Can You Build an ADU? What the Rules Actually Say

Accessory dwelling units, commonly called ADUs, granny flats, or in-law suites, have become one of the most discussed topics in residential real estate. They offer homeowners a way to add living space for family members, generate rental income, or increase property value. But the rules governing whether and how you can build one are complex, varying by state, city, and sometimes even by neighborhood. Here is a practical breakdown of what the regulations actually say.

State vs local rules: who has the final word?

The relationship between state and local ADU regulations depends entirely on where you live. California has been the most aggressive state in mandating ADU access, passing a series of laws that override many local restrictions. Under California law, cities must allow at least one ADU on any lot with an existing or proposed single-family home. Cities cannot require owner occupancy for ADUs permitted before 2025, cannot impose minimum lot size requirements beyond what is needed for the ADU itself, and cannot require additional parking for ADUs near public transit. Other states have followed with varying degrees of preemption. Washington state has its own ADU-enabling legislation. In contrast, states like Texas and Florida leave ADU regulation almost entirely to local governments, resulting in a patchwork where one city allows them freely and a neighboring city effectively prohibits them.

Lot size requirements vary widely

Where state law does not preempt local rules, cities often set minimum lot sizes for ADU construction. A city might require a minimum lot of 5,000 or 6,000 square feet before an ADU is permitted, which effectively excludes smaller urban lots. In California, state law prohibits cities from using minimum lot size as a barrier, so even small lots can qualify. But in cities in Texas, Arizona, and other states without preemption, lot size requirements are one of the most common barriers. Check your city's zoning code for the specific minimum lot size in your zone before you invest in architectural plans.

Setback rules determine where the ADU can go

Setbacks define how far a structure must be from property lines, and they are often the most practical constraint on ADU placement. A typical rear setback of 5 feet and side setbacks of 4 feet can significantly limit where a detached ADU can be placed, especially on narrow or shallow lots. California law limits rear and side setbacks for ADUs to 4 feet regardless of what the underlying zone would otherwise require, which opened up construction on many lots that were previously too constrained. Cities in other states may apply the same setbacks to ADUs as they apply to any other accessory structure, which can mean 10 feet or more from rear and side property lines.

Parking waivers make a real difference

One of the historic barriers to ADU construction was the requirement to provide additional off-street parking. Adding a parking space on an already-developed lot is expensive and sometimes physically impossible. California eliminated most parking requirements for ADUs, particularly those located near public transit, in areas with car-share availability, or on properties where the existing parking supply meets certain thresholds. Cities like Seattle have also reduced parking requirements for ADUs. Where parking is still required, it typically means one dedicated space for the ADU, which can be in tandem configuration in the driveway.

Utility connections add cost and complexity

A detached ADU needs connections for water, sewer, and electricity. Some cities require separate utility meters for ADUs, which adds cost but gives the ADU tenant their own utility accounts. Other cities allow ADUs to share connections with the main house. Impact fees for new utility connections vary widely and can add thousands of dollars to the project cost. California law caps utility connection fees for ADUs and prohibits cities from charging impact fees on ADUs under 750 square feet. In other states, the full range of impact and connection fees may apply.

Owner-occupancy requirements are changing

Some cities have historically required the property owner to live on-site, either in the main house or in the ADU, as a condition of permitting the ADU. This requirement limits ADU construction by investors and makes the decision more personal for homeowner-occupants. California suspended owner-occupancy requirements through 2025 for ADUs permitted under state law, making it possible for investors to build and rent both the main house and the ADU. Other states and cities vary on this point. Some require owner occupancy, some do not, and some have suspended the requirement temporarily as part of housing production efforts.

Size limits define the scope

Maximum ADU size is typically capped at a percentage of the primary dwelling's floor area or an absolute square footage limit, whichever is smaller. California allows detached ADUs up to 1,200 square feet regardless of the main house size. Attached ADUs are typically limited to 50 percent of the existing living area. Cities in other states may set lower maximums, commonly 600 to 800 square feet. These size limits directly affect the ADU's usefulness and rental value, so they are worth understanding before you begin design work.

Start with your city's planning department

The best first step is to contact your city's planning department and ask about ADU regulations for your specific property. Provide your address and parcel number, and ask what zone you are in, what the ADU rules are for that zone, and what permits are required. Many cities have dedicated ADU informational pages on their websites with checklists, fee schedules, and pre-approved plan sets. The regulations are complex, but the information is available if you know where to look.