2027 ADU Regulations: California Law Changes Explained
Four new California ADU regulations go into effect in 2027, and together they change how many ADUs you can build, what you pay in impact fees, and how long permitting takes.
Governor Newsom has signed all four bills: AB 956, SB 1117, AB 1621 and AB 2601. These aren't jargon cleanup bills. If you're researching the 2027 ADU regulations, whether you own a single-family home or you're an investor eyeing SB 9, these are the changes that will shape your next project. The headline: building two detached ADUs just got a lot easier.
I'm Joseph Peretz, ADU educator and founder of NeoBuilders. NeoBuilders has completed more than 100 ADUs and larger development projects, so below we walk through what each bill does, what it doesn't do, and how to think about it before you spend a dollar.
AB 956: Two Detached ADUs on a Single-Family Property
AB 956 lets owners of a qualifying single-family property build two detached ADUs instead of one. This is the bill everyone is talking about, and it's one of the biggest California ADU regulation changes in years.
For the last few years, a single-family property could have one detached ADU under 800 sq ft approved through ministerial approval. Ministerial approval means the city must approve your application if it meets objective state standards. There are no public hearings, no neighbor input and no discretionary judgment. Under AB 956, both detached ADUs can be approved ministerially as long as each one is under 800 sq ft.
That's on top of the conversion ADU and JADU you're already allowed to build. On the right lot, a property could look like this:
Add SB 9 potential on top of that, depending on the property. That's five independent living units on a lot that was originally permitted for one.
A few things to keep in mind:
Size matters. You can go larger than 800 sq ft, but you lose the ministerial pathway and may have to jump through extra permitting hoops. It can be done.
Setbacks still apply. This doesn't mean you can start cramming ADUs onto a property. Your tenants still need to live comfortably, with real privacy.
The JADU has a caveat. If you build two detached ADUs under this pathway, the city isn't required to also approve a JADU ministerially on that same property. For most people this isn't a big deal, but it's worth knowing.
The bigger shift is in the question we're asking. It used to be "Can I build an ADU?" Now it's "How much housing can I create on this property?" For families, that could mean housing multiple generations on one lot. For investors, it's a completely different way to evaluate the development potential of a single-family property.
SB 1117: The 750 Sq Ft Impact Fee Change
SB 1117 changes how impact fees are calculated on larger ADUs: for properties with up to two ADUs, only the square footage above 750 sq ft counts, not the whole unit. It answers one of the questions homeowners ask us most: how much are the fees going to be?
To see why it matters, start with how the old rule worked. An ADU of 750 sq ft or less was exempt from impact fees. Once an ADU went over 750 sq ft, fees could be charged proportionally, based on the ADU's size relative to the primary residence. That created what I call the 750-square-foot cliff. Make your ADU 751 sq ft instead of 750, and the fee could be based on the entire ADU, not just that extra square foot.
ADU size | Old calculation | SB 1117 (up to 2 ADUs) |
750 sq ft | Exempt | Exempt |
751 sq ft | Fee could be based on all 751 sq ft | Fee based on 1 sq ft |
1,000 sq ft | Fee could be based on all 1,000 sq ft | Fee based on 250 sq ft |
Two details to get right:
The 750 sq ft threshold is interior livable space. It's not the exterior footprint of the structure. This was clarified in SB 543, which we covered in our California ADU laws going into 2026 breakdown.
SB 1117 only addresses development and impact fees. You will likely still pay other city fees to build an ADU.
The catch: three or more ADUs
SB 1117's special calculation applies to properties with no more than two ADUs. This is where it connects to AB 956. If you build the two new detached ADUs, you can potentially use the new calculation on each one.
But say you also convert your garage into an ADU. That's three ADUs total, and the special treatment no longer applies. The proportional calculation can then apply to the full size of each ADU over 750 sq ft. With three ADUs, two of them at 800 sq ft, both could be calculated on their full 800 sq ft rather than just the 50 sq ft over the threshold.
The number of ADUs you build can change how your impact fees are calculated, so factor that in when you plan multiple units.
AB 1621: No More Endless Plan Checks?
