ADU Builder in San Dimas, CA: Permits, State Law and Costs

Why does state ADU law apply to a San Dimas ADU?

The California Department of Housing and Community Development sent the City of San Dimas a written ADU Findings Letter on January 29, 2024, concluding that the City's ADU Ordinance, No. 1281 adopted October 13, 2020, did not comply with the state ADU statute in effect at that time. HCD then sent the City a Technical Assistance Letter on March 18, 2025 stating that HCD followed up and was told that, rather than amending its noncompliant ordinance, the City intends to exclusively use State ADU Law to process ADU applications. HCD's reply noted that the proper statutory path to achieve that result is for the City to repeal the ordinance under Government Code section 66326, subdivision (b)(2), and that repealing it would have the additional benefit of clarity for potential applicants who may otherwise rely on an ordinance that is still in the local code but not compliant with state law. The practical effect for a homeowner is that California Government Code Sections 66314 through 66342 control the ADU review in San Dimas, and Section 66325(a) states that the state ADU approval article shall supersede a conflicting local ordinance.

What is the maximum size of an ADU in San Dimas?

The City of San Dimas Accessory Dwelling Unit (ADU) Requirements handout, published on files.sandimasca.gov, states that the maximum ADU size in San Dimas shall not exceed 1,000 square feet. That local cap sits below the state ceiling in California Government Code Section 66314(d)(5), which limits a detached ADU on a lot with an existing or proposed single-family dwelling to 1,200 square feet of floor area. Section 66321(b)(2)(A) and (B) set statewide minimum floors of 850 square feet for a studio or one bedroom detached ADU and 1,000 square feet for a multi bedroom detached ADU that a local ordinance cannot undercut. Section 66321(b)(3) requires every city to allow at least an 800 square foot detached ADU with four foot side and rear yard setbacks, and Section 66325(a) voids any local rule that conflicts with those state floors. Confirm the size of the unit that fits your lot with the San Dimas Planning Division before submitting a permit application.

What setbacks, height, and design rules apply to a San Dimas ADU?

The San Dimas ADU Requirements handout states that a minimum setback of 4 feet is required for the sides and rear of the property for ADUs, and that exterior doors may not be located on the facade that is closer than 15 feet to a property line. The handout also states that architectural features of the proposed ADU must match the architectural style of the main residence, and that an ADU must include washer and dryer hookups. The handout requires every project to be designed to the 2025 California Residential Code, the 2025 California Mechanical Code, the 2025 California Electrical Code, the 2025 California Energy Code, the 2025 California Green Code, the 2025 California Wildland Urban Interface Code, and the City of San Dimas Municipal Code. State Government Code Section 66314(d)(7) lets the City clear an existing structure and a same location rebuild from the 4 foot setback ceiling, Section 66321(b)(4) sets the detached ADU height ladder at 16, 18, or 25 feet depending on the lot, and Section 66325(a) voids any local design rule that conflicts with state law.

What permit process and timeline apply to a San Dimas ADU?

The Assembly Bill 2234 page on sandimasca.gov states that a Building Permit is required for all ADU projects, that the City of San Dimas Building and Safety Division maintains the submittal list for Detached ADUs with Solar, Junior ADU, and Residential Additions, and that the Master Fee Schedule on the City's website sets the current fee totals. The page lists the City Hall address as 245 East Bonita Avenue, San Dimas CA 91773, office hours as Monday through Thursday 7:30 a.m. to 5:30 p.m. and Friday 7:30 a.m. to 4:30 p.m., and links to the ADU Permit Application, ADU Building Permit, and ADU Solar Permit Application forms. State Government Code Section 66317(a)(3) requires the permitting agency to approve or deny a complete ADU or JADU application within 60 days from receipt, and states that if the local agency has not acted within 60 days the application is deemed approved. Section 66317(a)(2)(A) requires a written completeness determination within 15 business days. Confirm the submittal list and the current fee total with the San Dimas Building and Safety Division before filing.

What other site requirements apply to a San Dimas ADU?

The San Dimas ADU Requirements handout requires a geotechnical report from a California licensed geotechnical engineer with recommendations that consider the entire footprint of the proposed new structure and a minimum of 5 feet beyond the proposed footprint, and the report must be submitted with the architectural and engineering plans at plan submittal. The handout requires a photovoltaic system on every newly constructed, non manufactured, detached ADU under the 2025 California Energy Code, with the panels placed on the ADU or on the primary dwelling unit, and an existing primary dwelling with solar does not offset the ADU requirement. The handout states that the sanitary drainage system for the new ADU shall be connected to the building sewer of the main unit, that the sanitary sewer line is not allowed to connect to, pass through, or be installed under a dwelling unit, structure, garage, breezeway, patio cover, porch, steps or similar, and that the water heater must be shown on the plans. The handout confirms that the Los Angeles County Fire Department, not the City Fire Department, has authority over fire flow, fire hydrant location, and fire hydrant size, and that the California Wildland Urban Interface Code applies in any Fire Hazard Severity Zone. The assigned ADU address is the current legal address followed by the letter A.

