Riverside | Hillsides and Historic Districts

Riverside Hillsides, Arroyos and Historic Districts: Proposition R, Measure C, Grading and Title 20

Two sets of Riverside rules limit what owners may add to a lot beyond the base zone. On the hills and along the arroyos, voter-approved measures and the Grading Code (Title 17) keep lots large and grading small. In the older neighborhoods, the Cultural Resources Code (Title 20) puts changes to historic buildings through design review. Both shape where ADUs, SB 9 homes and new buildings can go. The Riverside overview sums up the whole guide.

Proposition R and Measure C

Proposition R and Measure C are measures that Riverside voters adopted to protect hillsides, arroyos and farmland. An arroyo is a steep-sided natural drainage channel that is dry for much of the year. The City's General Plan Land Use and Urban Design Element says Measure C was "passed by Riverside voters in 1987," and Policy LU-4.1 directs the City to "Adhere to the protections for hillside development set forth in Proposition R and Measure C." Policy LU-6.1 does the same for agricultural areas, and Policy LU-6.3 ties the Arlington Heights Greenbelt, whose "agricultural character" the policy protects, to "applicable provisions of the Subdivision and Zoning Codes in addition to Proposition R and Measure C."

The General Plan also names one neighborhood where the measures replaced older local rules. In Hawarden Hills, "many of the provisions of the plan were superceded by stronger, more protective measures passed by Riverside voters in the form of Proposition R and Measures C. The measures protect the Hawarden Hills ridgeline from development." Policy LU-54.1 tells the City to "Adhere to density limits set forth in Proposition R and Measure C."

This guide did not read the ballot text of either measure. The rules below are the City code sections that carry them out.

The RC Residential Conservation Zone

Section 19.100.010 B says the RC zone "is established consistent with General Plan objectives and voter approved initiatives (Proposition R and Measure C) to protect prominent ridges, hilltops and hillsides, slopes, arroyos, ravines and canyons." Table 19.100.040 A sets its density at 0.50 homes per gross acre, its height at 20 feet and one story, and its setbacks at 30 feet in front and 25 feet on the sides and rear.

Lot size depends on two things: the date the land was zoned RC and its average natural slope, the average steepness of the land before grading, in percent. Section 19.100.050 C gives the formula: S = 0.002296 × I × L ÷ A, where I is the contour interval in feet, L the length of the contours in feet, and A the acres of the parcel as it existed on November 13, 1979. Public Works approves the figure from survey maps.

Source: Riverside Municipal Code Section 19.100.050 A. The three slope rows apply to land zoned RC on or after May 15, 1979, where the map average must also be at least 2 acres.

Three exceptions matter. One house may be built on an RC lot that existed on May 15, 1979 and is smaller than these sizes, "if such individuals occupy the residence after construction." The RC standards do not apply to buildings that existed or were under construction on November 13, 1979, or to their repair and additions. And the lot size rules "shall not apply to lots created pursuant to California Government Code § 66411.7," the SB 9 lot split.

Building in RC takes extra review. No grading permit is issued until grading plans are approved, and "No building permit shall be issued for any building or structure in the RC Zone until slope planting and irrigation plans" and design review drawings are approved. Cut and fill slopes over five feet tall must be planted and irrigated. Every tentative map must show "at least one practical usable building site" for each lot. The July 1, 2026 fee schedule lists $3,419 for single-family design review in RC zones.

A PRD in RC clusters homes on the flatter part of a site. Lots must be at least one-half acre, the benchmark density is 0.5 homes per gross acre with a bonus of up to 25 percent (0.63), and open space must be held by "a recognized conservation group" with an endowment. Density may not be moved onto RC land from another zone, and Table 19.650.020 makes the City Council the final decision maker for RC PRDs. A detached ADU in RC is capped at 20 feet and one story. SB 9 applies in RC under Chapter 19.443, though the City's SB 9 checklist warns "Not all properties in this Zone will qualify."

Hillside and Arroyo Grading Under Title 17

Grading is moving earth to shape a building site. Section 17.12.020 applies the hillside and arroyo rules when "The property is in the RC - Residential Conservation Zone," when it "has an average natural slope ten percent or greater," or when it lies within or next to the Mockingbird Canyon, Woodcrest, Prenda, Alessandro, Tequesquite or Springbrook Arroyos. Under Section 17.28.020, grading on those sites is "limited to the minimum grading necessary to provide for an approved dwelling unit or units, driveway, garage and limited level yard," and the rest of the site stays natural under a recorded open space easement.

Source: Riverside Municipal Code Section 17.28.020 A.9 and A.12. The Director may move a pad size up or down by up to 25 percent based on site sensitivity.

Other limits in Section 17.28.020 shape the house and driveway. Slopes of 3.9:1 or steeper may be no more than 20 feet tall, and flatter slopes 25 feet. Driveway grading may be no wider than 15 feet and no steeper than 15 percent unless the Fire Department and Director approve. "No grading shall be permitted on slopes exceeding 40 percent unless findings can be made by the Planning Commission." Structures "shall be designed to fit into the hillside rather than altering the hillside to fit the structure."

