Minor Magical Focus: Man Made Structures (as in large scale constructions: buildings, monuments, aquaducts, canalways, walls, dams, etc.)
Too big? Too small? Just right? Is there a better way to describe it other than 'man made structures?'
Size - we're looking for something "slightly narrower than a single Te/Fo combo", so if you're going to apply this to any and every spell with Target:Room or :Structure, I'd say it's way too big. That's using the structure as an excuse to affect everything within it - and that sounds like a Major focus to me, (or maybe just a flat veto, as a Focus defined as a spell Parameter ~might~ smack of Lab Activities to me.)
However, if you're talking about the target solely being the structure itself - the walls, the floors, the roof, etc - then... it's still big, so it would depend on the Saga. But I'd imagine (not knowing you, nor talking with you about what you plan to do with this), that a series of spells like Conjure the Mystic Tower or Warm the Frigid Hall would not throw a game out of balance.
So, if you're affecting the room or structure, yes. If you're merely using that as a target to affect everyone within it, and then they go on about their business elsewhere, no.
As for the name, it's a bit awkward, but sounds accurate (at least for the second, more narrow choice, above).
Creating stone buldings would work. Destroying wooden ones would too.
Creating walls, I have accepted in my game ( that does not include wards, but does any form otherwise, as long as she's really creating a wall.
But buildings in general or, worse, any man-made structure I would reject. With a bit of argumentation I could accept it as major, but as the hound pointed out we would first have to agree on what it affects, precisely, and point out that it does not mean you can affect everything in a building, just the building itself (and things like boats would still be out)
It seems OK to me. Its no problem for me if someone want to create or destroy huge castles.
You should clarify with your player what kind of effects he can to do with the focus. It is more important than the wording.
He may be famous by killing his enemies with falling castles.
I admit, it was a half-formed thought in my head as I typed it in right before bed last night. I've been trying to pound out this architect idea that has been vaguely floating around my head for some days now, with little sucess. Now that I've had a night to sleep on it, perhaps man-made structures isn't quite right either. Perhaps just the building of structures, as in aiding in the creation of. Not sure I'm happy with that either. Maybe I'll throw the whole concept back into the closet that is my brain. 
My original thought would exclude Target: Room and Structure, and would have to be some sort of spell that only affected the building, but no items or people within it.
Not every concept has a clean, single- or couple-word phrase in English.
But for what you're describing, if I were SG, for an "architecture" type concept (and not an undefined and open-ended magical pwning of any structure), I'd let you go with Creo and Intellego - the one to Create, the other to Understand the design of an existing one. Wood, stone, ice, metal, clouds - if it's a structure, including bridges and walls but possibly(?) excluding ships*, the mage can create or discover the mundane architectural properties of them.
~For me~, that sounds about right - unless I'm overlooking something.
(* A true ship has "architecture" - a mere "boat" not so much. If allowed, it would be a SG ruling thing, case by case, no hard definition.)
Target structure can work here. No room. make it a thing that affects the structure instead of its contents and there we go. Easy to rule when it applies and when it does not by troupe fiat. It is a good focus from a character point of view so I'd allow it. I'd make it a minor focus, but that is me.
Cheers,
Xavi
Do you want only to create new buildings?
You need target: structure to destroy buildings.
With target: part por even target individual you can make them fall as well: just destroy a few blocks of stone here and there and the castle's donjon collapses. But then, the focus would not apply in those cases.
Cheers,
Xavi