Is there anything in canon which explains how complexity ramps up for created habitations? We know that a wizard's tower requires +3 complexity, but what requires a +1, or a +2?
I don't have it in front of me, but I'll bet HoH:TL would give a little more insight with the Tremere Wall magic-- but then you'd probably be pretty familiar with that. 
-Ben.
I'm not aware of anything like that. However, I'd point out that +3 complexity is not just a wizard's tower, is a customizable wizard's tower (the caster gets to choose the floorplans). The way we play it is:
Design complexity:
+0 magnitude for a basic wall, straight, circular or rectangular.
+1 magnitude for a complex, curving, crenellated wall, or a single enclosed space, bare except for a few openings, OR
+2 magnitudes for a complex enclosed space, with multiple floors and rooms per floor, stairways, ornate windows etc.
Customizability:
+0 magnitude for minor cosmetic modifications (ornamentation and position of the windows etc.)
+1 magnitude for more serious modifications to the same basic structure (arbitrary floorplans etc.)
+2 magnitude for complete reconfigurability within the scale (any building of that size)
+01 Intricacy , page 144 , Haunt of the Living Ghost & The Shadow of Human Life
+06 Mentem , page 144 , for a very elaborate effect
+01 Complex Effect , page 148 , Weight of a Thousand Hells
+03 Elaborate Design , page 153 , Conjuring the Mystic Tower
+01 Fancy Effect , page 156 , Creeping Chasm
+01 Complexity , page 61 Covenants , The Riches That are Rightfully Mine(d)
From a quick scan , there is no canon statute on the matter , not even in the Creo section on page 77.