You don't need the whole building to be immovable, though, just a few key load-bearing bits. Plank of wood under each wall, cross-sectional slice of each pillar, etc. Same principle as skytowers. Once the engineering constraints are sorted out, high-security stuff will probably settle into fewer, taller buildings, and then inter-clan competitiveness will promote outlandish architecture which would be flagrantly structurally impossible without 5SBs, as a "look how many sealsmith-hours per month I can afford to permanently commit to maintenance" status display.