Why does the converted garage or added room in my Westminster home never match the house temperature?

Because it was almost never given a fair duct. Additions were commonly fed by extending a run that was sized for the original room count, or by a tap into a trunk that had no spare capacity, so the added room receives the leftovers of the system's air. The diagnosis is an airflow read at every register. If the addition is starved, the honest options are a properly sized new run where the path allows it, or a ductless mini split head that serves the room independently, which on converted garages is usually the cleaner answer. A larger outdoor unit is not on the list, because the original rooms were never the problem.