Most homeowners pick an ADU size based on floor plans and rent. Under California law, a few specific square-footage numbers also decide which fees you pay and how your permit is reviewed. Crossing one of them by even a few feet can add costs that have nothing to do with construction.
This guide explains those numbers as they stand in 2026, what each one controls, and what they do not control. It covers state law and how Long Beach and Signal Hill apply it. Your parcel still matters, and we’ll point out where.
What changed on January 1, 2026
Since January 1, 2026, most ADU and JADU size limits in state law are measured in square feet of interior livable space. That change came from Senate Bill 543, signed October 10, 2025.
The law defines livable space as space intended for human habitation, including living, sleeping, eating, cooking, and sanitation. The older term, “living area,” covered the interior habitable area and could include basements and attics.
Some builders read the new wording to mean exterior wall thickness and unusable spaces no longer count. The statute does not spell out a measuring method, but Long Beach has. Its Planning ADU Bulletin says ADU floor area is measured only to the interior walls. Every other development standard still uses gross floor area, measured to the outside of exterior walls. Signal Hill has not published a comparable rule, so ask its Planning staff before you finalize a layout close to a limit.
The five numbers at a glance
| Size (interior livable sq ft) | What it controls | Gov. Code section |
|---|---|---|
| Under 500 | ADUs and JADUs this small are treated as not adding 500 sq ft of assessable space for school developer fees | 66311.5(c)(3) |
| 500 | Maximum size of a JADU; JADUs at or under 500 are exempt from impact fees | 66313(d), 66311.5(c)(1) |
| 750 | ADUs at or under 750 are exempt from impact fees; above 750, fees must be proportional | 66311.5(c)(1) |
| 800 | The size a city must allow for a detached new ADU with 4-foot side and rear setbacks | 66321(b)(3), 66323(a)(2) |
| 850 / 1,000 | A city’s maximum-size limit cannot be set below 850, or 1,000 for units with more than one bedroom | 66321(b)(2) |
Source: SB 543, chaptered text; HCD ADU Handbook, March 2026.
What each number means for your project
Under 500: school developer fees
School districts can charge developer fees on residential construction that adds more than 500 square feet of assessable space. Under SB 543, an ADU or JADU with less than 500 square feet of interior livable space is treated as not crossing that line. For a studio or compact one-bedroom, staying under 500 can remove this fee.
500: the JADU ceiling
A junior ADU must be no more than 500 square feet of interior livable space and sit entirely inside a single-family home. It can have its own bathroom or share one with the main house. Planning a JADU at 520 square feet is not a small overage; it no longer qualifies as a JADU.
750: the impact-fee line
Cities, special districts, and water corporations cannot charge impact fees on an ADU with 750 square feet of interior livable space or less. Above 750, impact fees are allowed but must be proportional.
HCD explains proportional as the ADU’s size compared with the primary home’s size. For example, a 900-square-foot ADU behind a 1,800-square-foot house would carry about half the impact fees a comparable new home would. If your design sits between 750 and 800, ask whether the extra space is worth that charge.
800: the size your city must allow
A city cannot use lot coverage, floor area ratio, open space, front setbacks, or minimum lot size to block an 800-square-foot ADU with 4-foot side and rear setbacks. Separately, one detached new-construction ADU on a single-family lot must be approved ministerially, and the city may cap that unit at 800 square feet of livable space. If your lot is tight, 800 is often the practical design target.
850 and 1,000: the floor under local maximums
Cities may set a maximum ADU size, but not below 850 square feet, or 1,000 square feet for a unit with more than one bedroom. Many local ordinances allow larger units than that. Larger units usually have to meet more local development standards, so confirm the rules for your zone.
How Long Beach and Signal Hill apply these numbers
Neither city currently applies a local ADU ordinance of its own. Both review ADUs under state law, which makes the state thresholds above the working rules on most lots.
Long Beach
Long Beach reviews every ADU and JADU strictly under state law and has published how it reads that law in its Planning ADU Bulletin (IB-PZ-001). For a single-family lot, the bulletin allows:
| ADU type (single-family lot) | Maximum size | Notes |
|---|---|---|
| Detached, new construction | 800 sq ft | One allowed; 4-ft side and rear setbacks, rear measured to alley centerline |
| Attached addition to the house | 50% of the house or 800 sq ft, whichever is more | Limit applies only to the added area |
| Conversion of part of the house | No limit | Keeps the existing height and setbacks |
| Conversion of a garage or accessory building | No limit | Up to 150 sq ft may be added, for entry only |
| JADU | 500 sq ft | Inside the house or attached garage; no addition allowed |
A single-family lot can combine one conversion ADU, one detached new ADU, and one JADU. Detached ADUs can be 16 feet tall, or 18 feet within a half mile of major transit.
