What an ADU can and cannot be in California
State law now overrides much local resistance to accessory dwelling units. The remaining constraints are mostly physical rather than political.
Accessory dwelling units — second units, granny flats, converted garages — went from broadly discouraged to broadly protected through a sequence of state laws that removed the tools cities used to say no.
Ministerial approval
The most important change is procedural. A compliant ADU application is approved ministerially, meaning it is checked against objective standards rather than judged at a hearing.
That removes the discretionary stage where projects previously died. There is no design review to fail and no neighbour objection to lose, provided the application meets the standards.
There is also a statutory clock. If a jurisdiction does not act within the required period, the application is generally deemed approved.
The limits that remain
- Size caps, which vary by whether the unit is attached, detached or a conversion.
- Setbacks from rear and side property lines, though reduced from typical residential standards.
- Height limits, which have been raised but still exist and vary with context.
- Building code and fire access, which are safety requirements rather than planning preferences.
Junior ADUs
A smaller category carved out of existing space within the main dwelling, with its own reduced requirements and typically an owner-occupancy condition. Useful where a full ADU does not fit.
“Most ADU projects that stall now stall on utilities and cost, not on permission.”
Practical points
- Design to the objective standards exactly; the ministerial path depends on compliance.
- Get the utility connection assessed early. Sewer capacity and panel upgrades are where budgets break.
- Check whether the jurisdiction has adopted pre-approved plans, which shorten the process further.
- Confirm the current size limits before drawing anything, since they have changed repeatedly.
