What States Allow ADU? Municipal Rules & Guidelines to Consider

TL;DR: Thinking about building an accessory dwelling unit (ADU)? This guide explores what states allow ADU development and what to know about San Diego ADU regulations specifically. Learn how state and local laws affect your ability to build.

Key Takeaways:

• Many states now allow ADUs, including California, Oregon, and Washington.
• California ADU laws provide protections and standardized rules that San Diego must follow.
• San Diego ADU regulations allow for detached ADUs on lots with a primary residence.
• Local rules can vary, so working with a design-build firm ensures code compliance.
• Property owners gain flexibility, rental income potential, and added property value with ADU construction.

When property owners consider to build an ADU, it’s essential to understand not just the local zoning but also how states across the country treat accessory dwelling units (ADUs). So what states allow ADU construction—and how do the municipal rules & guidelines differ? For a company like Lars Remodeling & Design serving homeowners in San Diego, clarity on these issues helps guide clients through the process of ADU development on their primary residence.

Which states allow ADU construction?

The simple answer: many. In fact, laws broadly allowing accessory dwelling units / ADUs have now passed in at least 18 states. Some of the most ADU‑friendly states include California, Oregon, Washington, Maine, and Connecticut. This means if you’re a homeowner outside of one of those states, you still might have local barriers or few state‑level protections—but inside those states, ADU development is actively encouraged.

California’s laws: What to know

If you’re in California—and especially in San Diego—you are operating in one of the most developed ADU landscapes. California’s accessory dwelling unit California law sets statewide minimums that local municipalities must allow. For example, recent changes allow property owners to add detached ADUs on lots (or multiple ADUs under certain conditions) and streamline the review process. According to the state ADU handbook, a local jurisdiction must permit at least one ADU on a lot with a primary residence.

Some highlights of California’s rules:

• Smaller ADUs (750 sq ft or less) often face no impact fees.
• The approval process for ADUs that meet state standards must be ministerial and completed within a defined time.
• Local regulations cannot impose owner‑occupancy requirements (in certain cases), minimum lot size or parking in all situations.

San Diego specific: city and county guidelines

For homeowners in San Diego looking at accessory dwelling unit San Diego or ADU rules for San Diego, local rules dovetail with state law—but still require careful attention. The city and county adopt ordinances that build on what state law requires. For instance, municipal code in San Diego must accommodate ADU rules in California but may apply additional design or setback requirements compatible with state law. For example, the state law provision applies to properties where there is an existing or proposed primary dwelling unit.

If you’re looking to build an ADU or add detached ADUs on your lot, working with a full‑service remodel firm like Lars means navigating city permits, zoning, design, construction and the integration of the new unit into your property. The ideal scenario: you own your lot with a primary residence, you engage experts, and you pursue ADU construction that follows both state and San Diego county/city guidelines.

Why this matters for homeowners & remodelers

For property owners, knowing that the state and local laws allow ADUs means you can leverage additional housing, rental income potential, or living flexibility. And Lars Remodeling and Design can help walk you through these requirements. Especially when designing and managing accessory dwelling units / ADUs as part of a broader remodel, being aware of which states allow ADUs (and how those rules vary) gives clients confidence you’ll handle everything end‑to‑end.

Check out our ADU building services!