In practice, 800 square feet is the key number for a new backyard unit in Long Beach. It is also above the 750-square-foot impact-fee line, so a design between 751 and 800 square feet will carry proportional impact fees.
Two local steps can add review: properties in the Coastal Zone need an administrative Local Coastal Development Permit, and properties in a historic district need a Certificate of Appropriateness. Neither requires a public hearing, and both are filed with the building permit.
The city has released a draft local ADU ordinance. If it is adopted, the bulletin stops applying, so check for updates before you design.
Signal Hill
Signal Hill’s published ADU summary says the city has no local ADU ordinance and follows state regulations, with no discretionary review. That means the state thresholds in this guide apply directly.
Parts of Signal Hill’s online ADU material appear to predate the 2025 and 2026 state law changes. Confirm current standards with Signal Hill Planning before relying on its older summaries.
Fees that staying small does not remove
“No impact fees” is not “no fees.” State law defines impact fees so that they do not include utility connection fees or capacity charges. Plan-check and permit processing fees also still apply.
Whether you pay utility connection charges depends mostly on the type of ADU, not its size:
- Conversions of existing space (a garage, a room, or part of the house, with up to 150 sq ft added for entry): the utility generally cannot require a new separate connection or charge a related connection fee, unless the unit is built with a new home or later sold separately.
- Detached new construction and most other ADUs: a separate connection can be required. Any fee must be proportional to the ADU’s burden on the system, based on its square footage or plumbing fixture count, and cannot exceed the reasonable cost of service.
The physical work of running water, sewer, and electrical service to a backyard unit is a construction cost, separate from any fee. On many lots it is one of the larger variables in an ADU budget, and it depends on distance, existing panel capacity, and where the lines run.
A real example: an ADU conversion in El Segundo
From July 2024 to January 2025, De Sisto Construction converted an existing structure on an El Segundo property into an ADU. The finished unit is 646 square feet. Because it was a conversion, that size was largely set by the building that was already there, and it falls under the 750-square-foot impact-fee line described above. The utility work was a different story.
The scope included a new 200-amp main electrical panel on the house and a new 100-amp subpanel for the ADU, new water supply lines, and a relocated drain line running to a sump. Moving that drain meant saw-cutting and re-pouring part of the interior slab. Existing walls were also furred out to make room for insulation that meets Title 24 energy standards, and a fire-rated stucco system was added at the eaves.
The lesson for planning: state law generally protects conversions from a new utility connection requirement, but that protection covers fees, not work. A conversion can still need significant electrical and plumbing upgrades, and those belong in your budget as separate line items from the start.
What these numbers do not decide
State law sets a floor that cities must allow. It does not tell you what fits on your specific lot or what your city allows above that floor. Before settling on a size, confirm:
- Your city’s ADU ordinance. Long Beach and Signal Hill currently apply state law, but either city could adopt a local ordinance that changes the details.
- Height limits. State law guarantees at least 16 feet for most detached ADUs, and 18 feet near major transit, with 2 more feet to match the main roof pitch.
- Site conditions. Easements, existing structures, trees, slopes, and utility locations can make an allowed size impractical to build.
- Special areas. Coastal Zone, historic, and fire-hazard properties can involve extra review.
- Existing unpermitted work. Older ADUs built before 2020 have their own legalization rules and fee protections.
None of this can be answered from a general article. It takes a look at your parcel, your city’s current rules, and usually a site visit.
Before you lock in a size: a short checklist
- Decide what you need the unit to do: rental, family, office, or future flexibility
- Note the thresholds near your target size: under 500, 500, 750, 800
- Ask your city how it measures interior livable space on plans
- Ask which fees apply at your size: impact fees, school fees, utility connection and capacity charges, permit fees
- Ask your water, sewer, and electric providers whether a separate connection will be required
- Get a scope that prices utility runs and panel work as separate line items
- Compare the cost of the extra square feet against the rent or use they add
A contractor’s estimate should make each of these visible. If fees and utility work are missing or bundled into one number, ask for them to be broken out.
Planning an ADU?
De Sisto Construction (CSLB #803545) has built in Signal Hill, Long Beach, and surrounding cities since 2001. We prepare line-item estimates and coordinate permitting, so fees, utility work, and construction costs are visible before you commit to a size. If you’re weighing ADU options, give us a call at 562-425-1050 or email us at info@desistogc.com to talk through your property.
This article explains California state law as of September 2026. It is general information, not legal advice or a determination about any specific property. Local rules and parcel conditions apply.