What you're looking at
The main residence keeps its existing HVAC. The detached ADU (or detached addition) has its own single-zone ducted or ductless mini-split: one indoor head + one outdoor condenser, connected by a refrigerant line run between the structures.
Why extended ducts don't work here
The supply trunk lives inside the main residence. Once you cross a property line or even a covered breezeway, you're routing conditioned air through outdoor space -- which the code does not allow and which would lose almost all of its thermal capacity to the ambient before it reaches the zone.
The compliance pipeline auto-coerces detached additions to topology = separate_minisplit regardless of the user's preference.
What you need to spec
- Mini-split rated to the addition's design load (Manual J in, Manual S out). - HSPF2 / SEER2 / EER2 that match or exceed the prescriptive baseline for the climate zone (or be ready to trade off elsewhere). - Refrigerant line set with the appropriate length / lift derating. - A dedicated 240V circuit for the outdoor unit.