As of January 1, 2026, your ADU’s square footage is measured by interior livable space only. Walls, stairwells, and mechanical rooms no longer eat into your cap — and that quietly changes what you can build.
The 60-second version
SB 543 (Senator McNerney) was signed October 10, 2025 and took effect January 1, 2026. It doesn’t hand you a bigger building — it changes how the building is measured.
Every square-footage limit in California’s ADU statutes — 800, 850, 1,000, 1,200, the 750 fee threshold, the 500 JADU cap — is now explicitly interior livable space. Exterior walls, interior stairways, and non-habitable areas like mechanical and storage rooms no longer count against your cap.
Practical effect on a typical 800 sq ft detached ADU: roughly 50–120 sq ft of additional livable interior within the same legal limit, depending on wall assembly. Plus a hard 15-business-day deadline for the City or County to review your application, and confirmed fee protections under 750 sq ft.
- Interior livable space = the new measuring stick
- Walls & mechanical excluded
- 15-business-day plan review
- Interior measure can keep you under the 750 fee line
- Effective Jan 1, 2026
Why this is a bigger deal than it sounds
For years, the way a city measured your ADU was a gray area. Many plan-checkers counted the gross footprint — exterior wall to exterior wall — against your size cap. California’s Department of Housing and Community Development (HCD) had long advised that the caps should be measured as interior livable space, but it was guidance, not statute. By HCD’s own count, more than 50 local governments were applying the rules incorrectly.
SB 543 closes that gap. It takes HCD’s guidance and writes it into the Government Code, so it’s now black-letter law a plan-checker can’t wave off. That’s the real story here — not a brand-new entitlement, but the end of an argument. When the City of San Diego, the County, or a North County jurisdiction reviews your plans in 2026, the measurement standard is settled.
What “interior livable space” actually means
Under SB 543, your cap is measured from the inside faces of the walls — the floor area you can actually live in. Anything that isn’t habitable interior is carved out of the calculation.
| Counts toward your cap | Does not count |
|---|---|
| Living and dining areas | Exterior wall thickness |
| Bedrooms | Interior stairways |
| Kitchen | Mechanical / utility rooms |
| Bathrooms & interior hallways | Built-in storage closets (non-habitable) |
| Habitable conditioned floor area | Garages, covered patios, porches |
The honest tradeoff. This isn’t “free” square footage. Your building footprint still grows to accommodate those walls and mechanical spaces — which means more materials, more lot coverage, and more cost. What SB 543 actually buys you is design freedom: you can now lay out a fuller, more livable interior without your usable rooms shrinking to make room for 2×6 walls and a furnace closet. On an 800 sq ft cap, that’s the difference between a cramped one-bedroom and a comfortable one.
The size caps, restated as interior space
SB 543 didn’t change the numbers — it clarified that the numbers are measured as interior livable space. Here’s the statewide baseline that applies in every San Diego-area jurisdiction (a local agency can be more generous, but never stricter):
| Unit type | Statutory minimum a city must allow | Measured as |
|---|---|---|
| Studio / 1-bedroom ADU | at least 850 sq ft | Interior livable |
| 2-bedroom or larger ADU | at least 1,000 sq ft | Interior livable |
| Detached ADU (by-right floor) | at least 800 sq ft / 16 ft / 4 ft setbacks | Interior livable |
| Attached ADU | up to 50% of the main home | Interior livable |
| Junior ADU (JADU) | up to 500 sq ft, inside the existing home | Interior livable |
Many San Diego-area jurisdictions allow detached ADUs up to 1,200 sq ft. Whatever cap applies to your lot, it’s now measured the same way — from the inside.
Two more SB 543 changes worth knowing
A 15-business-day clock on plan review
The City or County now has 15 business days from receiving your ADU or JADU application to either deem it complete or hand you a written list of exactly what’s missing. Miss the window, and the application is treated as complete. This codifies an HCD permitting guideline and puts real pressure on slow counters — though the full ministerial approval still runs on the usual ~60-day track (about 30 days for pre-approved plans).
Fee protections, confirmed
The ≤750 sq ft impact-fee exemption isn’t new — it has been law since SB 13 (2020). SB 543’s contribution is confirming it’s measured by interior livable space, which is what lets a careful design land under the line.
| Unit size (interior livable) | What applies |
|---|---|
| Under 500 sq ft | Exempt from school district developer fees; no impact fees |
| 500–750 sq ft | No local impact fees; school developer fees may apply per square foot |
| Over 750 sq ft | Impact fees may be charged, proportional to the primary home |
Because the threshold is measured as interior space, the wall-and-mechanical carve-out can be the difference that keeps a unit under 750 sq ft on paper — and impact-fee-free — while still giving you a genuinely usable interior. Keep in mind “impact fees” are only one line item: permit, plan-check, and water/sewer capacity fees still apply at any size, and in San Diego school district developer fees start at 500 sq ft, not 750.
