Why the scope distinction matters
The compliance baseline, the PV mandate, the equipment grandfathering rules, and the validator set that decides which HVAC + DHW combinations actually pass all branch off the scope determination. Pick the wrong scope and the entire compliance run is wrong — either over-built (NC rules applied to an addition unnecessarily) or under-built (Add+Alt grandfathering applied to a project that should be NC).
The decision starts with one question: is there an existing residence on the lot that's staying?
- No existing residence → Newly Constructed.
- Existing residence stays, no work on it → some flavor of Add-Alone (ADU or non-ADU).
- Existing residence stays, work happens on it → Addition + Alteration or Alteration Only.
From there, the type of new construction (detached vs attached, ADU vs non-ADU) determines which code path applies.
Newly Constructed (NC)
What it is. A brand-new low-rise residential building on a lot that doesn't have an existing residence (or where the existing residence is being torn down to the foundation and rebuilt). The whole building is new — every wall, every piece of equipment, every window.
Code reference. Title 24 Part 6 §150.1. The prescriptive baseline in Table 150.1-A applies to every component. The §150.1(c)14 PV mandate applies to the whole CFA.
What's different about it. No equipment is grandfathered — every HVAC, DHW, ventilation, and lighting line on the Title 24 report (CF1R) reflects a new install. The compliance baseline is the strictest of any scope; TDV multipliers in the 2025 code penalize gas-burning equipment heavily enough that NC compliance fails with gas in nearly every climate zone. Most NC projects ship all-electric (heat pump heating + cooling, HPWH water heating).
Common examples. Custom new home on a vacant lot. A spec home built by a production builder. A teardown rebuild where nothing of the original residence remains.
Addition + Alteration (Add+Alt)
What it is. Work on an existing residence that adds new conditioned floor area (the "Addition" part) and/or alters existing building systems (the "Alteration" part). The original residence stays. The addition is a new piece of conditioned space attached to or connected with the existing.
Code reference. §150.2(a) for additions, §150.2(b) for alterations. Both reference Table 150.1-A for the prescriptive baseline of the new work, but explicitly do not invoke §150.1(c)14 — no PV mandate.
What's different about it. Existing equipment is grandfathered. The customer can keep an existing gas furnace, an existing gas tankless water heater, an existing single-pane window assembly — and only the items being altered are subject to the new code. The compliance baseline for the new work is still based on the climate zone's prescriptive values, but the comparison is against a "standard design" that's a mix of existing-as-is and new-meets-prescriptive.
Our compliance engine models Add+Alt projects as two conditioned zones:
the existing residence with Status = Existing, and the addition with
Status = New. The two are connected by an interior wall when the
addition is attached; detached additions skip the interior wall and treat each face
as exterior. The conditioned floor area sent to the compliance engine is the existing
residence's CFA, with the addition's CFA added separately on the Addition zone — this
is how the compliance engine distinguishes which loads attribute to which compliance basis.
Common examples. A 600 sf bedroom addition on the back of a 1980s ranch home, where the existing HVAC is being extended to serve the new bedroom. A kitchen remodel that includes punching out a wall to add a 200 sf breakfast nook. A whole-house renovation that adds a second story.
Grandfathering caveats. Equipment is only grandfathered if the project scope doesn't replace it. If the customer is doing an Add+Alt that includes replacing the existing gas furnace with a new heat pump, the heat pump is the "new" equipment for compliance purposes and has to meet the prescriptive HSPF2 / SEER2 / EER2 tier for the climate zone. The grandfathering exemption is specifically for equipment the customer is keeping.
Alteration Only
What it is. Work on an existing residence that doesn't add any new conditioned floor area. Replacing windows, replacing HVAC, replacing a water heater, re-roofing, adding insulation to existing walls or attic, kitchen or bathroom remodels that stay within the existing footprint.
Code reference. §150.2(b). The prescriptive baseline applies to the altered components only; everything else is grandfathered. No PV mandate.
What's different about it. The compliance run uses a single conditioned zone (no addition zone needed). The standard design compares altered-component-to-prescriptive while leaving the rest of the building as-is. This is the cleanest of the Add+Alt-family scopes because there's no addition geometry to reconcile.
Common examples. Replacing a 1990s split-system AC with a new heat pump. Replacing all the windows in a 1970s home with NFRC-certified low-e glazing. Re-roofing with a cool roof in CZ10. Swapping a gas water heater for a HPWH.
Add-Alone — the special case
What it is. An addition or ADU on a lot that already has an existing residence, modeled as a standalone compliance run rather than as the addition zone of a two-zone Add+Alt. The existing residence isn't part of the compliance scope — the addition or ADU is treated as its own structure.