AB 1621 generally limits building permit applications to two plan check and specification reviews, and shortens certain appeal timelines. Every builder knows the cycle: you submit plans, the city reviews them, you make corrections, you resubmit, and then you get another round of corrections. AB 1621 puts some structure around that process.
Here's what it does:
Caps plan checks. For building permit applications, a local agency generally can't require or request more than two plan check and specification reviews. After that, it can't keep putting the application through unlimited rounds under the normal process.
Shortens appeals. The timelines drop as shown below.
Adds a path to compel approval. In certain circumstances, such as an appeal that's denied, a decision that isn't made on time, or an appeals process that isn't provided, the applicant can seek a writ of mandate to compel approval.
Limits inspection surprises. If a complete application was already determined to be compliant, the agency generally can't use a later building inspection to require something that deviates from the approved plans, except where the law allows it.
Housing development project size | Old appeal timeline | New appeal timeline |
25 units or fewer | 60 business days | 30 business days |
26 units or more | 90 business days | 45 business days |
Here's what it doesn't do. It does not mean your permit is approved after two reviews. If an application still doesn't comply with the applicable standards, the agency can deny it, and the applicant can request additional submittals. The goal is to keep a compliant project from getting trapped in a cycle of revisions.
AB 1621 also isn't an ADU-specific law. It's a broader housing and post-entitlement permitting law. It's relevant to ADUs because every ADU still goes through the building permit and inspection process. The takeaway: more structure around the review process, and stronger protection against unnecessary delays.
AB 2601: A Big One for SB 9 Projects
AB 2601 allows certain SB 9 housing development applications and urban lot split applications to be processed at the same time, instead of one after another. This one doesn't address ADUs, so if you're not interested in investment projects, skip ahead. If you are an investor, pay attention.
AB 2601 doesn't change the number of units you can build under SB 9. It changes how you get through the process. An SB 9 project has two major pieces: the housing development itself and the urban lot split. Under AB 2601, the housing development and the parcel map or urban lot split can move through review concurrently.
Why that matters: every time you have to wait for one approval before you can even submit the next application, you add time to the project. On a development project, cutting that unnecessary sequencing can make a real difference to cost and feasibility.
If you're considering an SB 9 lot split in California, put this law on your radar going into 2027.
How These California ADU Regulations Work Together
AB 956, SB 1117 and AB 1621 each tackle a different part of the ADU process: how many units you can build, how much you may pay in impact fees, and how permitting works. They make the most sense when you look at them together.
Imagine a single-family lot with an existing house. Under AB 956, you may now be able to build two detached ADUs through the qualifying ministerial pathway. Under SB 1117, if you have no more than two ADUs, any ADU over 750 sq ft can have its impact fee calculated on only the portion above 750 sq ft. And as you move through the building permit process, AB 1621 puts new limits on plan-check reviews and shortens certain appeal timelines.
Bill | What it does | Who it matters most to |
AB 956 | Allows two detached ADUs on a qualifying single-family property | Homeowners and investors |
SB 1117 | Reduces the impact fee burden on larger ADUs (up to two ADUs) | Anyone building over 750 sq ft |
AB 1621 | Limits plan-check rounds and shortens appeal timelines | Anyone going through building permits |
AB 2601 | Lets SB 9 development and lot split applications run concurrently | SB 9 investors and developers |
Honorable Mentions: Electrical Hookups and Coastal ADUs
Two more bills are worth knowing about, especially if utilities or a coastal location could affect your timeline.
SB 1196: Electrical Hookup
SB 1196 requires the California Public Utilities Commission (CPUC) to set timelines by September 30, 2027 for electric utilities processing certain smaller electrical projects, called small energization projects. Anyone who has built in California knows getting utilities connected can become a big part of the schedule. The goal here is more predictability. The timelines cover things like:
When the utility has to determine whether an application is complete
How deficiencies are communicated to the applicant
How long the utility has to process certain steps
Providing cost estimates, including potential infrastructure upgrades
Accountability if the utility doesn't meet the established timelines
Don't walk away thinking every ADU now gets its electrical hookup within a set number of days. SB 1196 doesn't do that. It requires the CPUC to establish timelines for qualifying small energization projects, which makes the process more predictable and accountable, not universally fast.
