Project Basis Gate
This article explains the 2024 IMC model-code path. Before project use, confirm the adopted code edition, local amendments, controlling administrative provisions, permit-application date, and current AHJ procedures. The locally adopted requirements control.
This section explains how and when the IMC applies-especially in cases of conflict, existing systems, and special situations.
Plain English:
Conflicts (102.1): Where there is a conflict between a general requirement and a specific requirement, the specific requirement is applicable. Where conflicts exist between different sections of the IMC, the most restrictive requirement governs.
Existing Systems (102.2): Lawfully installed systems can stay, but additions/repairs/replacements must meet today's code without making the overall system less safe than before.
Maintenance (102.3): All systems - new and existing - must be maintained in safe, sanitary, and working order. Maintenance follows ASHRAE/ACCA/ANSI 180.
Change in Occupancy (102.4): Any change in building use or occupancy must be confirmed with the code official - new occupancy may require mechanical system upgrades to comply with current code.
Referenced Standards (102.5): Chapter 15 standards apply at the edition listed, unless they conflict with an express IMC requirement - the IMC governs.
Gaps (102.6): If the IMC doesn't cover something, the code official determines the applicable requirements using nationally recognized standards, test data, or technical opinion.
Other Laws: Local, state, and federal law continue to apply independently.
On Plans: Why It Matters in Design & Construction:
Prevents shortcuts: new work must meet current code.
Clarifies how to handle alterations, renovations, and occupancy changes.
Keeps older systems running legally but ensures upgrades aren't unsafe.
Code Path: What to Show
M-001: Note whether this is new work, an alteration, or an addition - identify which portions are existing vs. new.
Flag any occupancy changes on the cover sheet with both the new use and the prior use.
For referenced standards: cite the Chapter 15 standard and edition relied on.
Check: Before You Submit
Confirm with the AHJ whether the scope disturbs existing conditions and triggers upgrade compliance.
For occupancy changes: confirm AHJ approval before submitting the permit.
Verify alterations don't make the overall system less compliant than it was before.
Review Risk:
Don't assume existing systems can remain without AHJ confirmation.
Don't skip occupancy change notification - it requires formal AHJ approval.
Don't omit the code edition when referencing Chapter 15 standards.
Field Tip:
When working on an existing building:
Key point: Always confirm with the AHJ whether your scope triggers new code compliance or if existing conditions can remain.
Skipping this step is one of the top causes of plan review delays.
Masterbuild QA Lens
Applicability disputes most often arise on existing-building projects where the team assumes prior-code work is grandfathered. It is not - additions and alterations must meet current code, and occupancy changes trigger a full review. Before submittal, identify every scope element touching existing systems and confirm the compliance basis for each.
Drawing / Submittal Check
For alterations and additions: M-001 must identify which portions are new work vs. existing-to-remain, the code edition applying to the existing system, and the code edition applying to new work. Occupancy-change projects must note the previous use and the proposed use with confirmation that the change has been approved or is being reviewed simultaneously.
Common Review Risk
Existing-building alterations generate more applicability comments than any other project type. The most common: reviewer cannot determine which portions are new vs. existing-to-remain, or the existing system does not comply with current code and the alteration triggers a full upgrade requirement that was not planned or budgeted. Identify and document all existing-condition assumptions on M-001 before submittal.
When To Escalate
Escalate when an AHJ determines that an existing system must comply with the current code edition - expanding scope or cost the project hasn't budgeted - or when a conflict between referenced standards cannot be resolved without a PE-documented interpretation.