Why it exists. Historically, modeling an addition as its own standalone run let designers skip the existing residence's geometry entirely (which they often don't have plans for) and focus on the new work. The Title 24 compliance software still supports this as a project type. But the 2025 ADU FAQ from the CEC reinterpreted which of these standalone runs count as "newly constructed" under §150.1(c)14:
- Detached ADU as standalone Add-Alone → treated as Newly Constructed. §150.1(c)14 PV mandate applies. Uses the NC validator set for equipment combinations.
- Attached ADU as standalone Add-Alone → treated as an addition. Our compliance engine reroutes the project through the Add+Alt path. §150.1(c)14 does not apply.
- Non-ADU addition as standalone Add-Alone → also treated as an addition. Same rerouting. §150.1(c)14 does not apply.
The reroute is automatic. When the project's confirmed scope is "Add-Alone" and the ADU type is anything other than Detached (or the project isn't an ADU at all), our engine patches the run scope to "Addition and/or Alteration", moves the user-supplied addition CFA into the addition-zone slot, and falls back to a CFA-rectangular default for the existing-residence geometry (since the LLM-extracted walls describe the new addition, not the existing residence).
Why the reroute matters. Treating an attached ADU as NC would invoke the PV mandate, the all-electric TDV penalty, and the NC validator set — none of which match the regulatory intent for what is, functionally, an addition to an existing residence. The reroute makes the compliance result match the regulatory framing without requiring the customer to relabel their project.
CFA convention for Add+Alt
One trap worth flagging: for Add+Alt projects, the conditioned floor area sent to the compliance engine is the existing residence's CFA, not the combined existing + addition CFA. The addition gets its own zone with its own CFA. Adding the two together and submitting that as the project CFA causes the run to over-attribute loads to the existing residence (which has older, grandfathered envelope assumptions) and produces compliance results that don't match either zone.
On the Confirm step of this site, the Addition geometry section is what captures the addition's CFA separately. The "total living area" readout that shows existing + addition is for the customer's reference only — the compliance engine never sees the sum as a single number. The same is true for rerouted Add-Alone projects (attached ADU and non-ADU): the addition's CFA goes into the addition-zone slot; the existing residence's CFA goes into the main-zone slot.
Scope decision cheat-sheet
| Project description | Scope | PV mandate? | Equipment grandfathered? |
|---|---|---|---|
| Brand new house on vacant lot | Newly Constructed | Yes | No |
| New house + new detached ADU together | Newly Constructed (NDU=2) | Yes | No |
| 600 sf bedroom addition + existing residence stays | Add+Alt | No | Yes |
| Window replacement + HVAC swap, no addition | Alteration Only | No | Yes (for items not replaced) |
| New detached ADU, existing residence already on lot | Add-Alone (NC path) | Yes | No (ADU itself is new) |
| New attached ADU, existing residence already on lot | Add-Alone (rerouted to Add+Alt) | No | Yes (for existing residence equipment) |
| Standalone 800 sf addition modeled without existing residence plans | Add-Alone non-ADU (rerouted to Add+Alt) | No | Yes (for existing residence equipment) |
Small-project exemptions in §150.2(a)
The 2025 §150.2(a) carries a couple of small-project exemptions worth knowing for Add+Alt scope:
- §150.2(a) Exception 1 — cool roof + radiant barrier waiver for additions ≤ 300 sf. The addition's roof doesn't need to meet §150.1(c)11. The existing residence's attic isn't a §150.1(c)11 target on Add+Alt regardless of size.
- §150.2(a)1B5 — QII waiver for additions ≤ 700 sf. Quality Insulation Installation verification (a pre-drywall HERS/ECC Rater inspection) is not required on additions below this size. Additions above 700 sf and all NC projects keep QII per Table 150.1-A.
Both are automatic in our compliance engine — when the addition CFA falls below the threshold, the corresponding requirement is gated off in the compliance model. No designer action needed beyond getting the addition's CFA correct on the Confirm step.
Practical takeaway
Most California residential projects under permit today are some flavor of Add+Alt or Add-Alone. Pure NC is roughly 20–25% of single-family permit volume; the rest are additions, renovations, or ADUs on existing residential lots. Knowing which scope your project falls under before you start the compliance run determines the entire cost structure — Add+Alt projects benefit from grandfathering and skip the PV mandate, while NC projects carry both. Mislabeling scope wastes money on either side of the decision.
On this site, the scope is captured in the Classify step right after upload. If you're not sure which applies, the safest fallback is "Addition and/or Alteration" — it accepts both attached and detached additions and is the path most ADUs end up on after the 2025 reinterpretation. The compliance engine will route correctly from the scope label plus the ADU type.