Module 5 · Lesson 2
The 4-Foot Setback Rule
Cities can't require more than 4 feet of side and rear setback for most new ADUs — and conversions keep their existing setbacks. The rule in plain English.
This is the shortest, most powerful rule in ADU law, so this is a short lesson.
The rule: for most new ADUs, cities cannot require more than 4 feet of setback from your side and rear property lines.
Why it's a big deal: typical zoning demands 5 to 25 feet for regular structures. The 4-foot rule means your ADU can use corners of your lot that nothing else legally could. On a modest lot, those reclaimed strips often are the difference between fitting an ADU and not.
Three fine points:
- It's a cap, not a mandate. Cities can allow less than 4 feet; they can't demand more for a qualifying ADU.
- Conversions do even better. An existing garage or structure converts in place with its existing setbacks — even zero.
- Front setbacks are separate. Local front-yard rules generally still apply — but they can't be used to prevent you from building at least an 800 SF unit.
The practical move: when sketching where an ADU could go, draw the 4-foot lines along your side and rear fences first. The space inside those lines (minus easements — Module 2) is your legal playground.
Key takeaways
- Cities can't require more than 4 feet of side/rear setback for most new ADUs.
- Converted existing structures keep their existing setbacks — even zero.
- Front setbacks follow local rules but can't block an 800 SF unit.
- Draw the 4-foot lines first when sketching placement — that's your legal envelope.