Is there a minimum unit count where ground-up actually starts to make sense financially

Twelve can work but your soft costs per door have to stay under roughly $18k to $20k or the deal bleeds out before you break ground. The number I would want to know is what your local jurisdiction charges for system development charges per unit, because in some PNW markets those alone run $15k or more and that is before a single architect invoice.

Soft costs aren't the killer, land basis per door at 12 units usually is.

Did a 16-unit ground-up in Tacoma in 2021 and the math worked, but just barely, and only because I already owned the dirt. Soft costs ran about $28k per door all in, architect, geotech, civil, permits, the whole pile, and on a 12-unit that same number would have made me walk. What saved us was getting the construction loan at a spread that didn't destroy the yield-on-cost, which took three lenders to find. The 30-to-40 number people throw around isn't wrong exactly, it's just that below 20 doors you have almost no room for a cost overrun before the deal is underwater on a refi or sale. If you're not controlling land basis, 12 units in a mid-size PNW market is a very hard ask.