SB 1077: Coastal ADUs
SB 1077 isn't a new 2027 law. It was enacted earlier, and we also covered it in our last bills breakdown. It set a July 1, 2026 deadline for the California Coastal Commission, working with the Department of Housing and Community Development, to develop guidance for local governments on how to facilitate and clarify the permitting process for ADUs and JADUs in the coastal zone.
That doesn't mean every coastal ADU is suddenly approved automatically. Coastal properties can still have additional requirements, and local coastal programs and permitting requirements still matter. If you're building near the coast, look at your specific property and jurisdiction.
Before You Build: Be Smart About What You're Building
Just because the law says you can doesn't mean you should start planning two detached 1,000 sq ft ADUs on your lot. Bigger isn't always better, and every property has constraints. Before you act on any of the 2027 ADU regulations, run through three checks:
Look at your finances. Can you afford the monthly payments for these projects? They are not cheap.
Look at your lot. Can you comfortably fit the new units without sacrificing comfort and privacy? Setbacks and layout decide this, not the new law.
Plan your 5, 10 and even 20 year goals. If these projects don't make sense for you in the long run, you may want to pivot.
I've completed more than 100 of these projects, both ADUs and large development projects, so I've seen how these decisions play out. The right plan comes from your lot, your budget and your goals, not from the maximum the law allows.
Catch Up on Previous Years' California ADU Regulations
California keeps stacking ADU changes year over year, and knowing what came before helps you read the 2027 changes in context. Our earlier breakdowns cover the laws that shaped the last two years:
Frequently Asked Questions About the 2027 ADU Regulations
How many detached ADUs can I build on a single-family property in 2027?
Under AB 956, a qualifying single-family property can have two detached ADUs instead of one, and both can be approved ministerially if each is under 800 sq ft. You can go larger than 800 sq ft, but you lose the ministerial pathway and may face extra permitting steps.
What is ministerial approval?
Ministerial approval means the city must approve your ADU application if it meets objective state standards. There are no public hearings, no neighbor input and no discretionary judgment from the city.
Can I also build a garage conversion ADU and a JADU?
The garage conversion ADU is allowed in addition to the two detached ADUs. The JADU has a caveat: if you build two detached ADUs under this pathway, the city isn't required to also approve a JADU ministerially on the same property.
Does SB 1117 reduce impact fees on every ADU?
No. It applies to properties with up to two ADUs, and it only changes the calculation for ADUs over 750 sq ft, where only the portion above 750 sq ft counts. The 750 sq ft measurement is interior livable space. With three or more ADUs, the proportional calculation can apply to the full size of each ADU over 750 sq ft. SB 1117 also only addresses development and impact fees, so other city fees will likely still apply.
Is my building permit approved automatically after two plan checks under AB 1621?
No. AB 1621 generally limits a local agency to two plan check and specification reviews, but it doesn't force approval. If the application still doesn't comply, the agency can deny it, and the applicant can request additional submittals.
Does AB 2601 change how many units I can build under SB 9?
No. AB 2601 doesn't change the number of units allowed under SB 9. It lets certain housing development applications and parcel map or urban lot split applications be processed concurrently instead of one after the other.
Does SB 1196 guarantee my ADU's electrical hookup within a set number of days?
No. SB 1196 requires the CPUC to establish timelines by September 30, 2027 for qualifying small energization projects. It aims to make the process more predictable and accountable, not to set one universal deadline for every ADU.
Is SB 1077 a new 2027 law?
No. SB 1077 was enacted earlier. It set a July 1, 2026 deadline for the California Coastal Commission to develop guidance for local governments on permitting ADUs and JADUs in the coastal zone. Coastal properties can still have additional requirements under local coastal programs.
How do I know what I can build on my property?
It depends on your lot, your zoning, setbacks and your budget. Book a call with NeoBuilders and we'll review your property together.
Talk to NeoBuilders About Your ADU or Multi-Family Project
The 2027 California ADU regulations open up real options, but the right move depends on your lot, your budget and your goals. If you're weighing two detached ADUs, a garage conversion, or an SB 9 multi-family development, book a call with NeoBuilders and we'll look at your property together and talk through what makes sense.
Comments