California ADU Impact Fees: What Homeowners Actually Pay

What state law guarantees on ADU impact fees

Government Code Section 66311.5(c)(1) is written as a flat prohibition with a single proportionality rule above it. The text says a local agency, special district, or water corporation shall not impose any impact fee upon the development of an ADU that has 750 square feet of interior livable space or less, or upon a JADU that has 500 square feet of interior livable space or less. Any impact fees charged for an ADU that has more than 750 square feet of interior livable space shall be charged proportionately in relation to the square footage of the primary dwelling unit.

Government Code Section 66311.5(c)(2) defines impact fee by pointing back to the Mitigation Fee Act, subdivision (b) of Section 66000, and adds that impact fee for ADU purposes also includes the Quimby fees specified in Section 66477. The same subdivision states that impact fee does not include any connection fee or capacity charge charged by a local agency, special district, or water corporation. Connection fees and capacity charges are governed by a separate subsection of the same section.

• ADUs at or below 750 sq ft of interior livable space pay no impact fee.
• JADUs at or below 500 sq ft of interior livable space pay no impact fee.
• ADUs above 750 sq ft pay impact fees proportional to ADU sq ft divided by primary dwelling sq ft.
• Impact fee for ADU purposes includes the Section 66477 Quimby fee.

How school fees are treated separately

School impact fees sit in a different statute, Education Code Section 17620, and Section 66311.5(c)(3) ties the two together for ADUs and JADUs only. Section 17620(a)(1)(C)(i) authorizes a school district to levy a developer fee on other residential construction only if the resulting increase in assessable space exceeds 500 square feet. Under the same Education Code provision, an ADU or JADU that contains less than 500 square feet of interior livable space is considered other residential construction that does not increase assessable space by 500 square feet, and is therefore outside the 500 square foot trigger for the school developer fee.

In practice this means a JADU is never subject to the school fee, and an ADU under 500 square feet of interior livable space is also not subject to it. An ADU at or above 500 square feet may be subject to school impact fees that the school district chooses to levy, collected through the city or county as the school district's collection agent under Section 17620(a)(4). Confirm the current school district fee with the city or county building division before signing a construction contract; the dollar figure is set by the school district, not the city, and changes on a different schedule.

School impact fees are distinct from the Section 66311.5 impact-fee category above. The cap in Section 66311.5 covers all other local impact fees and Quimby fees, and the school fee sits outside that cap, but only because Section 66311.5(c)(3) writes the school fee back into Section 17620 with the 500 square foot floor.

What fees are NOT impact fees under Section 66311.5

Section 66311.5(c)(2) is explicit: impact fee for ADU purposes does not include any connection fee or capacity charge charged by a local agency, special district, or water corporation. Those fees are addressed in subsections (b), (d), and (e), and they are governed by separate rules about when a separate utility connection is required.

Section 66311.5(b) states that an ADU or JADU shall not be considered by a local agency, special district, or water corporation to be a new residential use for purposes of calculating connection fees or capacity charges for utilities, including water and sewer service, unless the unit was constructed with a new single-family dwelling. Subsection (d) then says that for an ADU or JADU described in paragraph (1) of subdivision (a) of Section 66323, a local agency, special district, or water corporation shall not require the applicant to install a new or separate utility connection directly between the unit and the utility, or impose a related connection fee or capacity charge, unless the unit was constructed with a new single-family dwelling or is separately conveyed under Section 66342.

Subsection (e) covers the remaining case: for an ADU that is not described in Section 66323(a)(1), a utility may require a new or separate connection directly between the ADU and the utility. Consistent with Section 66013, that connection may be subject to a connection fee or capacity charge that is proportionate to the burden of the proposed ADU, based upon either its square feet or the number of its drainage fixture unit values, and the fee shall not exceed the reasonable cost of providing the service. The proportionate formula mirrors the impact-fee rule but applies to the utility, not to the impact-fee category.

Why local agency fee schedules still matter

Section 66311.5 caps and proportions the impact fee; it does not tell the city what dollar figure to publish in its fee schedule. A city still sets the underlying rate, and that rate is what the proportional calculation multiplies by. The underlying fee itself must comply with the Mitigation Fee Act, Section 66000 and following, which requires a nexus study and a rough proportionality finding on file with the city before the fee is charged on residential development.

For an Orange County or Los Angeles County homeowner, the practical reading is: the proportional cap protects the unit, not the dollar figure. A larger detached ADU on a typical lot pays a proportional share of whatever the city's impact fee is for the same unit, and a small garage conversion ADU on the same lot pays zero. The dollar figure still depends on the city and the district. L Square Design & Build confirms the actual, address-specific charges with the reviewing agency before signing a construction contract, so the budget is updated when the jurisdiction confirms the actual charges.

Section 66325 reinforces the point at the article level by stating that Article 2 (sections 66314 to 66331) supersedes a conflicting local ordinance, except that local agencies may adopt less restrictive requirements. A city ordinance that tries to charge a flat impact fee on a 500 square foot ADU conflicts with Section 66311.5(c)(1) and is preempted by state law. A city ordinance that charges nothing at all is less restrictive and is allowed.

How L Square applies this on real California lots

L Square Design & Build's published 2026 model lineup runs from the 600 sq ft two bedroom Model 600 at $180,000, to the 800 sq ft Models 800A and 800B at $240,000 each, to the 1,000 sq ft Models 1000A and 1000B at $295,000 each, up to the 1,200 sq ft Model 1200 at $354,000. Under Section 66311.5(c)(1), Model 600 at 600 sq ft of interior livable space is at or below the 750 sq ft floor and pays no Section 66311.5 impact fee; Models 800A, 800B, 1000A, 1000B, and 1200 are above the 750 sq ft floor and pay impact fees proportional to their square footage relative to the primary dwelling on the lot. Pricing for each model is listed on the ADU cost guide, and impact fees are listed as a separate permit and consultant line, not bundled into the construction number.

A 700 sq ft detached ADU with a vaulted ceiling built by L Square in Pasadena is documented on the project walkthrough page, with the finished great room, kitchen, bath, and bedroom shown. That unit sits at 700 sq ft of interior livable space, which is below the 750 sq ft floor in Section 66311.5(c)(1), so it does not trigger a Section 66311.5 impact fee based on the square footage alone. School impact fees still depend on the address and the school district, and are confirmed with the city building division during plan check.

For homeowners comparing ADU cost across California, the right questions for the reviewing agency are: (1) what is the published impact-fee schedule for this address, (2) is this unit at or below 750 sq ft of interior livable space, and (3) is there a separate school district fee. Those three answers drive the Section 66311.5 calculation. L Square Design & Build can be reached at (949) 374-7980 to discuss a specific lot in Orange County or Los Angeles County.