What piece of multifamily development does someone coming from the GC side underestimate most?

Ten percent on hard costs is real money, and a single quarter of extra carry can wipe it out. Run the sensitivity on a small deal and the input that moves the output most is the construction loan rate combined with how long the loan is carried before units start paying rent, well ahead of cost per square foot. A 24 unit project with, say, 5 million in debt costs meaningful money every month it sits unleased. The piece that deserves the same respect as your construction knowledge is the exit cap rate. Value at the end is net operating income divided by the cap rate, and nobody guesses a cap rate three years out with real confidence, so the deal has to work across a range rather than at one number.

Your cost advantage is real, just not where you think. The place a GC background actually pays is in not getting sandbagged on change orders and in knowing which subs will still be standing at month fourteen. Lenders also like a sponsor who can defend a budget line by line.

But yes, the assumption is wrong on its own. Land basis and financing cost swamp construction savings in most deals I've looked at. If you overpay 400k for a site, ten percent off a 4 million build gets you back to even and you've done all that extra work for nothing.

Also worth knowing: lenders will usually want a fixed price contract from a third party GC, or a very good reason why you self-performing is acceptable. That surprises people from your side of the business.