# §150.2(a): Additions to existing residences
What it requires
When you add conditioned floor area to an existing residence, the addition itself has to meet the §150.1(c) prescriptive package, for its own walls, ceiling, windows, and any HVAC capacity newly installed. The existing residence is left alone (it's grandfathered to the code in effect when it was built).
The compliance model treats the addition as a separate conditioned zone attached to the existing residence; the Title 24 2025 compliance engine emits two zones with a shared interior wall.
Key carve-outs from §150.1
- No §150.1(c)14 PV mandate, additions do not trigger solar PV. Period.
- Existing equipment is grandfathered, the existing furnace, AC, water heater stay unless explicitly replaced.
- Existing envelope is grandfathered, the existing walls, ceiling, windows stay unless explicitly altered.
The intent is to remove disincentives that would otherwise discourage homeowners from adding to existing buildings.
When it applies
- Any addition of conditioned floor area to an existing residence.
- Attached ADUs (treated as additions to the existing house, not as new construction).
- "Add-Alone" projects where the addition is built but no alteration is happening to the existing house.
When §150.2(b) applies alongside
If the addition project also alters the existing residence (replaces the furnace, swaps in new windows on the existing walls, etc.), §150.2(b) applies to the existing-residence work in parallel with §150.2(a) on the addition. See §150.2(b), alterations.
How we use it
When you select "Addition" or "Addition and/or Alteration" on the Confirm step, we apply §150.2(a) automatically:
- A separate "Addition" zone is added to the compliance model.
- The new conditioned floor area sizes the addition zone (NOT the existing+addition total).
- Existing equipment defaults to "Keep" unless you check the box to replace.
- PV is exempt, the CF1R shows "Not required (addition / alteration scope)" on the PV line.
See the Add+Alt paths article for the full decision tree.