ADU Builder in Diamond Bar, CA: DBADU Program, Permits and Costs

How many ADUs can a Diamond Bar single family lot have?

The City of Diamond Bar planning FAQ, hosted on diamondbarca.gov, states that all residentially-zoned properties are eligible to construct one ADU and one Junior ADU (JADU), regardless of lot size, with ADUs required to be at least 800 square feet. For studios and one bedroom ADUs the size limit is 850 square feet, and for units with two or more bedrooms the size limit is 1,000 square feet. A JADU is created within the walls of a single family residence, may share restrooms with the primary residence, and must contain a basic kitchen, with a 500 square foot interior cap. The state 60 day permit clock in Gov. Code 66317(a)(3) still applies to the ADU permit, so a completed application the city does not approve or deny within 60 days is deemed approved.

What is the Diamond Bar Pre-Approved ADU (DBADU) program?

The Pre-Approved Accessory Dwelling Unit (DBADU) Program page, hosted on diamondbarca.gov, says the City launched the program in January 2026 to comply with AB 1332 and to streamline permitting, and that pre-approved designs bypass the pre-screening stage and move directly into the plan check process. The ADU Opportunity for Designers page, also on diamondbarca.gov, says pre-approved plans meet the City building code and design guidelines and eliminate design costs for the homeowner. The DBADU page lays out the five step homeowner flow: catalog selection, contact the architect or designer, building permit application, departmental review, plan check and approval. Departmental review for pre-approved plans is limited to 7 days rather than the normal 10 days. The DBADU Catalog page states pre-approved ADU plans will be added once submitted by design professionals and reviewed by the City, so the catalog is being built as designers apply.

What are the size, height and setback rules for a Diamond Bar ADU?

The City of Diamond Bar planning FAQ, hosted on diamondbarca.gov, sets the size floor at 800 square feet for any ADU and gives 850 square feet for studios and one bedroom units and 1,000 square feet for units with two or more bedrooms, with a 500 square foot JADU cap. The DBADU Application Checklist, hosted on diamondbarca.gov, lists the design criteria: detached ADUs only (no conversions, no attached ADUs, no JADUs), a maximum 16 foot height, a minimum 220 square foot and maximum 800 square foot size for one bedroom, and a maximum 1,000 square foot size for two or more bedrooms, with Ranch, Spanish, or Contemporary style and roof pitch and fascia to match. State law in Gov. Code 66314(d)(5) caps detached ADU total floor area at 1,200 square feet, and Gov. Code 66321 requires every city to permit at least 800 square feet with 4 foot side and rear yard setbacks. Gov. Code 66314(d)(7) prohibits any setback requirement for a structure built in the same location and to the same dimensions as an existing structure being converted to an ADU.

What are the DBADU site conditions and Building and Safety rules?

The Pre-Approved Accessory Dwelling Unit (DBADU) Program page, hosted on diamondbarca.gov, lists the Building and Safety Division conditions that every DBADU project must meet. The ADU cannot be used near a slope within the height of slope divided by two, per California Building Code section 1808.7. The ADU is not allowed where a grading permit is required, per California Building Code Appendix J103.1. Where any portion of the ADU sits within 10 feet of an adjacent structure, fire rating of eaves and exterior walls is required. The ADU must be reliant on the existing main residence utilities and must not interfere with existing utilities, per Diamond Bar City Code Section 22.42.120(7)(m). Sewer elevations on the lot must be higher than the street sewer elevation. The work must not require the design or use of a retaining wall or planter to create a pad for the ADU. The ADU Opportunity for Designers page adds that the City will adopt California Building Code 2025 at the beginning of 2026, so any plan submitted under the DBADU program must be developed subject to CBC 2025.

How long does a Diamond Bar ADU permit take?

The City of Diamond Bar planning FAQ, hosted on diamondbarca.gov, states that once an application is accepted by the Building and Safety Division the plan check turnaround is 15 working days, and that every subsequent submittal is 10 working days until all comments and corrections have been addressed and the permit is issued. The Pre-Approved Accessory Dwelling Unit (DBADU) Program page adds that reviews for pre-approved plan sets are limited to 7 days rather than the normal 10 days, because Planning and Public Works only review pre-approved plans for site-specific items such as drainage, covenants, and will-serve letters. State law in Gov. Code 66317(a)(3) still applies, so if the City does not act on a completed application within 60 days the application is deemed approved.

How much does an ADU cost in Diamond Bar?

