This manga is weird with it's realism.
The laundry business is a really good idea for a pre-modern society. IRL it used to be a massive business that employed a lot of women and those household that did it themselves had the women devote about 2 whole days, from morning to evening, every week to get the laundry done. It's a good idea for making money, but if the society anything like those found on pre-modern Earth it will make a lot of people unemployed.
On the other hand, limestone plaster and whitewash was super common before modernity. It was used almost everywhere and even cavemen knew how to make and use it. It's quite unrealistic that it's not commonly used in the manga. Medieval Europe, for example, had almost all of it's stone building whitewashed, even the castles, and a lot of wooden buildings as well.
Also, building a stone house like that in a week by himself is a bit ridiculous. Something like that would take a team of skilled builders the better part of a year to build. Especially without modern equipment. For get the laundromat, he could make bank just by building houses. Especially since his work quality is supposed to be superior as well.