What you're looking at
A plan-view schematic of a single-story residence with a one-room addition. The existing air handler in the garage feeds the original rooms through the original duct trunk; a new branch extends from the same trunk into the addition.
When this works
- Attached additions only. The supply branch has to physically reach the new zone; if the addition is detached (or sits across a courtyard), a separate mini-split is required instead. - AHU has spare capacity. Title 24 §150.0(h) requires Manual J/S sizing on the whole new conditioned floor area, not just the addition -- if the existing equipment was sized exactly for the original CFA, the addition pushes it over. - Duct routes don't add 40' of leaky run. Extending ducts through unconditioned attic adds heat-gain + leakage that the prescriptive duct-R + duct-leakage limits can absorb up to a point.
When you'd switch to a separate mini-split instead
- Detached ADU or detached addition. - Existing AHU is undersized or running on emergency electric resistance. - Existing ductwork is severely undersized and a re-trunk would be more invasive than a new system.