Accessory Dwelling Unit Size Limit Calculator
Estimate the maximum allowable size of an Accessory Dwelling Unit (ADU) based on your primary dwelling size, lot size, and common regulatory standards.
Formulas Used
Step 1 — Percentage-of-Primary Cap:
Cap₁ = Primary Dwelling Size (sq ft) × Percentage Factor
Percentage factors: Detached/Attached: 35%–75% | JADU: 50% (jurisdiction-dependent)
Step 2 — Absolute Maximum Cap:
Cap₂ = Jurisdiction & Type Absolute Limit
Ranges from 500 sq ft (JADU) to 1,200 sq ft (CA Detached/Attached)
Step 3 — Lot Coverage Cap:
Cap₃ = (Max Coverage % − Current Coverage %) × Lot Size (sq ft)
Max coverage: 40%–50% depending on jurisdiction and type
Step 4 — Final Limit:
ADU Size Limit = min(Cap₁, Cap₂, Cap₃)
Subject to an absolute minimum floor (150–200 sq ft)
Assumptions & References
- California limits based on AB 68, AB 881, AB 3182 (effective 2020) and AB 2221 / SB 897 (2023): detached/attached ADUs up to 1,200 sq ft; JADUs up to 500 sq ft.
- Oregon limits based on HB 2001 (2019) statewide ADU mandate: up to 900 sq ft or 75% of primary, whichever is less.
- Washington limits based on HB 1337 (2023): up to 1,000 sq ft or 50% of primary dwelling.
- General / Model Code reflects common IRC-based local ordinance patterns (~35% of primary, 1,000 sq ft max).
- Lot coverage maximums are typical values; local zoning may differ significantly.
- Junior ADUs (JADUs) must be within the existing primary structure footprint — no new lot coverage is added.
- This calculator does not account for setbacks, height limits, parking requirements, owner-occupancy rules, or utility connection fees.
- Always consult your local planning or building department for jurisdiction-specific requirements.