ADU Questions · 2026
Can an ADU Be Bigger Than the Main House?
In most California cities, no. An ADU is generally required to be “subordinate” in size to the primary residence, meaning smaller than the main home, and many local ordinances cap ADU square footage as a percentage of the primary home’s floor area on top of the state’s flat 1,200 sq ft maximum.
The short version: ADUs are meant to be secondary units. State law caps them at 1,200 sq ft, but plenty of cities go further and tie the maximum to a percentage of the primary home’s size, so an ADU larger than the main house would be unusual and, in many jurisdictions, simply not allowed. Confirm the exact local rule for your property before assuming either way.
What “subordinate in size” actually means
California’s ADU statutes were written around a simple idea: an ADU is a second, smaller unit added to a property that already has a primary residence. It’s not meant to become the dominant structure on the lot.
That concept shows up in a few different ways depending on the jurisdiction:
- Flat state caps. No detached ADU can exceed 1,200 sq ft, no matter how large the primary home is.
- Percentage-of-primary caps. Many cities set a local maximum expressed as a percentage of the main house’s square footage (for example, a cap somewhere in the range of 50 percent of the primary home’s floor area), which can land well below 1,200 sq ft on smaller homes.
- The guaranteed minimum. Even with a percentage rule, state law requires every city to allow at least an 800 sq ft, 16 ft tall ADU with 4 ft side and rear setbacks, regardless of how small that would make the percentage cap.
Because these rules stack, the real ceiling on a given lot is often whichever is smaller: the flat state cap, the local percentage cap, or (as a floor) the 800 sq ft guarantee.
Why this makes an oversized ADU unlikely
If a city’s ordinance ties ADU size to a percentage of the primary home, a bigger ADU only becomes possible if the primary home is also bigger. A small percentage cap on a modest 1,400 sq ft house, for instance, could land well under the state’s 1,200 sq ft ceiling, keeping the ADU meaningfully smaller than the house it sits next to.
That’s by design. Local governments generally intend for the main home to remain the dominant structure on a single-family lot, both in scale and in how the property reads from the street. An ADU that outsizes the primary residence would work against that intent, which is exactly why so many local codes build the subordinate-size rule directly into their ordinances.
None of this is universal, though. Percentage caps, exact thresholds, and how they interact with the 800 sq ft guaranteed minimum vary by city and county. Some jurisdictions lean closer to the state’s flat caps with little added restriction; others layer on stricter local percentage limits. The only way to know your property’s real ceiling is to check the local ordinance or have it confirmed as part of a feasibility study.
The size rules at a glance
| Rule | What it means |
|---|---|
| State maximum | 1,200 sq ft for a detached ADU, everywhere in California |
| Guaranteed minimum | Every city must allow at least an 800 sq ft, 16 ft tall ADU with 4 ft side/rear setbacks |
| Local percentage caps | Many cities cap ADU size as a percentage of the primary home’s square footage, which can be stricter than the state maximum |
| JADU | Up to 500 sq ft, built within the walls of the existing single-family home |
| Practical takeaway | An ADU larger than the primary residence is uncommon and often disallowed locally, even where state law alone wouldn’t prohibit it |
How this plays out with Framework First
Framework First’s 14 models range from 405 to 1,200 sq ft, which covers the full spread from a compact JADU-style layout up through the state’s maximum detached ADU size. Because every model price includes a permitting budget along with plans, foundation, the factory build, delivery, utility hookups, appliances, and final inspection, the model size you choose has a direct, transparent effect on the total price, though your exact permit costs and any property-specific site work get confirmed in the feasibility study. You can browse the full lineup at /models/ or see current all-inclusive pricing at /pricing/.
Since local percentage caps depend on your specific primary home’s square footage, the right next step is confirming what your city or county actually allows on your lot. A feasibility study checks your property against the local ordinance, including any subordinate-size rule, before you commit to a model.
Frequently asked questions
Is there a specific percentage limit that applies statewide?
No. California’s state law sets the flat 1,200 sq ft maximum and the 800 sq ft guaranteed minimum, but it does not set one uniform percentage-of-primary-home rule. Percentage caps, where they exist, are set locally and vary by city and county. Confirm the specific figure with your local planning or building department.
What happens if my primary home is small, like 900 sq ft?
If your city applies a percentage-of-primary cap, a smaller primary home could push your allowable ADU size below what you’d otherwise expect, though the state’s 800 sq ft guaranteed minimum still applies as a floor in most cases. This is exactly the kind of detail worth confirming before choosing a model, since it can rule certain sizes in or out for your specific lot.
Does the subordinate-size rule apply to JADUs too?
Yes, in a different way. A JADU is capped at 500 sq ft and must be built within the walls of the existing single-family home, so by definition it’s a smaller unit carved out of the primary residence rather than a separate structure competing with it in scale.
Can I just build the largest model regardless of my home’s size?
Not necessarily. The 1,200 sq ft state cap is a ceiling, not a guarantee that every property qualifies for it. Local percentage rules, lot coverage limits, and setback requirements can all reduce what’s actually buildable on a specific property. A feasibility study is the most reliable way to find your property’s real maximum before selecting a model.
Who do I ask to confirm the exact rule for my property?
Your city or county planning department can confirm the local ordinance directly. For a faster, project-specific answer, Framework First’s feasibility study checks your property against local zoning, including any subordinate-size or percentage-based ADU rule, as part of the process.
Ready to find out what size ADU your property actually qualifies for? Start a feasibility study or browse our full model lineup to compare sizes and pricing.