ADU Builder in Walnut, CA: Permits, Pre-Approved Plans and Costs

Walnut pre-approved ADU plans through the SGVCOG

The City of Walnut Community Development Department runs a Pre-Approved ADU Plan Program on the ADU and Pre-Approved ADU Information page, adopted under Assembly Bill 1332. All plans are developed by private architects selected through the San Gabriel Valley Council of Governments and are accessible for a fee set by the architect of record. To get a set of plans, contact the architect listed on the city page and reference the plan name. The plans are Over the Counter eligible at the Walnut Planning Division for detached structures only and cannot be used for garage or guest house conversions. L Square reviews the lot for setbacks, slope, utility route, and Very High Fire Hazard Severity Zone status before recommending any plan, then submits the application package to the Community Development Department at 21201 La Puente Road, Walnut, CA 91789.

Walnut permit process and the 60 day state clock

Walnut ADU applications go to the Community Development Department, Planning Division. California Government Code Section 66317(a)(2)(A) requires a completeness decision within 15 business days, and Section 66317(a)(3) gives the city 60 days to approve or deny a complete application when an existing single family home is on the lot. If the city does not act within 60 days, the application is deemed approved under state law, less any applicant requested delay. Walnut also offers an Over the Counter review for applicants using the Pre-Approved ADU Plans; more complex sites are moved to standard plan review. L Square prepares the submittal package, schedules the intake at City Hall, and tracks both the 15 business day and 60 day clocks so the project does not slip into the next planning cycle. City Hall office hours are Monday through Thursday, 7 a.m. to 6 p.m.; the office is closed on Fridays.

Size, height, and setback rules in Walnut

Walnut uses the state rules for detached ADU size and setback under the Pre-Approved ADU program and the local ADU ordinance. California Government Code Section 66314(d)(5) caps a detached ADU on a lot with an existing or proposed primary dwelling at 1,200 square feet of floor area. The state floor in Government Code Section 66321(b)(2)(A) and (B) is 850 square feet for a studio or one bedroom detached ADU and 1,000 square feet for a multi bedroom detached ADU, and Walnut cannot undercut that floor. Section 66323(a)(2) creates a separate ministerial path for an 800 square foot detached ADU with 4 foot side and rear setbacks, and Section 66314(d)(7) lets the city clear existing structures and same location rebuilds from the 4 foot setback ceiling. Maximum height is 16 feet for a detached ADU, with 2 additional feet allowed for the roof pitch to provide adequate clearance. L Square confirms the local overlay and any hillside, slope, or VHFHSZ rules with the Walnut Planning Division before final submittal.

Parking, JADU, and legalization for older Walnut ADUs

Walnut Municipal Code Section 6.08.210(G)(2) provides a path to legalize unpermitted ADUs and JADUs built before January 1, 2020 in accordance with California Government Code Section 66332. The City of Walnut will not deny a permit to legalize a unit solely because it does not fully comply with State ADU and JADU requirements or the local ADU ordinance, so long as the unit meets public health and safety. For new construction, Government Code Section 66314(d)(10) caps required parking at one space per bedroom or per unit, Section 66314(d)(11) bars the city from requiring replacement parking when a garage, carport, or covered parking structure is converted to an ADU, and Section 66314(b)(2) lets a local agency fully exempt ADUs from parking. A Junior ADU is capped at 500 square feet, must be created within the walls of an existing single family home, and requires owner occupancy in either the primary home or the JADU with a recorded deed restriction before the permit issues. L Square verifies the unit against current code and the recorded covenant before any close in.

Impact fees, preemption, and the California state law ceiling

Under California Government Code Section 66311.5(c)(1), Walnut, special districts, and water corporations shall not impose any impact fee on an ADU that has 750 square feet of interior livable space or less. Any impact fees charged for an ADU larger than 750 square feet are charged proportionately in relation to the square footage of the primary dwelling. Section 66311.5(b) treats an ADU as not a new residential use for water and sewer connection fees, unless the ADU was built with a new single family home. Section 66325(a) states the ADU approval article supersedes any conflicting local ordinance, and Section 66325(b) does not authorize stricter standards than state law, only less restrictive ones. When the local overlay adds a rule that conflicts with state law on size, parking, setbacks, or approval timelines, state law controls. L Square confirms the local fee schedule with the Walnut Planning Division before the submittal package is final.

Walnut plan check fee and L Square cost ranges

Walnut's Development Services Fee Schedule, as published in the 2025 ADU Handout, lists a $3,374 planning fee per ADU application. Building and Safety permit review fees are collected separately and depend on the project valuation. The fee applies to the Pre-Approved ADU Plan review and is in addition to building permit and impact fees that are set when the application is complete. L Square model prices for ADUs that meet the 1,200 square foot state ceiling start at $180,000 for the smallest prefab layout, with $240,000, $295,000, and $354,000 covering the larger 2 bedroom plans published on lscontractor.com/adu-cost-orange-county/. Those prices are the same for Los Angeles County cities including Walnut, and they cover design, engineering, permits, and build for the published plan footprint. Site specific costs such as slope work, utility trenching, retaining walls, and code upgrades for Very High Fire Hazard Severity Zones are quoted after the lot review. Confirm the final fee number with the Walnut Planning Division. L Square holds California Contractors State License Board number 1095476 and writes every proposal in writing before any work starts.

Official Walnut ADU sources

• The City of Walnut Community Development Department ADU and Pre-Approved ADU Information page lists the Pre-Approved ADU Plan Program adopted under Assembly Bill 1332, with plans developed by private architects selected through the San Gabriel Valley Council of Governments. The page lists 21201 La Puente Road, Walnut, CA 91789 as the Community Development Department address, and City Hall office hours of Monday through Thursday 7 a.m. to 6 p.m. with the office closed on Fridays. Official source
• The City of Walnut 2025 ADU Handout, updated November 26, 2025, defines ADU and Junior ADU types, references Walnut Municipal Code Section 6.08.210(G)(2) for unpermitted ADU legalization (state code Section 66332), sets the legalization cutoff at units built before January 1, 2020, sets 850 square feet for a studio or one bedroom ADU and 1,000 square feet for a multi bedroom ADU, requires JADU within existing single family residence space, caps JADU at 500 square feet, sets heights at 16 feet (18 feet near transit, 25 feet attached), and requires side and rear yard setbacks of at least 4 feet. Official source
• Ordinance No. 25-04 is the current City of Walnut ADU ordinance, adopted as the local implementing ordinance consistent with state ADU statute. The 2023 Walnut Fee Schedule is published by the Planning Division and is referenced by the handout for the current fee numbers. Official source
• California Government Code Section 66317(a)(3) gives the city 60 days to approve or deny a complete ADU application when an existing single family home is on the lot, and Section 66317(a)(2)(A) requires a completeness decision within 15 business days. Official source
• California Government Code Section 66314 limits a detached ADU on a lot with an existing or proposed primary dwelling to 1,200 square feet of floor area, and Section 66314(d)(7) lets the city clear existing structures and same location rebuilds from the 4 foot setback ceiling. Official source
• California Government Code Section 66311.5(c)(1) bars a local agency, special district, or water corporation from imposing any impact fee on an ADU with 750 square feet of interior livable space or less, and impact fees above that floor are charged proportionately in relation to the primary dwelling square footage. Official source
• California Government Code Section 66325(a) states the ADU approval article supersedes a conflicting local ordinance, and Section 66325(b) does not authorize stricter standards than state law, only less restrictive ones. Official source