San Diego ADU Guide 2026: What an ADU Is and How to Start

An accessory dwelling unit (ADU) is a second, complete home on the same lot as a main house. It has its own place to live, sleep, cook, eat and bathe. Under California law, most residentially zoned San Diego lots with an existing or planned home are eligible to apply for one. Eligible isn’t the same as buildable, though: your yard, utilities and budget set the real limit. If you’re considering adding an ADU, this guide explains what one is, the types you can build, what Lars plans for each type, and how long the process really takes. It then points you to the detailed page for each next question.

Key takeaways

• ADU is short for accessory dwelling unit. Granny flat, casita, in-law unit and backyard cottage are informal names for the same thing.
• The main types are detached, attached, conversion (for example a garage) and junior ADU (JADU) inside the house.
• California sets a statewide floor for ADUs. At least 11 states now have statewide ADU laws.
• Lars planning ranges run from $120,000–$200,000+ for a JADU to $250,000–$450,000+ for a detached ADU.
• Real projects often take well over a year from first idea to move-in, even though the permit decision clock is 60 days.

Information checked October 25, 2026. Rules and programs change, so confirm current requirements with the agency that governs your lot.

What’s an ADU?

An ADU is an attached or detached home that gives one or more people complete, independent living space. California defines it as a residential unit with permanent provisions for living, sleeping, eating, cooking and sanitation, located on a lot with a proposed or existing primary residence (California HCD, ADU Handbook, summarizing Government Code §66313).

That definition is what separates an ADU from a home office, a pool house or a guest room. If the space lacks a kitchen or bathroom, or can’t function as independent living space, it isn’t an ADU, and it can’t legally be rented or lived in as one. A smaller “junior” version lives inside the main home. How large either one can be depends on the jurisdiction and is covered on the requirements page linked below.

State ADU law now sits in one chapter of the Government Code, sections 66310 through 66342, after SB 477 consolidated it in 2024 (HCD ADU Handbook). Each city, and the County of San Diego for unincorporated areas, writes its own ADU regulations, and local building codes still apply. Those local rules have to stay within the state’s minimums.

What does ADU stand for in housing?

ADU stands for accessory dwelling unit. “Accessory” means the unit is secondary to the main house on the same lot. “Dwelling unit” means it is a full home, not a room.

You’ll also see two related terms:

• JADU (junior accessory dwelling unit): a unit of no more than 500 square feet built entirely within a single-family home, with at least an efficiency kitchen (Government Code §66313(d) and §66333, per the HCD ADU Handbook).
• Companion unit: the City of San Diego’s older name for an ADU. You may still see it on older permits and in older handbooks.

What’s an ADU home? The main types

An “ADU home” is simply the second unit itself. The right type depends on who will live there, how much yard you want to keep, and which structures and utilities are already on the lot.

Granny flats, casitas and in-law units

Granny flat, casita, in-law unit, backyard cottage and companion unit are everyday names for an ADU. Current permits use only the legal terms “ADU” and “JADU”. If a contractor quotes a “casita”, ask which of those two permits the project will actually use.

Garage conversion or a new granny flat?

Lars has long helped San Diego owners compare these two routes, and the tradeoffs still hold:

• A garage conversion starts with walls, a slab and a roof. That can save money when the structure is sound. But residential use may still require slab, moisture, insulation, fire-separation, egress and utility work, and you lose the storage or parking the garage provided. Headroom can become a problem if the floor has to be raised to meet the house.
• A new granny flat costs more to build, but you choose the layout, daylight and orientation from scratch. It also keeps the garage. If the existing garage is in poor condition, a new structure can be the better long-term value. Have an architect and an inspector look at it early.

For the permit side of a conversion, see garage conversion permits in San Diego. The Poway garage-to-ADU conversion shows a finished example.

Lesson from a Lars project: an attached ADU that preserved a Del Mar yard

Lars completed a project in the incorporated City of Del Mar. The owners wanted a private place for guests without giving up the backyard they valued. Lars architects measured the house and lot. They also studied how the owners used the yard and tested the plan against Del Mar’s rules.

An attached ADU made better use of the link between the home and yard. A separate building would have filled the middle of the open space. Instead, the unit of about 800 square feet extended from the main home in a rectangular footprint. That shape kept more usable yard and gave guests a clear room plan.

The connection didn’t mean the two spaces should feel like one room: the outside needed to match the main house, while guests inside still needed a sense of privacy. Walls, openings, insulation, sound control, size, height and setbacks all shaped the plan. The finished ADU serves guests while protecting the outdoor space the owners value. This is a lesson from one Lars project, not a testimonial or a guaranteed outcome.

