RAW: How to increase height effects of any Area spell?

[whswhs]

This isn't an answer to your general question, but might address the specific case:

Suppose you yourself are standing on the roof of a tall building, one that puts you, say, twenty feet above the ground. You want to cast an area spell on your foes who are also on that roof. Do you need to cast it twice, once for ground level and once for a layer above that? I don't think you should. You certainly ought to be able to cast it at the level of your own footing.

(Compare a different case: If I dig a pit sixty feet deep, can I force every hostile wizard to case a spell five times before the sixth casting is based at the level of the boards I've used to cover the pit? Or can they just choose to cast from ground level?)

Now, it might make sense to say that if I'm on the ground, but I can look up and see the enemy standing on the roof of a tower, I can cast the spell on the tower at the level where they're standing. It would be a bit much, perhaps, to level me cast it at an imagined level of the air, but if there's actually footing there, why not allow it to be based specifically at the level of that footing?