The arroyo rules are the strictest. "No development or grading of any kind shall be permitted within 50 feet of the limits" of the six named arroyos and their mapped tributaries. An arroyo's limits include "the adjacent slopes having an average natural slope of 30 percent or greater." No private crossings may be graded, and "All land within the boundaries of these arroyos shall be included as an open space easement on final tract and parcel maps." The SB 9 sections bar two-unit projects and lot splits on a "mapped Arroyo."

Exceptions go through Chapter 17.32, which lets the Director grant a conditional exception for "unusual lot size, shape, or topography" and similar conditions; the fee schedule lists $5,265 for a grading exception. The code defines its trigger two ways: Section 17.08.120 uses "a straight-line slope of ten percent or greater" for the area to be graded, while Section 17.12.020 uses an average natural slope of ten percent for the property. A site can meet one test and not the other, so ask Public Works which one it applies.

Historic Designations Under Title 20

Riverside's historic sites page lists four local designations: "City Landmark - A cultural resources of the highest order of importance," "Structure of Merit," "Historic District," and "Neighborhood Conservation Area (NCA) - Similar to a historic district, but with resources of somewhat lesser significance and/or with a lesser concentration of resources." Section 20.50.010 defines a Historic District as an area with "A concentration, linkage, or continuity of cultural resources, where at least 50 percent of the structures or elements retain significant historic integrity." A contributor is a building that adds to a district's historic character; a non-contributor does not.

On age, the City page says "The general rule of thumb is 50 years old," but "a property of any age can be historically designated." The City's historic preservation page says the State Office of Historic Preservation has named Riverside a Certified Local Government, a city whose preservation program meets state and federal standards. The nine-member Cultural Heritage Board recommends designations, and the City Council designates Landmarks, Structures of Merit and Historic Districts after a public hearing. A designated property then gets the Cultural Resources (CR) overlay, which is "strictly an informational designation."

Certificates of Appropriateness

A Certificate of Appropriateness (COA) is the City's written approval of a change to a historic property. Section 20.25.010 requires one "before any person restores, rehabilitates, alters, develops, constructs, demolishes, removes, or changes the appearance of any" Designated or Eligible Cultural Resource, any element in a Historic District, contributing or not, or a contributor to a Neighborhood Conservation Area. "Non-contributors and Non-contributing features in Neighborhood Conservation Areas are not subject to the Certificate of Appropriateness requirements."

Sources: Riverside Municipal Code Sections 20.25.025 and 20.25.030; City Planning Services Fee Schedule, July 1, 2026.

Demolition gets its own notice. For the main building on a designated or eligible property, in a Historic District, or a contributor to an NCA, notice is given "at least 30 days prior to action," including a placard of at least 11 by 17 inches posted "within five feet of the front property line." Even for a building never surveyed, Section 20.25.010 D lets the City require a Cultural Resources Report before demolition to see if it is eligible.

Owners must keep historic buildings in repair under Section 20.35.010, and "Failure to maintain a Cultural Resource may result in a Determination of Nuisance and Summary Abatement." After unpermitted work, the City Council "may impose up to a five-year moratorium on any City approvals and permits" for the property.

ADUs, SB 9 and Incentives on Historic Lots

Riverside bars SB 9 on most historic land. Sections 19.443.040 and 18.085.030 bar two-unit projects and lot splits in "a Historic District or Neighborhood Conservation Area designated pursuant to Title 20," on property in the State Historic Resources Inventory, and on any "City or County Landmark or Structure of Merit." Movable ADUs are barred in historic districts and NCAs too.

Regular ADUs are allowed, under the Design Standards for ADU on Historic Properties, "Adopted February 19, 2025." The standards say "compliance with the applicable ADU Design Standards is mandatory for ADU and JADU on properties that are Cultural Resources," and they also apply to non-contributors in Historic Districts. A new detached ADU goes behind the house where possible, and in the front yard only "where there is no other space available on the property that could accommodate an ADU of at least 800 square feet and one bedroom." On height: "ADU shall be equal to or shorter in height than the primary dwelling, but at least 18-feet in height as allowed by State Law," and "Two-story ADU, greater than 18-feet in height, are allowable only if the primary dwelling is two stories in height." So a historic one-story house limits its ADU to 18 feet, below the 30-foot two-story ADU height in Chapter 19.442.

The main incentive is the Mills Act contract, which lowers property taxes in return for restoration and upkeep. Section 20.30.020 says a contract "is for a minimum period of ten years, renewed annually," that "Mills Act applications will be accepted between January 1 and May 31," and that the City Council acts before the end of the same year. The 2026 fees are $821 to apply and $1,016 to start a contract. The code is not uniform on who may apply: Section 20.30.010 limits preservation incentives to properties "designated as cultural resources pursuant to the provisions of Riverside Municipal Code Title 20," while Section 20.30.020 also names properties "listed on any official federal, state, or county register." Ask the Historic Preservation Officer before applying for a property listed only on a state or federal register.

The City posts design guidelines for the Heritage Square, Mt. Rubidoux, North Hill and Palm Heights historic districts, and Citywide Residential Historic District Design Guidelines, on its historic preservation page.

Where to Go Next

Related on VanPlex: the Riverside overview, California ADU law and property tax and insurance.

Frequently Asked Questions

Official sources (checked October 2026)