has many more videos detailing the process. However, being a youtuber does open up possibilities that aren't common.
-- https://www.theguardian.com/cities/2017/nov/16/japan-reusabl...
Of course, houses can be renovated - as the article explains. But going along with a country's traditional building methods is often cheaper than bucking the trends.