ADUs and additions: paying for a house you are not building
Fee schedules were written for full houses. Accessory units and additions generate a fraction of the demand and are frequently quoted the full number anyway.
The mismatch
Impact fees are calculated from service units: projected trips, projected students, projected gallons. A six-hundred-square-foot accessory dwelling unit does not generate a full house worth of any of those. Yet the simplest schedule to administer charges per dwelling unit, flat.
That gap is the argument, and it is a statutory argument, not a constitutional one. Nearly every act limits the charge to a proportionate share of the burden the development actually imposes.
Utility schedules often already have the answer
Some published district rate cards charge accessory dwelling units at a fraction of a full tap, and scale single-family system development fees by dwelling size, with smaller units at roughly half the standard charge.
Those tiers exist in the adopted schedule. Being quoted the standard number does not mean the tier does not apply to you. Ask for the schedule itself, not a verbal quote.
Additions and remodels
Impact fees are normally charged only on the net increase in service units. A kitchen remodel adds none. Converting an attic into two bedrooms may add some.
State the scope in writing when you apply. If you are not adding bedrooms, bathrooms, fixtures or floor area, say so on the application rather than waiting to dispute the invoice.
Waivers you can ask for
About half the enabling acts explicitly authorize waivers, most commonly for affordable housing and, less often, for economic development projects. Where a jurisdiction has an ADU incentive program, it usually lives in the same ordinance.
Waivers must be requested, and generally before assessment. Half of the acts that allow waivers also require the jurisdiction to backfill the fund from other revenue, which is worth knowing: it explains why the answer is sometimes a reluctant yes rather than a no.