L Square publishes configured model prices on the cost guide page: $180,000 for Model 600, $240,000 for Models 800A and 800B, $295,000 for Models 1000A and 1000B, and $354,000 for Model 1200. These are model prices, not a final total for a specific lot. Diamond Bar lots also carry the Building and Safety plan check fee set by the City fee schedule, the City filing fee under Diamond Bar City Code Section 22.44.040, the proportional impact fee on ADUs over 750 square feet under Gov. Code 66311.5, school district fees where the project exceeds the district threshold, and any utility and connection fees from the providers named in the City application materials. The DBADU Program page says pre-approved plans eliminate design costs for the homeowner, but the program does not waive City plan check fees, and additional permits such as electrical, plumbing, mechanical, fire safety, and utility connections may still be required. Garage conversions, attached ADUs, and JADUs are priced from a property specific scope because each one depends on the existing structure and the lot layout.

Official Diamond Bar ADU sources

• The City of Diamond Bar planning FAQ, hosted on diamondbarca.gov, states that all residentially-zoned properties are eligible to construct one ADU and one Junior ADU regardless of lot size, that an ADU is at least 800 square feet, that studios and one bedroom ADUs are allowed 850 square feet and units with two or more bedrooms are allowed 1,000 square feet, that a JADU is created within the walls of a single family residence and may share restrooms with the primary residence and must contain a basic kitchen, and that JADUs are allowed up to 500 square feet. Official source
• The City of Diamond Bar planning FAQ, hosted on diamondbarca.gov, states that once an application is accepted by the Building and Safety Division the plan check turnaround is 15 working days, and that every subsequent submittal is 10 working days until all comments and corrections have been addressed and the permit is issued. Official source
• The Pre-Approved Accessory Dwelling Unit (DBADU) Program page, hosted on diamondbarca.gov, says the City launched the program in January 2026 to comply with AB 1332, with pre-approved designs bypassing the pre-screening stage and moving directly into plan check, and that departmental review for pre-approved plan sets is limited to 7 days rather than the normal 10 days. Official source
• The ADU Opportunity for Designers page, hosted on diamondbarca.gov, states that the City launched a Pre-Approved ADU Program in January 2026, that pre-approved plans meet the City building code and design guidelines and eliminate design costs for the homeowner, and that plans shall be developed subject to California Building Code 2025 which will be mandated for Diamond Bar at the beginning of 2026, and the Building Division will collect plan check fees including Planning and Public Work fees before stamping the plans with the official pre-approval stamp. Official source
• The DBADU Application Checklist, hosted on diamondbarca.gov, limits pre-approved plans to detached ADUs only, requires a minimum of two applications per designer (one for one bedroom at 220 to 800 square feet and one for two or more bedrooms up to 1,000 square feet), requires a full kitchen, limits height to 16 feet, requires Ranch, Spanish, or Contemporary style, and requires an application fee under Diamond Bar City Code Section 22.44.040 to be submitted with the application. Official source
• The Accessory Dwelling Units page, hosted on diamondbarca.gov, defines an Accessory Dwelling Unit as an independent second dwelling unit on a property sometimes referred to as a granny flat, in-law unit, or backyard cottage, that ADUs can be attached or detached or located within the existing home, and that they must include living and sleeping areas, a kitchen, and bathroom, and that the City of Diamond Bar Accessory Dwelling Unit Ordinance can be found in its entirety on the MuniCode platform under Development Code Section 22.42.120. Official source
• Gov. Code 66317(a)(3), hosted on leginfo.legislature.ca.gov, requires a permitting agency to approve or deny a completed application to create or serve an accessory dwelling unit within 60 days from the date the permitting agency receives a completed application if there is an existing single family or multifamily dwelling on the lot, and states that if the local agency has not approved or denied the completed application within 60 days, the application is deemed approved. Official source
• Gov. Code 66311.5(c)(1), hosted on leginfo.legislature.ca.gov, states that a local agency, special district, or water corporation shall not impose any impact fee upon the development of an accessory dwelling unit that has 750 square feet of interior livable space or less or a junior accessory dwelling unit that has 500 square feet of interior livable space or less, and that any impact fees charged for an ADU of more than 750 square feet shall be charged proportionately in relation to the square footage of the primary dwelling. Official source
• Gov. Code 66314(d)(5), hosted on leginfo.legislature.ca.gov, states that the total floor area for a detached accessory dwelling unit shall not exceed 1,200 square feet, and Gov. Code 66314(d)(7) states that no setback shall be required for an existing living area or accessory structure or a structure constructed in the same location and to the same dimensions as an existing structure that is converted to an accessory dwelling unit. Official source