Nagayasu built a temporarily mansion when Ieyasu was on his way to subjugate Takeda, then, Ieyasu saw his excellent home construction ability in it that and allowed Nagayasu to become his officer.
家康が武田攻めのときに逗留するための仮館を長安が建設したが、このときに家康がその館を見て長安の作事の才能を見抜き、仕官を許したといわれている。