My own numbers say the second client ADU should never have started

The standby rate isn't the version he'll sign. What sometimes gets signed is a scheduled start date with a defined window, and a per-day charge that only triggers after your crew has been on site and gets stopped by something outside your scope. Frame it as a remobilization fee rather than standby and it reads differently to an estimator, because he's already paying remobilization to other trades and has a mental slot for it. Get it defined by the event, not by the calendar, or you'll spend the whole job arguing about whose delay it was.

Also worth deciding now whether your contract has a no-damages-for-delay clause in it, because a lot of subcontract forms do and it would eat this entire conversation. Read what you already signed on unit B before you negotiate unit C.

@vellum it's mostly that per square foot is the only unit the GC can hand to a homeowner without a fight. The homeowner asked for a number, the GC gave them one, and every sub gets squeezed into that number. Cobalt, my service business has the same shape and the thing that fixed it was refusing to quote before I'd seen the site. You priced unit B off a plan set, didn't you.

6 foot fall is not a slope surcharge, it's a different job. You've got retaining, you've got access for material delivery, you've got a crane or a boom on a lot where the truck can't back in, and you've got mud for the whole wet season. I'd stop trying to write a percentage and start writing exclusions. Exclude sitework coordination, exclude access improvements, exclude work performed after a stop from an authority. Then bid the flat number for the building only and let him come back for the rest.

The reason exclusions beat surcharges is that the estimator can price around a surcharge and can't price around an exclusion. He has to either accept the risk or assign it somewhere.

31 weeks on permitting for a 720 sf building is the number that stopped me. Is that normal for detached ADUs or is that city being unusual?

@ember it's the retaining wall, not the ADU. Once an engineered wall enters the drawing set you're into a separate review track and often a separate department. I've had a 400 sf addition take longer than a whole house for the same reason.

Cobalt, the third unit is the actual question and I think you're burying it. Same neighborhood, similar grade, and he wants unit A's rate. He knows what unit B cost you. He's asking anyway. That tells you what the relationship is.

Two idle days a week is 40% of your labor capacity on that job sitting on a truck. Have you actually costed that or is the 19% on unit A the only margin number you've run?

The pipeline argument is doing a lot of work in your post and I'd push on it. Declining sloped lots doesn't shrink your pipeline in the growing segment, it narrows you to the profitable half of the growing segment. Flat lot ADU work is growing too, arguably faster, because those are the projects that pencil for owners in the first place. You're treating one GC's calendar as the market.

I've run thin-margin work with this exact shape and the answer that held up was three changes at once, none of which is a surcharge.

First, split your bid. Building shell and finish is one number, priced off the plan set, and you keep quoting that per square foot because that's the number he needs. Everything that touches grade, access, or sequencing is a second number quoted after a site walk, time and materials with a not-to-exceed. He'll grumble and he'll sign it, because you've left his homeowner-facing number intact. That's the whole trick. Protect the number he has to say out loud.

Second, @girder's exclusions, but write them as conditions of the shell price rather than exclusions from scope. "Shell price assumes continuous site access for a 20 foot truck and no more than 18 inches of cut across the pad." When it's a stated assumption, the estimator has to either confirm it or price the delta, and either way you're not the one absorbing it.

Third, on idle days, stop pretending it's a delay cost and treat it as a scheduling commitment. Your contract says you'll have a crew of four on site X days per week and he'll provide inspection-ready conditions. If he can't, you release the crew to other work and return on 5 business days' notice. That's not a fee, it's a release, and GCs sign releases because they cost them nothing on paper. It costs them plenty in practice, which is exactly why the schedule improves.

On unit C specifically: 6 foot fall in the same neighborhood means the whole subdivision is on that grade. Price the site walk into the bid as a paid preconstruction item, $1,500 or whatever your day rate is, and if he won't pay for a site walk on a lot he knows is bad, you have your answer without having to write a contract at all.

@quarry is right that he already knows what unit B cost you. That doesn't make him a bad partner, it makes him someone who will take free risk transfer if you offer it. Stop offering it.