ADU, JADU or non-dwelling accessory structure?

The project name has to match the legal use. An ADU and a JADU are both housing. A non-dwelling accessory structure serves the main property, for example as a pool house, studio or storage building, but it isn’t an independent residence. A sink, a bathroom or a kitchenette doesn’t by itself make a structure an ADU. If you’re weighing a junior unit against a backyard unit, see JADU vs. detached ADU.

Lesson from a Lars project: choosing an accessory structure in Spring Valley

One Spring Valley family first explored a small pool house that could double as an ADU. Once the goals and the approval requirements were separated, independent housing wasn’t the best match. The homeowners wanted pool facilities, light food preparation, an outdoor shower and flexible gathering space. They didn’t need another legal dwelling.

Lars designed and built a permitted non-dwelling accessory structure with a kitchenette, pool bathroom, outdoor shower, built-in storage and adaptable interior space. The Spring Valley accessory structure project shows the completed work. The structure is not approved sleeping space or rental housing, and it should never be used that way without a new approval.

What states allow ADUs?

The useful version of this question is which states require cities and counties to allow ADUs, rather than leaving it to each town. A Virginia Housing Commission review identified nine states with statewide laws broadly allowing or requiring ADUs: California, Connecticut, Maine, Montana, New Hampshire, Oregon, Utah, Vermont and Washington (Virginia Housing Commission). In 2025, Arkansas and Iowa joined them. Iowa now requires cities and counties to permit at least one ADU on every single-family lot (American Planning Association, August 19, 2025).

That makes at least 11 states with statewide ADU laws. More states are debating them, so the count keeps changing. Outside those states, whether you can build an ADU depends entirely on your city or county.

If you’re building an ADU in California, you have one of the most detailed state frameworks behind you. State law sets the minimum every city and county must allow:

• No minimum lot size for qualifying ADUs.
• Review that follows written standards rather than a discretionary hearing.
• A decision within 60 days of a complete application.

These floors come from Government Code §66314 and §66317 (HCD ADU Handbook). San Diego’s rules build on them, which is why most residentially zoned lots here are eligible to apply for at least one unit.

Can I build an ADU on my property?

Usually, yes. If your lot is zoned for residential use and has an existing or proposed home, California law entitles you to apply for a building permit for a new ADU. The ADU permit process follows written standards instead of a hearing. The agency has 15 business days to say whether the application is complete and 60 days to approve or deny a complete one (HCD ADU Handbook; City of San Diego IB-400).

Being allowed to apply doesn’t tell you what the lot can hold. Before you pay for an ADU design, screen five site-specific requirements:

1. Who regulates the lot. A San Diego mailing address may be in the City, another incorporated city such as Del Mar or Encinitas, or unincorporated San Diego County. Each has its own ordinance.
2. Overlays. Coastal, very high fire hazard, historic and slope overlays can add review time or setbacks.
3. Utilities and access. Sewer or septic, water, electrical capacity and construction access can limit the size or raise the cost.
4. HOA documents. California Civil Code §4751 limits how far an HOA can restrict an ADU on a single-family lot, but reasonable design rules can still apply (HCD ADU Handbook).
5. Existing permits. Unpermitted work on the main house or garage can complicate approval.

Size limits, setbacks, height, parking, coastal review and City-versus-County differences each have their own answer. Those ADU rules and regulations are covered in detail, with section numbers, in San Diego ADU requirements and permits.

Lesson from a Lars project: fitting 1,200 square feet to an irregular yard

A City of San Diego family asked Lars to explore a long-term home for their adult daughter. The goal was greater independence for her while keeping family close. They didn’t know whether the yard could hold a useful ADU or how much private space the lot would allow.

The design team began with measurements and a site review rather than a stock floor plan. The City’s size limit for this kind of unit, 1,200 square feet (City IB-400), set an upper boundary. The yard’s shape, the main home, paths and outdoor uses set the real design. An L-shaped footprint used the open area better than a simple box. It became a two-bedroom, two-bathroom ADU of about 1,200 square feet. HOA review was part of the work: the team compared the exterior and site plan with the association’s rules while following City requirements.

The lesson is not that every yard can hold a 1,200-square-foot ADU. Measurement, jurisdiction, HOA documents, utilities, privacy and household priorities have to work together.

What does a San Diego ADU cost?

Costs for ADUs in San Diego vary most with type, site and utilities. Lars publishes planning ranges by type. These are company planning references, not market averages or fixed quotes. The “+” means a project can exceed the upper figure.