What this means in San Diego specifically
SB 543 is state law, so it binds every jurisdiction we work in — the City of San Diego, unincorporated San Diego County, and North County cities like Encinitas, Carlsbad, and Oceanside. A few local notes:
City of San Diego. The standard ADU track and the ADU (Home Density) Bonus Program both have to honor the interior-livable-space measurement. On Bonus Program lots, where you’re stacking multiple units, measuring each unit by interior space rather than gross footprint can meaningfully change how many units fit your numbers and which ones land under the 750 sq ft fee line.
County (unincorporated). Larger rural lots already had room for bigger detached ADUs where lot coverage allows. The interior-measurement rule mostly helps you spend your cap on living area instead of structure.
North County coastal. In Encinitas, Carlsbad, Oceanside and other coastal parcels, the measurement change stacks on top of the existing coastal permit and pre-approved-plan (PRADU) processes — it doesn’t replace them.
How we use this on your project
- Feasibility & cap math We confirm the exact size cap and fee thresholds for your specific lot and jurisdiction, measured the SB 543 way.
- Design to the interior We lay out the unit to maximize livable rooms, pushing walls, stairs, and mechanical out of your cap calculation.
- Permit strategy We submit complete plans that start the 15-business-day clock clean — and document the measurement basis so a plan-checker can’t quietly revert to gross.
- Fee positioning Where it makes sense, we design to keep the unit under the 750 sq ft interior threshold to avoid impact fees, without sacrificing function.
- Build Same team carries the approved design through construction — no handoff, no scope gaps.
Want to know what SB 543 means for your lot?
We’ll run your specific jurisdiction, cap, and fee thresholds in a free feasibility check.
SB 543 FAQ
Does SB 543 mean I can build a bigger ADU?
Not a bigger legal cap — the size limits (800, 850, 1,000, 1,200 sq ft) didn’t change. What changed is that those caps are now measured as interior livable space, so walls, stairways, and mechanical rooms don’t count against them. On a typical 800 sq ft detached ADU that’s roughly 50–120 sq ft more usable interior within the same limit. Your building footprint still grows, so it isn’t free space — it’s a more livable layout for the same cap.
When does SB 543 take effect?
It was signed by Governor Newsom on October 10, 2025 and took effect January 1, 2026. It applies to ADU and JADU applications in every California jurisdiction, including the City of San Diego, San Diego County, and North County cities.
What exactly doesn’t count toward my ADU’s square footage now?
Exterior walls, interior stairways, and non-habitable areas such as mechanical and utility rooms are excluded from the size calculation. Garages, covered patios, and porches don’t count either. Your habitable interior — living areas, bedrooms, kitchen, bathrooms, and interior circulation — is what’s measured against the cap.
What is the 15-day rule in SB 543?
The City or County has 15 business days from receiving your ADU or JADU application to either deem it complete or give you a written list of what’s missing. If they miss that deadline, the application is treated as complete. It speeds up the front end of permitting; the full ministerial approval still runs about 60 days (roughly 30 for pre-approved plans).
Will I owe impact fees on my ADU under SB 543?
No local impact fees apply to an ADU with 750 sq ft or less of interior livable space. An ADU or JADU under 500 sq ft of interior livable space is also exempt from school district developer fees. Over 750 sq ft, fees may apply and are charged proportionally to the size of the primary home. Because the threshold is measured as interior space, the wall-and-mechanical carve-out can help keep a functional unit under the fee line.
Does SB 543 change the City of San Diego ADU Bonus Program?
It doesn’t replace the Bonus Program, but the program has to honor SB 543’s interior-livable-space measurement. On multi-unit Bonus Program lots, measuring each unit by interior space rather than gross footprint can change how the unit count and the 750 sq ft fee thresholds pencil out. We model this lot by lot.
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Not legal advice. This page summarizes California SB 543 (2025–2026) and related ADU law as we understand it for general informational purposes. ADU rules change frequently and local implementation varies — we verify the current requirements for your specific jurisdiction and lot before design. Prepared by IL Total Design & Build (CSLB #1058676). Last updated June 2026.
Sources: California SB 543 (McNerney), Government Code §66311 et seq.; California HCD ADU Handbook (2026); City of San Diego Information Bulletin 400 & ADU Bonus Program; County of San Diego PDS ADU Handbook; California YIMBY SB 543 summary; Atkinson, Andelson, Loya, Ruud & Romo and Holland & Knight legal analyses (2025).
