Merced County ADU Requirements (2026)
Size limits, setbacks, required documents, and fees for building an ADU in Merced County, California — covering unincorporated county areas, the City of Merced, Atwater, and Los Banos.
Who this guide is for
If you're planning to build an ADU in Merced County — in unincorporated county land, the City of Merced, Atwater, Los Banos, Livingston, or another city — this guide covers the key requirements before submitting plans.
Merced County spans the San Joaquin Valley with a mix of agricultural land and growing residential communities driven in part by UC Merced. This guide focuses on unincorporated Merced County; city rules may vary.
Jurisdiction overview
| Jurisdiction | Planning authority | Notes |
|---|---|---|
| Unincorporated Merced County | Merced County Planning Division | Agricultural and rural areas |
| City of Merced | Merced Development Services | Home of UC Merced; active ADU market |
| City of Atwater | Atwater Community Development | Suburban; former Castle AFB area |
| City of Los Banos | Los Banos Community Development | Western county, growing area |
| City of Livingston | Livingston Community Development | Contact city for standards |
Size limits
| ADU type | Maximum size |
|---|---|
| Detached ADU | Up to 1,200 sq ft |
| Attached ADU | Up to 50% of existing living area, or 1,200 sq ft — whichever is less |
| JADU (Junior ADU) | Up to 500 sq ft, within existing structure |
| Garage conversion | Full existing garage area |
A single-family property may have both one ADU and one JADU.
Setback requirements
| Setback | Requirement |
|---|---|
| Side and rear (new construction) | 4 ft minimum (state law) |
| Front | Per underlying zoning district |
| Conversion of existing structure | No additional setback required |
| Height limit (detached) | 16 ft |
Rural parcels in unincorporated Merced County may have larger setbacks based on zoning classification (agricultural, residential, or mixed). Confirm with county planning before siting your ADU.
Required documents
A complete ADU application for unincorporated Merced County typically requires:
- Permit application form — Merced County Planning Division
- Site plan — parcel boundaries, setbacks, existing and proposed structures, utilities, access, drainage
- Floor plan — dimensioned with room labels, fixtures, egress
- Elevations — all four sides, heights, materials
- Structural plans — foundation, framing, lateral bracing
- Title 24 energy compliance — CF1R energy forms
- CalGreen checklist — mandatory green building requirements
- Proof of ownership — deed or title report
- Septic/well documentation — capacity evaluation for parcels not on public utilities
Fees
California limits ADU fees:
- No impact fees for ADUs under 750 sq ft
- Proportional impact fees for ADUs 750 sq ft and above
- No school fees for ADUs
Merced County permit fees are lower than coastal counties. As of 2026:
- Building permit fee: $1,500–$3,500 depending on project valuation
- Plan check fee: Typically 65% of building permit fee
- Utility connection fees: Vary by location
The City of Merced has its own fee schedule, which may differ from county fees. Check with the applicable jurisdiction.
Castle Airport Development Authority (CADA) Area
The area around the former Castle Air Force Base (now Castle Airport) in Atwater has specific land use considerations managed through the Castle Airport Development Authority. If your property is in the CADA influence area, confirm compatibility with CADA planning guidelines before proceeding.
Common reasons ADU plans are sent back
Based on patterns from Merced County and similar Central Valley jurisdictions:
- Septic documentation missing for rural parcels
- No Title 24 energy forms included
- Site plan missing north arrow, scale, setback dimensions, or utility locations
- Structural plans incomplete — no lateral resistance details
- Floor plan not dimensioned sufficiently
- Elevations missing one or more sides
- No construction type or occupancy listed on cover sheet
Running a pre-check before you submit catches most of these issues in minutes.
Sources
Frequently asked questions
Yes. California state law requires Merced County and its cities to allow ADUs on residentially-zoned properties. The county Planning Division handles unincorporated parcels; the cities of Merced, Atwater, Los Banos, Livingston, and others have their own building departments.
Yes. The UC Merced campus has driven significant housing demand in the City of Merced and surrounding areas. The City of Merced actively processes ADU permits, and properties near campus are frequently targeted for ADU construction to house students and faculty. City of Merced rules apply within city limits — the county handles unincorporated areas.
Yes. Many unincorporated Merced County parcels use septic systems and private wells, particularly in agricultural and rural areas. A septic capacity evaluation is typically required for new ADUs on parcels not connected to public sewer. The system must be demonstrated to have adequate capacity for the additional dwelling.
State law requires jurisdictions to act on complete ADU applications within 60 days. The City of Merced has seen increased ADU activity in recent years and has dedicated resources to permit processing. Unincorporated county turnaround depends on application completeness.
Merced County has extensive agricultural land. ADU rules apply to residentially-zoned parcels — agricultural zoning has different provisions. Some parcels near city limits are in a mix of zoning designations. Confirm your parcel's zoning and eligibility with Merced County Planning before proceeding.
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