I’m no expert, but my understanding is that cost per floor generally decreases until you cross certain thresholds. For example:
- 1-4 floors – light frame wood (type V) construction
- 5-7 floors – have to add a fire separated concrete podium (type I) under either Type V or Type III construction
- 75’ above the level of fire access – triggers IBC classification as a high rise, which has a ton of additional fire protection/life safety requirements
- 8+ floors – requires noncombustible construction, so no more wood unless it’s mass timber
- 15ish?? floors – at some point you’ll hit the maximum floor area allowed by type 1B construction and will have to move to type 1A construction. the main distinction is that floor slabs need a 2 hour fire rating.
- Then somewhere around 30-35ish floors lateral bracing will become significantly more expensive, vertical transportation of materials during construction becomes more complicated, elevator design becomes more complex. Foundations also get more expensive
- 420’ building height triggers a few more requirements that were added after 9/11, like adding a third fire stair and a second riser for each sprinkler zone
So, yeah, it’s definitely not a linear cost increase, and there’s a sweet spot in there once you hit type 1A construction – where RUSBUS happens to sit – in which I feel like adding floors should decrease cost per unit. At least in theory. 