Thinking about this....
Personally I don't want a game about the "Ars" of magic to get into the science of geology, physics, whatever. But I'm generally happy to suggest that foundations should be relative-ish to tower height, so a higher magnitude spell to make a bigger tower would be reasonable.
But then again, there is this:
Cleverbastardiam: "You see, Cynicula, I told you we didn't need a higher magnitude spell to make a double-height tower. I've done it! One Mystic Tower atop another!"
Cynicula: "Okay... but... does it look a bit... wonky?"
Cleverbastardiam: "Wonky? What do you mean?"
Cynicula: "Like, maybe you didn't quite put the top one on straight?"
Cleverbastardiam (whose Optimism trait is higher than his Finesse score): "Oh, well, umm, I'm sure nobody else will notice.... Do you, err, think maybe we wasted seven pawns of vis?"
Cynicula: "No."
Cleverbastardiam: "Really?! Great!"
Cynicula: "We wasted fourteen."
Cleverbastardiam: "What?"
Cynicula: "Sure. Because now if we target the top half, then actually our Perdo Terram will end up encompassing the bottom of the structure, too. So if we do Perdo it, we'll trash both halves of the tower."
Cleverbastardiam: "Oh... well I suppose it does look pretty weird... and nobody's going want to walk on the wonky floors up at the top, are they? Alright, well, fourteen pawns, easy come and easy go - here comes a Perdo Terram....!"
Cynicula: "Hell no - stop!"
Cleverbastardiam: "Really? So we should just leave it as it is?"
Cynicula (wondering how much rubble would fall on the covenant if they did Perdo the whole thing, and marvelling at how catastrophic a Perdo-Terram-the-structure botch would be inside a stone-build-covenant): "Sure. The foundations aren't deep enough anyway, and now it's crooked towards one side it will definitely fall over. I'm just going to go and tell the people in the north range to evacuate as soon as possible. And then I'm going to go and hire a mundane master mason for the rebuilding work...."