California ADU Size, Height & Setback Rules: 2026 Guide

Before anyone sketches a floor plan, every ADU project starts with three questions: how big can it be, how tall can it be, and how close to the property line can it sit? The good news for California homeowners is that state law sets a floor under all three, so a city can't zone your backyard cottage out of existence. Here's how those rules work in 2026, and what they mean for a lot in Carlsbad or anywhere else in San Diego County.

How Big Can an ADU Be in California?

State ADU law works with minimums that cities must allow, not with one statewide size. According to California's Department of Housing and Community Development (HCD), a city or county:

• Can't set a maximum size below 850 square feet for a studio or one-bedroom ADU, or below 1,000 square feet for an ADU with more than one bedroom.
• Must allow at least an 800-square-foot ADU with four-foot side and rear setbacks, even if lot coverage, floor area ratio, open space or front-setback rules would otherwise block it.
• Can't impose a minimum lot size for ADUs.

Where a city doesn't have a compliant local ordinance, the state defaults apply: up to 1,200 square feet for a new detached ADU, and up to 50 percent of the main home's floor area for an attached ADU (with at least 800 square feet always allowed). Cities may choose to be more generous than the state, so your local limit can be higher.

What counts toward the square footage?

Since January 1, 2026, the law (via SB 543) says ADU and JADU size limits apply to interior livable space. HCD's handbook defines that as space meant for living, sleeping, eating, cooking or sanitation, including bedrooms, bathrooms, kitchens, laundry rooms, closets and interior stairs. In practice, that's why your designer measures the interior so carefully: a few inches of wall thickness won't eat into your allowance the way it once could.

Junior ADUs are different

A junior ADU (JADU) is built entirely inside the walls of an existing or proposed single-family home, which can include an attached garage. It's capped at 500 square feet, and only one is allowed per single-family lot. If you're still deciding between unit types, our process page walks through how we help homeowners weigh the options early.

How Tall Can an ADU Be?

Height is where your lot's location starts to matter. Under state law, as summarized in HCD's 2026 handbook, a local agency can't set a height limit lower than:

• 16 feet for a detached ADU on a lot with a single-family or multifamily home.
• 18 feet for a detached ADU when the lot is within a half-mile of a major transit stop or a high-quality transit corridor, plus up to two more feet so the roof pitch can match the main house.
• 25 feet, or the primary dwelling's height limit if that's lower, for an attached ADU.

HCD also notes that cities must allow at least a two-story ADU where it fits within the applicable height limit and building standards. That opens the door to a second-story unit over a garage, which is often the smartest way to get more space on a smaller lot.

Tip: Not sure if you're near qualifying transit? HCD points homeowners to the California State Geoportal, which has an interactive map for checking whether a parcel qualifies for the 18-foot allowance.

Setbacks: The Four-Foot Rule

Setbacks are the required distance between a structure and your property lines, and this is the rule that unlocks most backyards:

• Side and rear setbacks: Cities can't require more than four feet from side and rear lot lines for a new-construction ADU.
• Conversions get even more room: No setback is required for an ADU created inside an existing living area or accessory structure, such as a garage conversion, or for a new structure built in the same location and to the same dimensions as an existing one.
• Front setbacks: A city may apply its front-yard setback, but per HCD, that setback can't stop an ADU of at least 800 square feet from being built. If the backyard can't fit one, the ADU may go partly or wholly in the front setback.

Exceptions San Diego Homeowners Should Know

State law sets the baseline, but a few local factors can change the picture:

• The Coastal Zone. HCD's fact sheet notes that provisions of a certified Local Coastal Program that conflict with state ADU law take priority inside the coastal zone, although the city must still approve or deny the application within 60 days. That matters for many coastal lots in Carlsbad, Oceanside, Encinitas and the City of San Diego.
• Building and fire codes still apply. Size and setback minimums don't override health-and-safety rules. Lots in fire hazard severity zones, for example, can face extra construction requirements.
• Easements and utilities. A recorded easement, a sewer lateral or an overhead power line can limit where an ADU can actually go, even when zoning would allow it.
• Local ordinances can be more generous. Some jurisdictions allow larger units or taller structures than the state minimums. Always check the current local code. The City of San Diego posts its code changes on its Development Services regulatory updates page.

Putting It Together for Your Lot

The numbers above tell you what a city must allow. What actually fits your property depends on its shape, slope, utilities, existing structures and local overlays. As a construction management agency, that's where we start every project: we map out what your lot can support, then help you choose a design that fits, whether it's a custom plan or one of our pre-approved ADU plans.

If you'd like a clear answer for your own property, book a free site walk. We'll look at your setbacks, height options and the best spot for an ADU, and talk through what size makes sense for your goals and budget.

For the full legal detail, HCD's Accessory Dwelling Unit Handbook (updated March 2026) is the best official reference.

The Bottom Line

California has made it much harder for local rules to block a well-planned ADU. Most homeowners can count on room for at least an 800-square-foot unit, four-foot side and rear setbacks, and a height of 16 feet or more. Start with those guarantees, layer in your lot's specifics, and you'll know what's possible before you spend a dollar on drawings. We're always happy to help you figure it out.