Residential Code Definitions of the Day 18 Sep 2025

[jar546]

[RB] SUBSTANTIAL DAMAGE.
Damage of any origin sustained by a structure whereby the cost of restoring the structure to its before-damaged condition would equal or exceed 50 percent of the market value of the structure before the damage occurred.

[RB] SUBSTANTIAL IMPROVEMENT.
Any repair, reconstruction, rehabilitation, alteration, addition or other improvement of a building or structure, the cost of which equals or exceeds 50 percent of the market value of the structure before the improvement or repair is started. If the structure has sustained substantial damage, any repairs are considered substantial improvement regardless of the actual repair work performed. The term does not, however, include either:

1. Any project for improvement of a building required to correct existing health, sanitary or safety code violations identified by the building official and that are the minimum necessary to assure safe living conditions.
2. Any alteration of a historic structure provided that the alteration will not preclude the structure’s continued designation as a historic structure. For the purposes of this exclusion, a historic building shall be any of the following:

2.1. Listed or preliminarily determined to be eligible for listing in the National Register of Historic Places.
2.2. Determined by the Secretary of the US Department of Interior as contributing to the historical significance of a registered historic district or a district preliminarily determined to qualify as a historic district.
2.3. Designated as historic under a state or local historic preservation program that is approved by the Department of Interior.
For the definition applicable in Chapter 11, see Section N1101.6.

 

[Paul Sweet]

I see where these terms were added to R322.1 in the 2015 IRC. This appears to align the code with FEMA regulations.

 

[Yankee Chronicler]

I think it is significant, and important to note, that both definitions are based on market value, NOT on construction cost. For those among us who are old enough to remember the pre-ICC days, for example the BOCA code used to base the degree of required code compliance for alterations repairs on a 25% / 50% rule, where the percentage was a percentage of the "physical value" (i.e. construction cost) of the structure, not the market value. Work under 25% of the "physical value" could be altered or repaired using the same construction as existing, without regard for compliance with current code. Between 25% and 50% of the "physical value" was left to the BO to decide how much conformance should be required but usually boiled down to requiring that new work must meet the current code and existing was allowed to remain if unaltered. Above 50%, the entire building had to be brought up to current code requirements.

All that took up less than a single 6" x 9" page. Now we have an entire book -- the IEBC -- to address that.