Others have discussed having a focus covering buildings, the creation of. I might argue that it should be Major, covering both Herbam and Terram. But those effects are of fairly low levels, really. Consider Conjuring the Mystic Tower which is only level 35, compared to the effect levels necessary to manipulate Aurae and Regiones.
If we're looking at a Focus which covers those effects, things change slightly.
If you're only looking to Strengthen Aurae, it's clearly a Minor virtue - these effects are Creo Vim, and a fairly limited subset thereof.
If you're looking to do more things with them...
A few years ago, for the (now very defunct) saga of Via Experimenta, I talked Jonathan Link into allowing a minor focus covering exactly Regiones and Aurae, for my character Titus Aurelius. This focus was fairly explcitly intended intended to cover essentially the guidelines on p. 14 of RoP: M, the Mystery Guidelines of TMRE p. 100 and a few InVi effects from the core book.
This was judged to be a minor focus, due to being pretty narrow, and it really is.
It covers (very) restricted areas of of several TeFo combinations, but that's not a problem (ArM5, p. 46).
So yes, I think a minor focus qualifies.
I don't really see how you'd define a single focus to cover both the building and the aura/regio manipulations though.
I should probably also recommend the article in Sub Rosa (issue 11, p. 32ff) dealing with the Hermetic Architect. I'm working on a respons to that article, but I have been for a while, and it's still nowhere near done. 