Utility trenching is the cost most often hidden in an early number. Trenching is the digging needed to run water, sewer, power and gas underground to the new unit. Lars recommends showing it as its own line, separate from:

• piping and connections
• meter or electrical-panel work
• permit fees
• repairing paving or landscaping afterward

On one Lars ADU project, the permitting team had to check whether sprinkler demand and plumbing fixture counts required a larger water meter and a right-of-way permit for work under the public sidewalk. A floor plan couldn’t answer that question; a site visit could.

The San Diego Housing Commission’s ADU pilot shows how large this line can get. At one of its five sites, splitting utilities would have required street trenching that added more than $100,000 and more than six months. The team shared utilities instead. The same report recommends a 15 to 20 percent contingency for ADU construction (SDHC ADU Pilot Program report, October 2021).

Units smaller than 750 square feet are exempt from impact fees under state law, but plan check, school and utility charges can still apply (HCD ADU Handbook). The $40,000 CalHFA ADU grant is closed: CalHFA says its last round ran out of funds on December 28, 2023 (CalHFA). For full budget categories, financing options and fee detail, see the San Diego ADU cost guide.

How long does an ADU take?

Plan in months, not weeks. The 60-day state decision clock covers one step: an agency’s review of a complete application. It doesn’t cover feasibility, design, corrections, financing, utility coordination, construction or final inspection.

The best public San Diego data on the full timeline comes from the Housing Commission’s pilot. It built five ADUs and recorded expected and actual time for each phase:

Those projects ran under 2021 rules. State law has since tightened review deadlines, and the City offers a 30-day review for qualifying pre-approved plans (City IB-400). Check current review times on the San Diego Development Services Department’s permit timeline dashboard close to when you submit. A contractor’s build time should never stand in for the full idea-to-occupancy timeline.

The process of building an ADU, step by step

An ADU moves through linked decisions. A fact missed early tends to come back later as a redesign or a delay.

1. Define the use. Who will live there, how many bedrooms, and what accessibility, privacy and storage do they need?
2. Confirm feasibility. Check the jurisdiction, zoning, overlays, records, boundaries, access, existing structures and utilities.
3. Set a complete budget. Match the scope with cash or financing, allowances and contingency.
4. Choose the type and design options. Compare detached, attached, conversion and JADU options on the actual site.
5. Prepare plans. Coordinate the ADU design with structure, energy, civil, fire and utility information.
6. Submit and respond. The agency checks completeness, reviews the plans and may issue corrections.
7. Build and inspect. Complete site work, foundation, framing, systems, finishes and inspections.
8. Close out the permit. Keep the final approval, plans, warranties and records.

Common ADU mistakes to avoid

• Buying a stock plan before confirming the lot. A pre-approved floor plan is not a pre-approved lot.
• Budgeting from construction cost alone. Design, permits, utilities, site work, financing and contingency belong in the same number.
• Assuming the legal maximum fits. Setbacks, easements, slope, trees and access often shrink the usable footprint.
• Treating the permit clock as the schedule. Corrections, utility work and long-lead items set the real finish date.
• Counting on renting out your ADU as a vacation rental. The City generally prohibits renting an ADU for fewer than 31 consecutive days (City of San Diego short-term residential occupancy rules). Long-term rental and sale rules are in the requirements guide linked below.

Next steps for homeowners in San Diego

This guide is the starting point. Each next question has its own page:

• Rules for your lot: San Diego ADU requirements and permits covers size, setbacks, height, parking, coastal and City-versus-County differences.
• Budget: the ADU cost guide covers costs by type, fees and financing.
• Junior units: JADU vs. detached ADU.
• Garage conversions: garage conversion permits in San Diego.
• Family use: building an ADU for aging parents.
• Design and construction: Lars’s San Diego ADU builders handle feasibility, design, permitting and construction under one team.

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About this guide

Written by TJ Kidder, a home remodeling specialist at Lars Remodeling & Design who focuses on homeowner education about planning, budgeting, design and construction. Lars is a San Diego design-build firm; learn more about Lars or contact the team with questions about a specific property.

Research method: this guide draws on California HCD, the City of San Diego, the San Diego Housing Commission, CalHFA, the Virginia Housing Commission and the American Planning Association. Sources were checked on October 25, 2026. Lars cost ranges are company planning references. Completed Lars projects are presented as examples, not as promises of the same approval, size, schedule or result. This article is general planning information, not legal, tax, lending or permit advice.