I calculate the carrying cost month by month (interest, municipal and school taxes, insurance, electricity, heating, maintenance) then project it over a longer period than you’re targeting. A project should still work after three unsold months.
Many investors take the best posted rate without looking at exit conditions, then discover the penalty at resale. I compare lenders on what matters for a short project: repayment flexibility, the ability to fund work in stages, and total cost over the real holding period. And if the numbers don’t hold up, I say so before the purchase.