# §150.2(c): Existing-plus-addition
What it is
§150.2(c) is an optional whole-building compliance path that combines:
- The existing residence (which would normally stay grandfathered under §150.2(b))
- The new addition (which would normally be modeled separately under §150.2(a))
into a single Title 24 performance model. The compliance engine then compares the proposed combined building against a "standard design" that treats the existing portion as prescriptive-equivalent and the addition as its own §150.1(c) target.
It's a niche path. Most addition projects pick the simpler split-zone treatment under §150.2(a) + §150.2(b), where the addition is its own zone and the existing residence is left alone. §150.2(c) only pays off when the designer wants to trade compliance margin across the boundary — e.g., a high-performance addition envelope buying back an alteration to the existing furnace that would otherwise fail §150.2(b) on its own.
When it applies
- Addition + alteration projects where the designer wants to combine compliance.
- Voluntarily — neither §150.2(a) nor §150.2(b) forces it.
- Only on the performance path under §150.1(b) — the prescriptive path doesn't accept whole-building trades across new + existing.
When you'd choose it (vs the default split)
- The addition's envelope is dramatically better than the existing — e.g., 2x6 R-21+R-5 CI new walls next to 2x4 R-13 cavity-only existing. The performance margin from the new walls can buy a furnace upgrade or DHW upgrade in the existing portion.
- The existing residence is undergoing substantial alteration (multiple components replaced) and §150.2(b) component triggers are getting tight on their own.
The default split is easier and almost always works — §150.2(c) is the escape hatch when the math is tighter than the simple split allows.
How we use it
Our service runs the simpler default: §150.2(a) for the addition, §150.2(b) for any altered components on the existing residence. The addition is modeled as a separate zone and the existing residence is referenced only at the IntWall boundary.
If a project clearly needs §150.2(c) treatment — heavy whole-house alteration plus a substantial addition — we flag it during plan review and discuss with the designer before generating the compliance verification report. It's a per-project conversation, not the default.
Common gotchas
- Designers picking §150.2(c) thinking it lets them avoid §110 mandatory measures on the existing residence — it doesn't, §110.x always applies to altered components.
- Whole-building models that look better on paper because the existing residence's energy use is now averaged across more floor area — the standard-design baseline rises proportionally so the apparent win is gone.
- Mixing §150.2(c) for energy compliance with §150.2(b) for ventilation calc — the two have to use the same scope definition or the AHJ flags it.