Mastering engineers often work in acoustically untreated domestic rooms.

Evidence

According to Hinksman (2021), mastering engineers might have been relegated to non-descript rooms, although a few lucky ones got acoustically treated spaces. Others set up operation in their homes, a normal domestic space, an acoustically treated domestic space, or a modified garage. There is the belief that intimate familiarity with the room is more important than the nature of the room itself. Again, there are no standards. — Toole, Floyd E, Sound reproduction- the acoustics and…, 340