Building Code Glossary
Accessory Structure: Building Code Definition
How the IRC 2009 define accessory structure, where the wording differs, and which chapters depend on the term.
On this page
Last updated September 7, 2026 · Definitions quoted from the 2009 International Codes as transcribed in this site's building-codes collection, with the source chapter linked for each quotation. Usage counts are computed over the chapters hosted here, one copy per code. The [ICC code library](https://codes.iccsafe.org/codes) publishes the current model text and the [ICC adoption maps](https://www.iccsafe.org/advocacy/code-adoption-maps/) show which edition a jurisdiction has adopted; those sources control.
Direct answer
Accessory structure is a defined term in the International Residential Code (IRC) 2009, in Definitions. The code's own words are: “A structure not greater than 3,000 square feet (279 m2) in floor area, and not over two stories in height, the use of which is customarily accessory to and incidental to that of the dwelling(s) and which is located on the same lot.” No other I-Code in this corpus defines it, so the IRC 2009 definition is the one to work from. The term is used in 14 chapters of this corpus, most often in Roof Assemblies (IRC 2009), Roof Assemblies and Rooftop Structures (IBC 2009), Scope and Administration (IBC 2009). These pages transcribe the 2009 model text; verify the definition against the edition and amendments your authority having jurisdiction has adopted.
Content and cited inputs reviewed by Scott Barrington, Harth founder and architectural technology specialist · Published 2026-09-07 · Updated 2026-09-07.
What does this term mean?
Accessory Structure — IRC 2009, Definitions:
A structure not greater than 3,000 square feet (279 m2) in floor area, and not over two stories in height, the use of which is customarily accessory to and incidental to that of the dwelling(s) and which is located on the same lot.
What must be verified before using this guidance?
Accessory Structure is defined once in this corpus, in the Definitions chapter of the International Residential Code (IRC) 2009:
A structure not greater than 3,000 square feet (279 m2) in floor area, and not over two stories in height, the use of which is customarily accessory to and incidental to that of the dwelling(s) and which is located on the same lot.
Where accessory structure is used in the code
The term appears in 14 chapters of the 2009 I-Codes hosted here. The chapters that lean on it hardest are the ones to read next.
| Chapter | Code | Mentions |
|---|---|---|
| Roof Assemblies | IRC 2009 | 23 |
| Roof Assemblies and Rooftop Structures | IBC 2009 | 18 |
| Scope and Administration | IBC 2009 | 4 |
| Building Planning | IRC 2009 | 4 |
| Foundations | IRC 2009 | 4 |
| Floors | IRC 2009 | 3 |
| Accessibility | IBC 2009 | 2 |
| Means of Egress | IFC 2009 | 2 |
| Scope and Administration | IMC 2009 | 2 |
| Scope and Administration | IPC 2009 | 2 |
| Definitions | IRC 2009 | 2 |
| Scope and Administration | IRC 2009 | 2 |
2 further chapters mention the term.
The terms accessory structure is built out of
The definition above leans on 2 other defined terms. Each carries its own code definition, and a local amendment to any of them changes what accessory structure covers:
- Structure — IPC 2009: That which is built or constructed or a portion thereof.
- Dwelling — IBC 2009: A building that contains one or two dwelling units used, intended or designed to be used, rented, leased, let or hired out to be occupied for living purposes.
Accessory Structure: what to open, and in what order
Every row is a document this term depends on. The ICC code library and the ICC adoption maps cover the middle rows.
| Check | Where | Confirmed |
|---|---|---|
| Read the definition in the adopted IRC | Definitions | |
| Re-check the requirement that uses it | Roof Assemblies (IRC 2009) | |
| Re-check the requirement that uses it | Roof Assemblies and Rooftop Structures (IBC 2009) | |
| Re-check the requirement that uses it | Scope and Administration (IBC 2009) | |
| Re-check the requirement that uses it | Building Planning (IRC 2009) | |
| Confirm the dependent definition | Structure | |
| Confirm the dependent definition | Dwelling | |
| Local amendment to any of the above | Jurisdiction's amendment list |
A research aid, not a code determination. The authority having jurisdiction decides which edition and which amendments apply.
Where can you explore related guidance?
- Explore all glossary guidance
- Structure: Building Code Definition
- Dwelling: Building Code Definition
Sources and method
Definitions quoted from the 2009 International Codes as transcribed in this site's building-codes collection, with the source chapter linked for each quotation. Usage counts are computed over the chapters hosted here, one copy per code. The ICC code library publishes the current model text and the ICC adoption maps show which edition a jurisdiction has adopted; those sources control.
Source links
Frequently asked questions
What is the code definition of accessory structure?
The International Residential Code (IRC) 2009 defines it in Definitions as: “A structure not greater than 3,000 square feet (279 m2) in floor area, and not over two stories in height, the use of which is customarily accessory to and incidental to that of the dwelling(s) and which is located on the same lot.” The 2009 model text is reproduced here; the definition that binds your project is the one in the edition your jurisdiction has adopted, as amended locally.
Which parts of the code depend on accessory structure?
In this corpus the term is used across 14 chapters, led by Roof Assemblies in the IRC 2009. Those are the sections whose scope moves if the definition is amended locally, so read them together with Chapter 2 rather than on their own.
What is considered an accessory structure?
In the 2009 I-Codes hosted here it is a defined term rather than a description. The IRC 2009 states, in Definitions: “A structure not greater than 3,000 square feet (279 m2) in floor area, and not over two stories in height, the use of which is customarily accessory to and incidental to that of the dwelling(s) and which is located on the same lot.” It is defined in IRC 2009. Because it is a defined term, whatever the adopted edition says it means is what every section using it means — so check Chapter 2 of that edition and any local amendment before relying on this wording.
What is considered a structure in building code?
Building is a separate defined term. The IBC 2009 defines it as: “Any structure used or intended for supporting or sheltering any use or occupancy.” That is not the same as accessory structure, which the IRC 2009 defines as “A structure not greater than 3,000 square feet (279 m2) in floor area, and not over two stories in height, the use of which is customarily accessory to and incidental to that of the dwelling(s) and which is located on …” — read both before applying either, and confirm the wording in the edition your jurisdiction has adopted.
Related pages
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Addition — IBC 2009 definition: An extension or increase in floor area or height of a building or structure.