The permit desk wants a stormwater plan I never budgeted for

What is a detention feature? I gather it holds water, but is it something a buyer would ever see or is it under the yard?

@juniper both exist. A dry basin is a shallow depression in the yard that fills during a storm and drains out slow. Underground chambers are plastic vaults under the driveway or lawn, invisible, much more expensive. The county cares that runoff leaving your lot after the build is no worse than before it.

The thing I'd want to know is why the first house didn't trigger it. Smaller disturbed area? Different zone? If it was just that nobody looked, that's a different problem, and you might have a house standing that's out of compliance.

@compass smaller lot and I only disturbed maybe a third of it. Total build was 1,600 square feet with a short drive. So probably genuinely under the threshold.

The five weeks versus three weeks gap is only a real conflict if the crew is doing the thing the calcs affect. Site work and grading are exactly what the plan governs. Could he start on something else, or is his crew a single sequence?

The decision here rests on one assumption: whether the 0 to 40k range is really uniform. It usually is not. Detention sizing follows from soil infiltration rate, existing slope, and where you can discharge. Your engineer already knows the soil type and whether there's a receiving ditch or storm main to tie into, so ask him for his expected case rather than his range. A number like "probably 12 to 18k unless the perc comes back bad" is a budgetable number. A refusal to narrow it tells you he hasn't looked at the site closely.

On the sequencing question, starting site work before the plan is approved carries a specific risk beyond cost. If the approved plan later requires a different finished grade or a basin where you have already placed fill, you pay to move dirt twice, and in some jurisdictions work performed ahead of an approved grading permit can trigger a stop work order or a violation. Whether that applies to you depends on your county's ordinance, so read it or ask the permit desk directly before anything moves.

Your margin math: 84 plus 383 is 467 against comps of 520 to 555. Before soft costs you have roughly 53 to 88 of spread. Soft costs at 22 was a guess, and you now have 6,800 of engineering plus permits, survey, utility connections, insurance, and sales cost at closing. If detention lands at 40 you are underwater at the low comp. That single unknown is larger than your entire remaining cushion, which is the argument for waiting three weeks.

The cost of losing the crew is a carry cost on 84k of your own money plus a delayed sale. Price that against the risk of building blind and the comparison gets easier.

@rena-expert the point about asking for the expected case rather than the range is the most useful thing I've read this week. Engineers give ranges because ranges are safe for them.

Underground chambers are the answer when you don't have room, and they're brutal on a spec budget. Excavation, stone bedding, geotextile, the vaults themselves, and then an inspection before you backfill. On a suburban lot I'd assume 25 to 35 all in if it goes that way. A surface basin on a lot with room can be 8 to 12. So the real question is whether your lot has a corner you can sacrifice. Look at your survey and find out if you can give up 1,200 square feet of yard. If you can, tell the engineer to design for surface and stop.

Also, a basin in the front yard hurts resale. Put it in the back if the grade lets you.

@girder does the basin come out of the buyer's usable yard in a way that shows in the appraisal?

@kestrel it shows up as a feature buyers ask about, in my reading. The maintenance obligation is the part I'd want to understand. Somebody has to mow it and keep the outlet clear, and depending on how the county wrote the approval that obligation can run with the land as a recorded agreement. That varies by state and by county, so it's a question for whoever handles your closing.

@anchor is right that the recorded piece matters. If there's a stormwater maintenance agreement recorded against the lot, it becomes a disclosure item at sale and your buyer's lender may ask about it. Worth knowing the shape of before you design the thing, and confirm the specifics with your closing attorney because recording practice differs by state.

The 22k soft cost number was a guess, and nobody told you it was low until the county did. That's what strikes me here, even though I'm nowhere near a build myself.

Numbers I'd want fixed before you decide anything. Your 178 a foot fixed, is that fixed with an escalation clause on materials? Because a fixed price that reopens if lumber moves more than some percentage is not fixed, and lumber has been moving. Second, what is your carry if you wait until spring? 84k of your equity idle for six months plus a spring start pushing the sale to late summer. Third, at the 520 comp with 40k of detention and honest soft costs I get you somewhere near break even. So this isn't a scheduling question, it's whether the deal survives the bad case. If it doesn't, you want to know that now, while the only sunk cost is 6,800.

@ledger the escalation clause question is the one I'd chase today. Every fixed bid I've seen in the last two years had something in it about material price movement.

Ask the permit desk what the actual disturbed area threshold is, in numbers. Sometimes you can redesign the driveway and drop under it. That's a free conversation and it takes twenty minutes.

@halyard that's a good angle but be careful, shrinking the drive to dodge a threshold can hurt you with buyers in a market where everyone wants two cars off the street.