What is the difference between an addition and an ADU?

The legal distinction drives almost every downstream decision — permit path, financing, zoning, rental rights, and resale value.

Addition:

• Extends the primary residence square footage.
• Typically shares the primary home's kitchen, entry, and utility systems.
• Local building department handles plan review under the International Residential Code.
• Zoning rules apply: setbacks, lot coverage, floor area ratio, height limit.
• Property appraises as one larger single-family residence.
• Cannot be rented as a separate unit in single-family-only zones.

ADU:

• Legally separate dwelling: kitchen, bathroom, sleeping area, independent entrance.
• Attached, detached, garage-conversion, or Junior ADU (JADU within primary).
• California state law (Gov. Code §65852.2) mandates ministerial approval within 60 days; local zoning cannot prohibit.
• Most states now have state-level ADU laws (California, Oregon, Washington, Maine, Connecticut, Vermont, New Hampshire, Massachusetts, more pending).
• Can be rented long-term in most jurisdictions. Short-term rental rules vary by city.
• California AB 1033 (2024) allows ADUs to be sold separately as condominiums in cities that opt in.
• Fannie Mae and Freddie Mac allow ADU rental income in mortgage qualification.

Cost comparison — on a per-square-foot basis, an ADU typically costs 20-40% more than a comparable addition because it includes a full kitchen, bathroom, and utility separation. However, an ADU often returns higher resale value per dollar due to independent rental income.

Financing comparison:

• Additions usually financed via cash-out refinance or HELOC against primary residence equity.
• ADUs can use the same, plus renovation loans (Fannie Mae HomeStyle, FHA 203(k)) that allow borrowing against post-completion value including ADU rental income.
• California-specific: the CalHFA ADU Grant Program offers up to $40,000 for pre-development costs to eligible borrowers.

AskBaily's scoping chat asks whether you want shared living space (addition) or a rentable unit (ADU) up front, because the downstream scope and cost are very different.