01Define the work before choosing the boundary
Addition-alone and existing-plus-addition are modeling approaches, not labels to choose based only on which produces a passing result. First separate existing unchanged components, altered components and new conditioned area. The 2025 CEC single-family manual’s Chapter 9 describes additions, alterations and their performance paths, including existing + addition + alterations.
Under the Energy Code definition discussed in that manual, an addition increases conditioned floor area and conditioned volume, including conditioning previously unconditioned space. A conversion can therefore be an energy-code addition without extending the building’s footprint. Planning or zoning labels do not by themselves settle the energy modeling scope.
Compare addition-alone and combined boundaries in the energy-modeling service example
02A boundary-selection decision tree
Discuss this sequence with the documentation preparer. Eligibility and treatment must follow the applicable edition and software rules.
- +Draw the proposed conditioned boundary and mark the existing house, newly conditioned area and altered systems.
- +If documenting the addition alone, identify the new scope and the required information about its connections to existing systems.
- +If evaluating existing + addition + alterations, identify what existing features must be modeled and how their conditions will be established.
- +If claiming improvement to an existing feature, ask what evidence and third-party verification the selected path requires.
- +If existing conditions cannot be established, document that limitation and compare permitted assumptions or another path rather than inventing favorable values.
03Worked example: a room addition and an older roof
Hypothetical example: an owner proposes a new bedroom and is also considering work on the existing house’s roof insulation. For an addition-alone study, the team identifies the new room’s assemblies, glazing and systems and the applicable interfaces with the house. It does not automatically claim the existing roof upgrade as a credit outside the chosen modeling scope.
For an existing + addition + alterations study, the team gathers existing-building information and asks how the proposed roof work can be represented under the manual and software rules. The comparison includes documentation and possible verification obligations, not just the compliance result. Neither alternative can be declared compliant from this narrative alone.
04Existing-condition evidence checklist
Separate observed facts, records, proposed changes and unknowns. Keep the basis for each modeled condition available for review.
- +Existing and proposed dimensioned plans with conditioned floor areas, volumes and dwelling-unit boundaries.
- +Assembly descriptions and available evidence of insulation; flag concealed or inaccessible conditions.
- +Window information, equipment nameplates, duct location and water-heating scope.
- +Prior permit documents or energy reports, with a note about whether they match today’s building.
- +A list of proposed upgrades and the party responsible for any required existing-condition verification.
05Compare the construction commitments, not just the model
A combined model can require more existing-building information and may depend on upgrades that the owner must actually construct. Do not quietly add a measure to make a model pass if it is absent from the drawings or budgeted scope. Obtain a coordinated decision and carry the selected measures into the final report and plans.
The current default is the 2025 Energy Code for applications on or after January 1, 2026; an older active permit needs the matching rules. Report preparation remains separate from installer certification, ECC verification and mechanical design. Consult Chapter 9 and its referenced existing-feature verification guidance rather than treating a general article as an engineering determination.