First came accross herrschaftlich in Der Vorleser
" Ich erwartete, daß in dem herrschaftlichen Haus auch herrschaftliche Menschen wohnten. "
In the English translation:
" I assumed that grand people would live in such a grand building. "
Also in Tonio Kröger :
" und seiner Vorfahren großes altes Haus war das herrschaftlichste der ganzen Stadt…"
I have two English translations:
1) " his large, old ancestral home was the finest house in all the town. "
2) " whose large ancestral home was the most magesterial in the eintire city ..."
Seems as though there are a number of ways to translate the term. I like the suggestions from mikefm #2: stately, manorial.