The short answer: ADU impact fees are limited in California: a city or special district cannot charge impact fees on an ADU of 750 sq ft or less of interior livable space, or on a JADU of 500 sq ft or less (Gov. Code section 66311.5(c)(1)). Above those sizes, impact fees must be proportional to the ADU’s size compared with the main house. Washington caps ADU impact fees at 50% of the fee for the principal unit. “Avoiding” impact fees is therefore mostly about design choices: size, unit type and timing.
Last verified: October 2, 2026 ยท Written by the ADU Scope editorial team
Key fact: Impact fees are not the same as permit fees, plan-check fees or utility connection charges. State limits on impact fees do not remove the other costs, so a “fee-free” ADU can still have a meaningful permit bill.
What ADU Impact Fees Are
An impact fee is a one-time charge that local agencies collect to fund public facilities that new development uses: parks, roads, fire protection, schools and similar services. California’s HCD ADU Handbook says impact fees for ADUs and JADUs must be determined in accordance with the Mitigation Fee Act (Gov. Code sections 66311.5(c)(2) and 66000), which requires a reasonable relationship between the fee and the burden the development creates.
For a new house, these fees can be substantial and are often the largest single government charge after the permit. Applying the same fee to a small ADU would make many projects uneconomic, which is why the state restricts them.
Fees the exemption does not cover
- Building permit and plan-check fees, which cities set locally.
- Utility connection and capacity charges, which are separately limited (see our article on utility meters).
- Planning or review fees for zoning checks.
- Coastal or special-area fees where they apply.
ADU Impact Fees: California’s Size Thresholds
The rule is a simple size test based on interior livable space. The 2025 legislative changes clarified that proportionality now references interior livable space specifically (Gov. Code sections 66321(b)(2)-(b)(3)), so exterior features such as porches generally do not count.
| Unit | Size | Impact fees |
|---|---|---|
| ADU | 750 sq ft or less | None |
| ADU | More than 750 sq ft | Proportional to size compared with the main house |
| JADU | 500 sq ft or less | None |
| School district fee | Under 500 sq ft | None (does not increase assessable space) |
| School district fee | 500 sq ft or more | May be levied under Education Code section 17620 |
Two points deserve attention. First, the school-fee threshold is lower than the general one: an ADU of 600 sq ft can be exempt from city impact fees yet still face a school district fee. Second, the exemption is about the city’s impact fees, so a separate agency such as a water district can impose its own charges under different rules.
Attention: A 750 sq ft ADU is exempt, but 751 sq ft is not. Do not design to the exact limit without a margin; construction changes and measurement differences can push you over.
How Proportional ADU Impact Fees Work Above the Threshold
When the ADU exceeds 750 sq ft, the fee is a share of what a new primary dwelling would pay. HCD’s handbook gives an example: a 2,000 sq ft primary dwelling with a proposed 1,000 sq ft ADU could result in 50 percent of the impact fee that would be charged for a new primary dwelling on the same site. SB 1117 (Chapter 809, Statutes of 2026), approved on September 29, 2026, changes this calculation for lots with up to two ADUs so that only the area above 750 sq ft is charged. It is expected to take effect on January 1, 2027, so confirm the effective date and how your city applies it. The worked illustration below reflects the calculation under the law before that change.
A worked illustration
The numbers below are hypothetical and only show the arithmetic. Your city’s actual fee will differ.
- Suppose the full impact fee for a new house on your site is $30,000.
- Your main house is 2,000 sq ft and you build a 1,000 sq ft ADU.
- The ADU is 1,000 / 2,000 = 50% of the house.
- The proportional fee would be 50% of $30,000 = $15,000.
- If you instead build a 750 sq ft ADU, the impact fee is zero.
In this illustration, 250 extra square feet cost $15,000, or $60 per extra square foot, before construction. That is a useful number to compare against what the extra space would earn or save you.
Tip: Ask the city for a written fee estimate for both a 750 sq ft and a larger version of your design. The difference tells you whether the larger unit is worth it.
Legal Ways to Reduce or Avoid ADU Impact Fees
These are not loopholes. They are the situations the law is designed around.
- Stay at or under 750 sq ft of interior livable space for a new or additional ADU.
- Convert existing space. A conversion ADU or JADU is generally not treated as a new residential use for utility connection or capacity charges, unless it is built with a new single-family home (Gov. Code section 66311.5(b)).
- Keep a JADU at 500 sq ft or less.
- Keep school-fee exposure low by staying under 500 sq ft where the design works.
- Check local waivers. Some cities and counties offer additional fee waivers or reductions; availability changes, so verify.
What does not work is splitting one project into pieces or misreporting square footage. Cities check plans, and an inaccurate measurement can lead to a revised fee or enforcement.
Washington and Other States
Washington takes a different approach. According to the Municipal Research and Services Center, ADU impact fees inside urban growth areas cannot exceed 50% of the fee for the principal unit. There is no 750 sq ft exemption, so even a small ADU can pay a fee, but the cap limits the cost.
Other states handle fees differently, and many leave them entirely to local governments. Portland, Oregon, for example, runs a separate system development charge (SDC) waiver program for ADUs, and the City notes that waivers restrict short-term rental use for 10 years. Check current terms before assuming a waiver is available.
| Jurisdiction | Impact fee rule |
|---|---|
| California | None up to 750 sq ft (ADU) or 500 sq ft (JADU); proportional above |
| Washington (urban growth areas) | Capped at 50% of the principal unit’s fee |
| Portland, OR | Separate SDC waiver program; confirm current terms |
Frequently Asked Questions
Is a 700 sq ft ADU exempt from impact fees in California?
Yes, impact fees cannot be charged on an ADU of 750 sq ft or less. Permit and utility charges can still apply.
Do school districts charge fees on ADUs?
They may for units of 500 sq ft or more. Units under 500 sq ft are exempt.
How are fees calculated above 750 sq ft?
As a share of the new-house fee, based on the ADU’s size relative to the main house.
Does a garage conversion pay impact fees?
If it is 750 sq ft or less, it is exempt from impact fees, and conversions are generally not treated as new residential use for utility charges.
Next Steps
- ADU Permit Cost by City
- Do ADUs Need Separate Utility Meters?
- California ADU Laws 2026
- ADU Setback Requirements Explained: California, Washington, Colorado and Portland
General information, not legal advice. Local rules vary and change; confirm with your planning department.
Sources: HCD ADU Handbook, March 2026 ยท MRSC: Accessory Dwelling Units ยท City of Portland: Temporary SDC exemptions