How much does an ADU cost in San Dimas?

L Square's configured model prices, published on the L Square cost guide page on lscontractor.com, start at $180,000 for the 600 square foot Model 600, run to $240,000 for the 800 square foot Model 800A and Model 800B, to $295,000 for the 1,000 square foot Model 1000A and Model 1000B, and to $354,000 for the 1,200 square foot Model 1200. The same model prices apply to Los Angeles County cities including San Dimas and are the design-build price, not a final project total. San Dimas lots that sit in a Fire Hazard Severity Zone often carry additional Wildland Urban Interface Code work, and canyon or hillside lots can add grading, drainage, and a geotechnical report beyond the model price. California Government Code Section 66311.5(c)(1) bars the State, any special district, and any water corporation from imposing any impact fee upon the development of an ADU that has 750 square feet of interior livable space or less, and any impact fees above 750 square feet are charged proportionately in relation to the primary dwelling square footage. L Square holds California Contractors State License Board number 1095476 and prepares the submittal package and the AB 2234 list of required documents for the homeowner.

Official San Dimas ADU sources

• The California Department of Housing and Community Development sent the City of San Dimas a written ADU Findings Letter dated January 29, 2024, addressed to Luis Torrico, Planning Manager, 245 East Bonita Avenue, San Dimas, California 91773, concluding that the City's ADU Ordinance, No. 1281 adopted October 13, 2020, did not comply with the state ADU statute in effect at that time, and listing the items to amend with a written response due no later than February 29, 2024. Official source
• The California Department of Housing and Community Development sent the City of San Dimas a Technical Assistance Letter dated March 18, 2025, signed by Jamie Candelaria, Senior Housing Accountability Manager, stating that HCD followed up and was told that, rather than amending its noncompliant ordinance, the City intends to exclusively use State ADU Law to process ADU applications, and that the proper statutory path is for the City to repeal the ordinance under Government Code section 66326, subdivision (b)(2). Official source
• The City of San Dimas Accessory Dwelling Unit (ADU) Requirements handout on sandimasca.gov states that the maximum ADU size shall not exceed 1,000 square feet, that architectural features of the proposed ADU must match the architectural style of the main residence, that exterior doors may not be located on the facade that is closer than 15 feet to a property line, that an ADU must include washer and dryer hookups, that a minimum setback of 4 feet is required for the sides and rear, and that a geotechnical report from a California licensed geotechnical engineer is required at plan submittal. Official source
• The Assembly Bill 2234 page on sandimasca.gov states that a Building Permit is required for all projects that involve alterations of structures, that the City of San Dimas Building and Safety Division maintains the submittal list for Detached ADUs with Solar, Junior ADU, and Residential Additions, lists the City Hall address as 245 East Bonita Avenue, San Dimas CA 91773, lists office hours as Monday through Thursday 7:30 a.m. to 5:30 p.m. and Friday 7:30 a.m. to 4:30 p.m., and links to the ADU Permit Application, ADU Building Permit, and ADU Solar Permit forms. Official source
• California Government Code Section 66317(a)(3) requires the permitting agency to approve or deny a complete ADU or JADU application within 60 days from receipt, and states that if the local agency has not approved or denied the completed application within 60 days, the application shall be deemed approved. Section 66317(a)(2)(A) requires a written completeness determination within 15 business days. Official source
• California Government Code Section 66314(d)(5) limits a detached ADU on a lot with an existing or proposed single-family dwelling to 1,200 square feet of floor area. Section 66314(d)(10) caps required parking at one space per ADU or per bedroom, whichever is less. Section 66314(d)(11) bars a local agency from requiring replacement parking when a garage, carport, or covered parking structure is converted to an ADU or demolished in connection with ADU construction. Official source
• California Government Code Section 66321(b)(2)(A) and (B) set statewide minimum floors of 850 square feet for a studio or one bedroom detached ADU and 1,000 square feet for a multi bedroom detached ADU that a local ordinance cannot undercut. Section 66321(b)(3) requires every city to allow at least an 800 square foot detached ADU with 4 foot side and rear yard setbacks. Official source
• California Government Code Section 66311.5(c)(1) prohibits the State, any special district, and any water corporation from imposing any impact fee upon the development of an ADU that has 750 square feet of interior livable space or less, and requires any impact fees on a larger ADU to be charged proportionately in relation to the primary dwelling square footage. Section 66311.5(b) treats an ADU as not a new residential use for water and sewer connection fees. Official source
• California Government Code Section 66325(a) states that, except as provided in subdivision (b), the ADU approval article shall supersede a conflicting local ordinance. Section 66316(a) makes a noncompliant ADU ordinance null and void in its entirety and applies state standards in place of the local ordinance. Official source