Conjuring the Mystic Basement

I was wondering what options there might be for adding a level below ground to and existing structure. I did look at CtMT and saw that it builds foundations 20' below the surface. Realizing that it is a legacy spell, could a variation of CtMT be used below ground then clear out all of the dirt from the 'structure'?

Adding a Perdo or Rego requisite should work. Or you can just hand wave it like conjuring the mystic tower. Perhaps the act of creating the tower displaces the ground.

I'd say it would be a Rego Terram, but of a higher magnitude than Conjuring the Mystic Tower, because the Rego guidelines require more work to shove dirt around than to make more of it (wouldn't necessarily need vis, though).

Trick is how to make the subterranean levels without undermining the foundation of the existing building. That's probably adding an order of magnitude or requiring a Finesse roll.

I would also vote for ReTe. The existence of a previous building on top also conditions this, since you want the previous structure to remain sound structurally speaking. Using Perdo you would probably make the building above collapse.

The best thing is to do it in one spell. Then there is no need to deal with the tower.

I would disagree if it were part of a single spell. If the dirt were being destroyed while the Creo magic was making the foundation then the tower would not move.

Given that CtMT makes a basement/foundation without Re or Pe requisites. I think Base 3, R: Touch +1, T: +1 Part, D: Momentary, +3 Size, +1 Complexity for CrTe 25 would work. The part would target the tower to add the basement. Otherwise I could add a Re req. on to Base 3, R: Touch +1, T: Ind, D: Momentary, +3 Size, +1 Complexity (CrTe20)