I'd say when it's either fallen over or fallen in. At this point, you could STILL creo-repair the covenant (I'd say a ritual spell)....but it would be full of the detritus and debris of the collapse and damage.
I'd say when a building has taken about 60% plus damage then it needs to have at lease SOME mundane (or craft magic) attention or the creo spell will just not hold.
A
(Somewhat dodging the question) Creo magic can't repair the building once its Essential Nature has changed from that of "building" to that of "ruin".
For example, once it has been overgrown, colonized by animals, etc...
In The Dragon and The Bear, there was the idea that Pagans couldn't be brought back to life once the seasons had turned, because by that time, they had "set in" their new state as inhabitants of the faerie realm. You could use the same principle for buildings.
Stray thought: you could actually use Creo to bring a corpse or a ruin closer to perfection... i.e. to being a "perfect corpse" or a "perfect ruin". 
Isn't a perfect ruin a bit of an oxymoron? 
And the core book p.190 gives "ruined and decaying versions of any buildings" as an example of the mundane level of a magic regio.
So I'd go all the way to "no longer recognizable as a ruin" before I ruled a building non-creo-repairable, I think. Which has interesting implications.
I think we have started to come around to the intent behind my initial question. My intent is to "heal" the ruins back to the original shape and I was envisioning a building's moment of perfection as the moment right after the building was completed. And I wanted to do this without having to know what the initial structure looked like. My stumbling block is that the main book never addresses this though it seems that it is encompassed by the Creo technique which does not seem to constrain this concept to only a subset of forms, but when looking at CrTe guidelines it is never addressed, only creation is addressed.. I have noticed a few of these gaps in several guidelines but was unsure if it was intentional and rationalized as hermetic theory had not yet "figured that out" or if it was merely an oversight.
Using the creation guidelines, how about replacing "create" with "restore" or "heal"? Then, restoring a stone structure would have a base level of 3.
Well, going by the parallels to Corpus and healing, and the fact that it's much easier to play with inanimate objects than animate (Creating a fully functional stone is base 3, a barely functional human is base 70) ...
Subtract two magnitudes from the CrTe bases to get the "repair" guideline, with the assumption that most of the thing remains and then add a magnitude if it's utterly in ruins. Thus repairing a broken gold goblet becomes CrTe20 (Base 5, +1 touch, ritual).
To fix your house (and assuming that wooden beams are involved along the way):
CrTe(He) 25(Base 3, +1 Touch, +1 Complexity since it's a ruin, +3 (or 4 size), +1 Requisite)
As an aside, the Order must know of Hypocausts, and with Hermetic magic, they're trivial to implement safely and easily. Popularising this spell, and playing on the Hermetic fondness for Roman mysteries, I can see a lot of old Roman forts rising again and disturbing the locals.
I think Creo can repair dead things. It's the limit of the soul that prevents magi from resurrecting people, not the inability of Creo magic to repair the body.
I might be thinking of Fourth Edition, e.g. Shadow of Life Restored, or whatever it was, but that's my recollection.