Existing + Addition + Alteration: How the Performance Method Works
The software models the whole house and sorts each feature as existing, altered or new. The standard design leaves unaltered parts as they are, sets altered parts to the better of existing or Section 150.2(b)2 values, and gives the addition the prescriptive package. The project passes if its long-term system cost is no higher.
- Choose your report
- Upload your plans
- Pay securely
- Permit-ready report by email
What goes in the standard design
Section 150.2(a)2B builds the standard design from three groups: unaltered existing components, altered components at the more efficient of their existing condition or the Section 150.2(b)2 level, and the addition meeting Section 150.2(a)1. The proposed design uses the real features of each. Existing structures with at least R-11 framed walls are excused from the mandatory wall insulation of Section 150.0(c).
Labels decide the answer
Each modeled feature is entered as existing, altered or new, and the compliance manual calls that identification critical (Example 9-52). Removed walls and windows are ignored. A garage ADU plus a house addition is modeled as separate zones (Example 9-7a). Inputs come from scaled plans, sections and elevations.
The budget and the margin
Additions and alterations are judged on long-term system cost (LSC), not on the source energy used for new homes. The proposed design complies when all mandatory measures are met and its total LSC is the same as or lower than the standard design. The headroom left over is the compliance margin, and every departure from the prescriptive package reduces it.
Why upgrades do not always count
Credit needs a better-than-baseline feature. Once ducts are altered, the standard design already assumes sealing, so sealing earns nothing (Example 9-54). Once the attic or roof is altered, the standard design becomes the Table 150.1-A package, so a new radiant barrier earns nothing (Example 9-55). To take credit for an old window or wall, an ECC-Rater verifies existing conditions before construction, and the registry's CF3R-EXC-20-H must agree before the CF1R can be registered.
The paperwork
The result is a CF1R-PRF-01-E with two lists, special features and modeling assumptions, and features that need ECC verification. It replaces the prescriptive addition and alteration forms. The prescriptive alternative is compared in prescriptive or performance for additions.
Background: prescriptive versus performance, the CF1R, room additions and residential reports.
Common Title 24 Questions
- When is Title 24 required for a home?
- Does a remodel need Title 24?
- Does a room addition need Title 24?
- What plans are needed for a residential Title 24 report?
- How much does a residential Title 24 report cost?
- How long does a residential Title 24 report take?
Frequently Asked Questions
No. The manual states there is no requirement to make alterations to the existing building under this approach, though plans and data for the existing building are needed.
Modeling it lets improvements to the existing house offset a shortfall in the addition, which an addition-alone model cannot do.
Explore Performance Calculations
- Prescriptive vs Performance Compliance for Home Additions
- Prescriptive vs Performance Compliance
- How Much Compliance Margin Should I Keep Before Construction?
- Title 24 Trade-Offs: A Worked Example of What Can and Cannot Move
- What Does Compliance Margin Mean on a Title 24 Report?
View all Performance Calculations guides → · Reports and Calculations · Title 24 guide