Merced County — Overlay Districts

Last reviewed: July 6, 2026

Overview

In unincorporated areas, Merced County implements “overlays” through the Special Planning Zone (SPZ) framework in the Merced County Zoning Code (Title 18). An SPZ can be applied as an overlay that adds regulations on top of a base zoning district; where the overlay and base standards conflict, the overlay controls (§ 18.18.010(C)).
Each SPZ is created by ordinance, mapped on the County’s official Zoning Map, and tailored to the place or resource it protects, with its own permitted uses, review procedures, and development standards set in the adopting ordinance (§ 18.18.010(B); § 18.06.030).

Plain-English: An SPZ overlay is a second zoning layer in unincorporated areas; if its rules differ from the base zone, the overlay’s rules win (§ 18.18.010(C)).

Linking context: Base zoning and allowable uses live under Merced County Zoning and Merced County Land Use. Parcel-level dimensional rules are in Merced County Development Standards. Overlay-driven conditions often interact with Merced County Design Review, Merced County Parking, and Merced County Signage. If an overlay creates a conflict with prior rights, see Merced County Nonconforming Uses and, where relief is needed, Merced County Variances and Exceptions.

How Merced County Overlays Work

• The County’s SPZ may be mapped two ways: by enumeration (e.g., SPZ (1), SPZ (2)) or by special use name (e.g., Airport SPZ, Historic SPZ, Wetland SPZ) (§ 18.18.010(B)(1)–(2)).
• An SPZ may be applied as an overlay to any base zone and is labeled together with the base zone on the Zoning Map (e.g., A‑1 SPZ, R‑1‑5000Historic SPZ) (§ 18.18.010(C)).
• The Zoning Map is the controlling map for zone and overlay boundaries in unincorporated areas (§ 18.06.030).
• Establishing an SPZ (including overlays) requires a zone-change ordinance processed like other Zoning Map amendments (§ 18.18.010(B), citing Chapter § 18.142).
• Each SPZ’s allowed uses and standards are set in its adopting ordinance; the County can include review procedures, development standards, and design guidelines within the SPZ regulations (§ 18.18.010(B)).

District-by-District: Merced County Overlay and Combining Districts (Unincorporated Areas)

Overlay SPZ — General Overlay Rules

• Purpose: To protect unique land uses/resources beyond typical zoning and to add regulations to base zones as needed (§ 18.18.010(A), (C)).
• Typical permitted uses: Determined by the specific SPZ ordinance; overlays may add, limit, or condition uses beyond the base zone (§ 18.18.010(B)).
• Key dimensional standards: Set by the SPZ ordinance; may modify setbacks, heights, site design, or performance standards (§ 18.18.010(B)). Not found in retrieved materials for any countywide “generic” overlay.
• Where it applies: Any mapped area on the County Zoning Map labeled as an SPZ overlay together with a base zone (§ 18.18.010(C); § 18.06.030).

Airport SPZ (overlay reference)

• Purpose: Identified in the code as a possible “special use” overlay (Airport SPZ) for airport-related compatibility and operations (§ 18.18.010(B)(2)). Detailed countywide overlay standards not found in retrieved materials.
• Typical permitted uses: Not found in retrieved materials.
• Key dimensional standards: Not found in retrieved materials.
• Where it applies: As mapped by the adopting ordinance on the Zoning Map (§ 18.06.030). Verify with the jurisdiction.

Historic SPZ (overlay reference)

• Purpose: Identified as a possible overlay (e.g., “R‑1‑5000Historic SPZ” label) to protect historic resources (§ 18.18.010(C)). Specific standards not found in retrieved materials.
• Typical permitted uses: Not found in retrieved materials.
• Key dimensional standards: Not found in retrieved materials.
• Where it applies: As mapped by ordinance on the Zoning Map (§ 18.06.030). Verify with the jurisdiction.

Wetland SPZ (overlay reference)

• Purpose: Identified as a potential SPZ overlay for wetland resource protection (§ 18.18.010(B)(2)). Standards not found in retrieved materials.
• Typical permitted uses: Not found in retrieved materials.
• Key dimensional standards: Not found in retrieved materials.
• Where it applies: As mapped by the adopting ordinance on the Zoning Map (§ 18.06.030). Verify with the jurisdiction.

Castle Commerce Center and Airport SPZ

• Purpose: A named SPZ that governs the Castle Commerce Center and Airport area; it ensures coordinated development per the Castle Air Force Base Reuse Plan, addresses airport-oriented operations, and sets standards/guidelines for quality development (§ 18.18.020(A)).
• Typical permitted uses: The SPZ includes internal zones with intended uses:
  • A‑M Aviation/Manufacturing: aviation operations, aircraft manufacturing/repair, and related industrial activities.
  • BP Business Park: research and development, offices, light manufacturing.
  • C Commercial: retail, entertainment, offices, services (§ 18.18.020(B)(1)–(3)).
• Key dimensional standards: Set by the SPZ’s adopting ordinance and design guidelines; not specified in retrieved materials (§ 18.18.010(B)).
• Where it applies: Within the mapped Castle SPZ area consistent with the Reuse Plan; see the County Zoning Map (§ 18.06.030; § 18.18.020(B)). Verify with the jurisdiction.

