Well, Juantxo does.
For the incredulous, here he is building (brick by brick) the walls of the bedrooms.
The exterior walls were also done this way and, in fact, they're load-bearing walls. That is, they form the structure of the house - together with two wooden pilars -, so they hold the weight of the entire house. There are no reinforced-concrete pilars anywhere in this house.
The other two rooms that you see opposite, whose walls Juantxo has just finished building, are the bathroom (right) and the dressing room (left).