Can I Build an ADU? A Complete Guide to Accessory Dwelling Units

Accessory dwelling units -- commonly called ADUs, granny flats, in-law suites, or backyard cottages -- have become one of the most popular ways to add housing in the United States. Whether you want to house an aging parent, generate rental income, or simply make better use of your property, an ADU can be a practical and valuable addition.

But before you start sketching floor plans, you need to understand the rules. ADU regulations vary dramatically by state and city, and the landscape has changed rapidly in recent years as state legislatures have stepped in to make ADUs easier to build.

What Is an ADU?

An ADU is a secondary dwelling unit located on the same lot as a primary single-family home (and increasingly, on multi-family lots as well). To qualify as an ADU under most codes, the unit must be a complete, independent living space with its own:

• Kitchen or kitchenette (with cooking facilities)
• Bathroom
• Sleeping area
• Separate entrance

An ADU is smaller than the primary residence and is considered "accessory" to it. Unlike simply renting out a room in your house, an ADU is a legally distinct dwelling unit with its own address (or unit number) for mail, utilities, and sometimes separate utility meters.

Types of ADUs

There are four main types of ADUs, and the type you choose affects cost, complexity, and which regulations apply.

Detached ADU

A detached ADU is a standalone structure built separately from the primary home, usually in the backyard. This is what most people picture when they think of an ADU -- a small cottage or studio behind the main house.

Advantages: Maximum privacy and independence for occupants, flexible placement on the lot, can be designed from scratch.

Challenges: Highest construction cost, may require new utility connections, takes the most yard space, may trigger setback complications.

Typical size: 400 to 1,200 square feet depending on local limits and lot size.

Attached ADU

An attached ADU shares at least one wall with the primary residence. This can be a home addition (a new wing or second story) or the conversion of an existing attached garage.

Advantages: Lower cost than detached since it shares a wall and foundation, easier utility connections, smaller site impact.

Challenges: Construction disruption to the main house, shared walls mean shared noise, may require structural modifications.

Typical size: 400 to 1,000 square feet.

Garage Conversion

Converting an existing detached or attached garage into an ADU is often the most affordable path. The basic structure already exists -- you are adding insulation, interior walls, plumbing, a kitchen, and a bathroom.

Advantages: Lowest construction cost, fastest build time, uses existing footprint so setback compliance is usually easier.

Challenges: Loss of garage parking (some jurisdictions have waived replacement parking requirements), existing structure may need significant upgrades to meet building code, limited size and layout options.

Typical size: 250 to 600 square feet depending on garage size.

Junior ADU (JADU)

A junior ADU is a small unit created within the existing footprint of the primary home -- typically by converting a bedroom, master suite, or other interior space. JADUs are a specific category recognized in California law and a few other jurisdictions.

Advantages: Very low cost (often under $50,000), no new construction footprint, can coexist with a full ADU on the same lot in California.

Challenges: Very small (limited to 500 sq ft in California), must be within the existing house, may share a bathroom with the main house in some cases, limited kitchen (efficiency kitchen allowed in California).

Typical size: 200 to 500 square feet.

Typical ADU Requirements

While requirements vary significantly by jurisdiction, here are the standards you will encounter in most cities that allow ADUs.

Size Limits

Most cities cap ADU size based on the lot size or the size of the primary residence:

• Minimum: 150 to 220 square feet (studio minimum)
• Maximum: Commonly 800 to 1,200 square feet, or a percentage of the primary home's floor area (often 50%)
• California statewide minimum: Cities must allow ADUs of at least 800 square feet regardless of lot size or FAR limits

Height Limits

ADU height is often more restrictive than the zone's general height limit:

• One-story detached ADUs: Typically 16 feet maximum
• Two-story detached ADUs: Typically 20 to 25 feet where allowed
• California: State law mandates cities allow at least 16 feet for one-story ADUs, 18 feet for ADUs near transit, and 18 feet for two-story ADUs where applicable

Setbacks

ADU setback requirements are often reduced compared to the primary residence:

• Rear and side setbacks: 4 feet is the standard minimum in California and increasingly common elsewhere
• Front setback: ADUs are typically required to be behind the primary residence, so the front setback is usually not an issue for detached ADUs
• Garage conversions: Existing structures that already encroach into setbacks can usually be converted without needing to comply with current setback rules

Parking

Parking requirements have historically been one of the biggest barriers to ADU construction. Many cities required one or two additional off-street parking spaces per ADU, which was impractical on smaller lots.

The trend is strongly toward reducing or eliminating ADU parking requirements:

• California: No parking required for ADUs within half a mile of public transit, one block of a car-share vehicle, in a historic district, when the ADU is part of a conversion, or in many other circumstances
• Many other cities have reduced requirements to one space or zero spaces

Owner Occupancy

Some jurisdictions require the property owner to live in either the primary residence or the ADU. This is intended to ensure neighborhood accountability but limits the use of ADUs as investment properties.

California suspended owner-occupancy requirements for ADUs through state law. Check your local jurisdiction for current rules, as this is an area of frequent legislative change.

State Laws That Mandate ADUs

The most significant development in ADU regulation over the past decade has been state-level intervention. Several states have passed laws that override local barriers to ADU construction.

