How do you model carrying costs on a conversion when the permitting timeline is genuinely unknowable

Ran into almost exactly this on a mill conversion in Spartanburg, permitting came in at 14 months after I modeled 10, cost me about $58k in unplanned carry and nearly blew my contingency. What I do now is model the worst credible case, not the theoretical worst, meaning I call the city historic preservation office directly and ask what the longest permitted project in that use category closed in the last 24 months, then I add two months to that number. If the deal does not survive that number I walk, full stop, because the downside is not symmetric, a bad permitting outcome can crater the whole stack while a fast permitting outcome just means you made more money. The weighted average approach sounds rigorous but you are weighting outcomes you do not actually have probability distributions for, so you are dressing up a guess in math clothing.

The $4,200 a month number is actually the starting point I'd use, not the problem. What I did on my first conversion prep was go directly to the city planning office and ask them to show me the permit log for the last five adaptive reuse applications, not the finished projects people brag about, the actual submission-to-approval timestamps. Greensboro is not huge and that data is public record. The 22-month project probably hit a specific snag, a historic overlay review or a fire egress dispute, that yours may not trigger at all, and knowing which one tells you whether your project looks more like the 4-month comp or the 22-month one. I would not model a weighted average because the distribution is not random, it's conditional on project type and reviewer caseload. What I actually do, and I am not in a deal yet but I have stress-tested this on paper repeatedly, is run two columns: one where I fund 14 months of carry out of reserves before I even close, and then decide whether the deal still pencils at that carry cost treated as a hard cost, not a variable. If it doesn't pencil with 14 months baked in as a sunk number, I'm out, because $58,800 of cushion above your midpoint is not enough buffer on a mill that size in a market with that kind of variance.

Greensboro's permitting variance is almost entirely a function of whether you hit asbestos or structural surprises mid-review, so I'd want the environmental Phase II in hand before I locked any schedule.