The lift station was full and nobody told me until after closing
Small one, 1.1 acre infill lot in a first ring suburb, zoning already allowed 12 units by right. I paid $310k, closed with a 21 day due diligence window because there were two other offers and I wanted it.
What I did in those 21 days: title, survey, a phase one, a soils report, and a call to the planning department where a nice person confirmed the zoning. What I did not do was write to the utility and ask for a will-serve letter.
Six weeks after closing my civil engineer sends the capacity request in. The lift station serving that basin is at capacity. Anything over about 4 units needs an offsite upgrade. The utility's number for my share of that upgrade was $480k and the queue for the work was roughly 14 months, with no promise the queue held.
Four units does not cover $310k of land in that submarket. I tried the 4 unit version for two months and it came out negative before I paid myself anything.
Sold it eleven months later for $268k to a builder doing two detached houses. Add $41k of architecture and civil, about $9k of carry, taxes and insurance, closing costs both directions. All in I was down about $118k.
What I'd do differently, plainly: the will-serve or capacity letter is a closing condition, same as title. If the seller won't wait for it, I ask for a shorter feasibility period with a bigger nonrefundable deposit and I send the utility request on day one instead of day thirty. Zoning tells you what you may build. Utilities tell you what you can build, and those are separate questions.