California: The National Leader

California has passed the most comprehensive ADU legislation in the country through a series of bills:

• AB 68, SB 13, AB 881 (2019): The landmark trio that transformed California ADU policy. These laws established ministerial (by-right) approval for ADUs meeting basic standards, mandated a 60-day permit review timeline, removed owner-occupancy requirements, limited impact fees, and prohibited local ordinances that effectively block ADU construction.
• AB 2221 (2022): Clarified and strengthened previous laws, including barring cities from requiring dedications or other exactions as conditions of ADU approval.
• AB 976 (2023): Made the removal of owner-occupancy requirements permanent (previously a sunset provision).
• AB 1033 (2023): Allows cities to permit ADUs to be sold separately from the primary residence as condominiums.

Under California law, cities must approve an ADU application ministerially (without discretionary review or public hearings) within 60 days if it meets objective standards. Cities cannot impose requirements that make ADU construction infeasible, such as excessive fees, overly restrictive design standards, or unreasonable owner-occupancy rules.

California also allows both a full ADU and a JADU on a single-family lot, effectively enabling three units on what was formerly a single-family parcel.

Other States with ADU-Supportive Laws

• Oregon: HB 2001 (2019) requires cities with populations over 10,000 to allow ADUs on lots where single-family homes are permitted.
• Washington: HB 1337 (2023) requires most cities to allow at least two ADUs per lot.
• Vermont: Act 47 (2023) legalized ADUs statewide on any lot with a single-family home.
• Montana: SB 528 (2023) legalized ADUs statewide.
• Connecticut, Massachusetts, and other states have passed or are considering similar legislation.

If your state has not passed an ADU mandate, your options depend entirely on your local city or county ordinance. Some cities have embraced ADUs voluntarily (Portland, Seattle, Austin), while others still effectively prohibit them through restrictive standards.

The ADU Permit Process

Here is a typical workflow for getting an ADU built, from concept to completion.

Step 1: Research (1-2 weeks)

Before spending money on design, verify that your property is eligible for an ADU:

• Confirm your zoning allows ADUs (or your state mandates them)
• Check setback, height, and size limits
• Look for overlay districts (historic, flood, hillside) that might add requirements
• Review HOA CC&Rs if applicable
• Contact your city's planning department for a pre-application consultation

Step 2: Design and Engineering (2-4 months)

Hire an architect or designer to create construction documents. You will need:

• Site plan showing the ADU location relative to the primary home and property lines
• Floor plans, elevations, and sections
• Structural engineering (especially important for garage conversions and two-story ADUs)
• Title 24 energy calculations (in California) or equivalent energy code compliance
• Possibly a soils/geotechnical report depending on site conditions

Many companies now offer pre-designed ADU plans that can be customized to your lot. Some cities have even created pre-approved ADU plan libraries to speed up permitting.

Step 3: Permit Application (1-4 months)

Submit your plans to the city building department for plan check. The review process includes:

• Zoning compliance review (planning department)
• Building code review (building department)
• Fire code review (fire department or fire marshal)
• Utility clearances (water, sewer, electrical)

In California, the city must approve or provide corrections within 60 days. Other jurisdictions may take longer. Expect at least one round of corrections and resubmittal.

Step 4: Construction (3-12 months)

Once you have your building permit, construction can begin. A garage conversion might take 3 to 4 months. A new detached ADU typically takes 6 to 12 months. Key milestones include:

• Foundation and framing inspections
• Rough plumbing, electrical, and mechanical inspections
• Insulation inspection
• Final inspection and certificate of occupancy

Step 5: Final Approvals

After passing the final building inspection, you will receive a certificate of occupancy (or equivalent). At this point, your ADU is legally habitable. You may also need to:

• Set up a separate address or unit number with the post office
• Arrange utility connections or separate metering
• Register the unit with the city if required for rental tracking

Costs and Financing

Construction Costs by Type

These ranges reflect national averages. Costs in high-cost metro areas (San Francisco, Los Angeles, New York, Seattle) can be 30-50% higher.

Additional Costs to Budget For

• Design and engineering: $5,000 to $20,000
• Permit fees: $2,000 to $15,000 (California limits impact fees for ADUs under 750 sq ft)
• Utility connections: $5,000 to $20,000 for new sewer, water, and electrical service
• Site work: Varies widely depending on grading, tree removal, and access

Financing Options

• Home equity loan or HELOC: Borrow against existing home equity
• Cash-out refinance: Refinance your mortgage and pull out cash for construction
• Construction loan: Short-term loan specifically for building, which converts to a permanent mortgage
• ADU-specific loan products: Several lenders now offer loan products designed for ADU construction
• CalHFA ADU Grant (California): California offers grants up to $40,000 for ADU pre-development costs

Is an ADU Right for You?

Building an ADU is a significant investment of time and money. Before committing, consider these factors:

• Rental income potential: Research rental rates for small units in your area. In many markets, a well-located ADU can generate $1,000 to $3,000+ per month in rental income.
• Property value impact: Studies consistently show that an ADU adds value to a property, often exceeding the construction cost in high-demand markets.
• Your lot's constraints: Not every lot is well-suited for an ADU. Very small lots, lots with challenging topography, or lots with extensive tree coverage may make construction impractical or prohibitively expensive.
• Neighbor relations: While you have the legal right to build an ADU (especially in ADU-mandate states), being a good neighbor by communicating your plans and addressing concerns goes a long way.
• Long-term plans: Consider how the ADU fits your life trajectory. Housing a family member now? Generating rental income? Increasing resale value? Your goals should drive your design decisions.