Planned Development (PD) Combining Zone (overlay-like)

• Why included here: While not labeled an “SPZ overlay,” the PD is a combining district the County applies to achieve master-planned outcomes that may modify or supplement base zone rules—functionally overlay-like. It appears in the Zoning Code’s list of zones (§ 18.06.030/Table 1‑1) and has its own process (§ 18.20.030).
• Purpose: To approve a master plan and subsequent development plan that can justify deviations from standard zoning where public benefits are shown (§ 18.20.030(B)).
• Typical permitted uses: Set in the PD master plan implementing the underlying land-use designations; uses, densities, and intensities must be consistent with the General Plan/specific plans (§ 18.20.030(C)(1)).
• Key dimensional standards: Established in the PD master plan/development plan (§ 18.20.030). Not found in retrieved materials for any specific PD.
• Where it applies: Parcels rezoned to PD via a Zoning Map amendment; master plan processed concurrently with the PD zone change (§ 18.20.030(B)(2)).

Decision-Relevant Overlay Rules (Quick Reference)

Practical tip: Overlay rules often trigger project-specific conditions through Merced County Design Review, and can affect Merced County Parking, site Landscaping and Screening, and Merced County Signage. Confirm these early to avoid redesign.

Checklist

• Confirm your site is in the unincorporated area and identify the base zone via the official Zoning Map (§ 18.06.030).
• Check if an SPZ overlay applies (look for labels like “SPZ,” “Historic SPZ,” etc.) and obtain the adopting SPZ ordinance (§ 18.18.010(B), (C)).
• Read the SPZ ordinance for permitted uses, dimensional standards, and any special review procedures (§ 18.18.010(B)).
• Where both sets of rules apply, apply overlay standards first when in conflict (§ 18.18.010(C)).
• For Castle SPZ parcels, identify which internal zone (A‑M, BP, C) you’re in and match uses accordingly (§ 18.18.020(B)).
• If mapped PD combines with your base zone, locate the approved master plan/development plan (§ 18.20.030).
• Coordinate any overlay-driven conditions with Merced County Design Review, Merced County Parking, and Merced County Landscaping and Screening.
• If existing uses or structures don’t meet new overlay rules, review Merced County Nonconforming Uses; if needed, explore Merced County Variances and Exceptions.
• Building design still must comply with the California Building Standards Code separately from zoning.

Risks & Ambiguities

Plain-English Summary

In unincorporated Merced County, overlays are applied through Special Planning Zones (SPZs). They sit on top of your base zoning and override it where they conflict. Each overlay is created by ordinance, shown on the County Zoning Map, and comes with its own allowed uses and standards. If you’re in the Castle SPZ, your sub‑zone (A‑M, BP, or C) guides what you can build. When in doubt, look up the Zoning Map, find the overlay’s ordinance, and apply the overlay rules first (§ 18.18.010; § 18.18.020; § 18.06.030).

Information Gaps

• Countywide standards for “Airport SPZ,” “Historic SPZ,” and “Wetland SPZ” beyond their identification as possible SPZ overlays: Not found in retrieved materials.
• Parcel-specific Castle SPZ dimensional standards or design guidelines: Not found in retrieved materials.
• The full text of Chapter § 18.142 (Amendments): Not found in retrieved materials.
• Nonconformities article section numbers: Not found in retrieved materials.
  Verify with the jurisdiction.

Source References

• Merced County Zoning Code Title 18 — Applicability to unincorporated areas (§ 18.02.030).
• SPZ purpose; creation; overlay mechanics; precedence; labeling (§ 18.18.010(A)–(C), citing § 18.142).
• Castle Commerce Center and Airport SPZ purposes and internal zones (§ 18.18.020(A), (B)(1)–(3)).
• Zoning Map authority for boundaries (§ 18.06.030).
• List of zones including SPZ and PD (Table 1‑1) (§ 18.06.030 context).
• PD combining zone procedures and standards (§ 18.20.030).
• County planning authority and review bodies (Article VII overview) (§ 18.140.010–.040).

Retrieved passages

• Merced County Zoning Code (§ 2) High relevance show source text
• Merced County Zoning Code Medium relevance show source text
• Merced County Zoning Code (§ 2) Medium relevance show source text
• Merced County Zoning Code (§ 2) Medium relevance show source text
• Merced County Zoning Code (§ 2) Medium relevance show source text
• Merced County Zoning Code (§ 2) Medium relevance show source text
• Merced County Zoning Code (§ 2) Medium relevance show source text
• Merced County Zoning Code (Section 18.20.30) Medium relevance show source text

Cited sections

• Merced County Zoning Code Title 18 — Applicability to unincorporated areas (§ 18.02.030). (Title 18)
• SPZ purpose; creation; overlay mechanics; precedence; labeling (§ 18.18.010(A)–(C), citing § 18.142). (§ 18.18.010)
• Castle Commerce Center and Airport SPZ purposes and internal zones (§ 18.18.020(A), (B)(1)–(3)). (§ 18.18.020)
• Zoning Map authority for boundaries (§ 18.06.030). (§ 18.06.030)
• List of zones including SPZ and PD (Table 1‑1) (§ 18.06.030 context). (§ 18.06.030)
• PD combining zone procedures and standards (§ 18.20.030). (§ 18.20.030)
• County planning authority and review bodies (Article VII overview) (§ 18.140.010–.040). (Article VII)
• MercedCounty_ZoningCode.md

• Official Merced County code library