How SB 543 Changes the Way ADU Square Footage Is Measured – And Why It Matters for Your Project

If you’re planning to build an ADU or Junior ADU (JADU) in California, one of the most important numbers in your project is the square footage limit. It determines what you can build, what fees you’ll pay, and whether your project qualifies for key exemptions. Until now, there has been real confusion about how that number is measured. Senate Bill 543, signed by Governor Newsom on October 10, 2025, and effective January 1, 2026, finally puts that confusion to rest.

What Changed: “Interior Livable Space” Is Now the Standard

Before SB 543, California’s ADU laws referred to size limits in terms of “square footage” without clearly defining what that meant. The California Building Code defines floor area as the space measured from the interior perimeter of the exterior walls, but several local jurisdictions interpreted the limit as applying to the entire building footprint, including exterior walls, stairways, and other non-livable areas.

SB 543 resolves this by amending the Government Code to specify that all statutory references to ADU and JADU square footage now refer to interior livable space. Exterior walls, interior stairways, and other non-habitable areas are excluded from the size calculation.

This means the size caps that matter for your project – 850 square feet for a studio or one-bedroom ADU, 1,000 square feet for a two-bedroom or larger ADU, and 500 square feet for a JADU – are all measured based on the usable living area inside the unit, not the overall building footprint.

Why This Matters for Homeowners and Builders

This seemingly technical clarification has real consequences for anyone planning an ADU project in Los Angeles or anywhere else in California.

You May Be Able to Build a Larger Unit

Under the old interpretation used by some jurisdictions, your exterior walls and stairs counted against the size cap. A standard 2×6 framed wall with exterior sheathing and siding can be 7 to 8 inches thick. On all four sides of an ADU, that adds up. For instance, an 850-square-foot footprint could have yielded only around 750 to 780 square feet of actual living space once wall thickness was subtracted.

Now that the limit is measured by interior livable space, you can design to the full 850 or 1,000 square feet of usable area. The actual building footprint may be slightly larger, and that’s allowed.

Design Calculations Become More Predictable

Previously, architects and designers had to account for varying local interpretations of how square footage was measured. One city might measure from the exterior; another from the interior. SB 543 creates a single statewide standard. This reduces confusion, cuts down on plan check rejections, and streamlines the permitting process.

Fee Exemptions Are Now Clearer

SB 543 also clarifies that development impact fee (DIF) exemptions are based on interior livable space. Here’s how the updated fee structure works:

Unit Type	Impact Fee Exemption	School Fee Exemption
ADU ≤ 750 sq ft interior livable space	Exempt	Only if ≤ 500 sq ft
ADU > 750 sq ft interior livable space	Proportionate fees	Standard fees apply
JADU ≤ 500 sq ft interior livable space	Exempt	Exempt

Because the measurement is now based on interior livable space rather than the building footprint, some ADUs that would have previously exceeded the 750-square-foot threshold may now fall under it – saving homeowners thousands of dollars in fees.

A Practical Example

Let’s say you’re designing a detached one-bedroom ADU and your architect draws a unit with a building footprint of 820 square feet. Under the old interpretation used by some LA area jurisdictions, that footprint would have been your “square footage” – safely under the 850-square-foot cap, but getting close.

Under SB 543, what matters is the interior livable space. Subtract the wall thickness on all sides and you might be looking at around 740 to 760 square feet of interior livable area. That’s your number. This also means you could potentially push the footprint a bit larger and still stay within the 850-square-foot interior livable space limit, giving you more room to work with in your floor plan.

Bonus: SB 543 Also Clarifies Combining ADU Types

Beyond the measurement change, SB 543 resolves another long-standing question: whether you can combine different types of ADUs on a single lot. The answer is now clearly yes.

• Single-family lots: You can have a detached ADU, a converted ADU (such as a garage conversion), and a JADU – all on the same property.
• Multifamily lots: You can combine multiple converted ADUs with up to two detached ADUs.

This is a significant opportunity for property owners looking to maximize the housing potential of their lot.

What You Should Do Now

If you’re in the planning or design phase of an ADU project, this is a good time to revisit your plans with the new measurement standard in mind. Projects that were borderline on size or fee thresholds under the old rules may now have more room, both literally and financially.

Keep in mind that local agencies are required to update their ADU ordinances to comply with SB 543. If your city hasn’t done so yet, their existing ordinance may be at risk of being voided entirely for noncompliance with state law. The California Department of Housing and Community Development (HCD) has oversight authority, and noncompliant local ordinances can be invalidated.

Need help navigating the new rules?

At CCS Inc., we help homeowners and property owners across Los Angeles navigate every phase of the ADU process from design and permitting to construction and compliance. If you’re unsure how SB 543 affects your project, schedule a free 15-minute consultation with our team and we’ll walk you through it.

Disclaimer: This blog post is provided for informational purposes only and does not constitute legal advice. For guidance specific to your project, consult with a licensed professional or your local planning department.