For many homeowners, adding an Accessory Dwelling Unit (ADU) to their property is an exciting opportunity to increase living space, generate rental income, or accommodate family members. However, building an ADU isn’t always as simple as it seems—especially when the project involves converting a two-family home into a three-family dwelling.
One of our recent clients encountered this exact challenge. At the very beginning of ever project we do a preliminary zoning and code analysis and talk to our contacts at the building department to clarify any gray areas. The local zoning official approved the addition of an ADU to the two-family home, but the building department official flagged a significant issue: the transition from a two-family dwelling to a three-family dwelling triggered a shift in building code compliance, moving the project from the International Residential Code (IRC) to the International Building Code (IBC).
Here’s what that means and how it could affect your ADU plans.
The Key Difference Between the IRC and IBC
The IRC applies to one- and two-family dwellings and townhouses up to three stories, offering simpler, more homeowner-friendly guidelines. However, as soon as a property becomes a three-family dwelling, it falls under the scope of the IBC, which introduces more rigorous requirements.
The IBC is designed to ensure the safety of larger, multi-family buildings, meaning it includes stricter provisions for fire safety, accessibility, and structural design.
What Triggers IBC Compliance for ADUs?
When adding an ADU, whether or not you must comply with the IBC depends on how the project changes the classification of your property. In the case of our client:
- Zoning Department Approval: Their town’s zoning department classified the ADU as an allowable third unit under local ADU laws, ensuring the project complied with lot size, setbacks, and use regulations.
- Building Department Reclassification: The building department determined that adding a third unit to the two-family home changed the building’s classification to a three-family dwelling. This triggered IBC compliance requirements, including the necessity for:
- Sprinkler Systems: Multi-family buildings under the IBC typically require fire sprinkler systems to enhance fire safety.
- Accessibility Features: Depending on the project, the IBC may also introduce accessibility requirements, such as adaptable entrances or routes for units.
What If the ADU Is Detached?
This requirement may not apply if the ADU is a separate, standalone structure. Detached ADUs often remain classified as single-family dwellings under the IRC, avoiding the additional requirements triggered by a building reclassification.
However, this depends on your jurisdiction’s interpretation of the building codes and zoning regulations, so it’s crucial to consult your local building department early in the design process.
Steps to Navigate the Process
If you’re considering adding an ADU to your property and want to avoid surprises, here’s how to proceed:
- Engage Early with Local Officials: Meet with both the zoning and building departments to understand how they interpret your project and any specific requirements.
- Work with Design Professionals: An experienced architect or designer can help ensure your plans meet IBC requirements, including fire safety and accessibility, or explore alternative solutions. They can also assist you in seeking variance approval from the building department if you are unable to comply with a code requirements due to the conditions of your property.
- Consider Detached vs. Attached ADUs: If sprinklers or other requirements under the IBC pose a challenge, consider whether a detached ADU is a viable option.
- Understand Costs: Adding sprinkler systems or meeting other IBC requirements can increase project costs. Ensure you budget accordingly and discuss financing options if needed.
Conclusion
For homeowners with two-family properties, adding an ADU can bring transformative benefits but also introduce regulatory complexities. At Dwelly, we specialize in helping homeowners navigate these challenges, from understanding local zoning laws to designing ADUs that meet code requirements.
If you’re planning an ADU project, reach out to us to learn how we can help streamline the process and deliver a high-performance design tailored to your needs. Let’s make your ADU vision a reality!
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