Plan Your ADU in San Diego

San Diego ADU Planner is an ADU San Diego guide for homeowners who want to add an accessory dwelling unit: a backyard home, a garage conversion, or a small unit inside the house. The City of San Diego allows attached and detached ADUs from 150 to 1,200 square feet [1]. National cost guides list $100 to $300 per square foot for many ADU types [3], and newer reports show prefab site work alone can add $65,000 to $160,000 [4]. When you ask for quotes here, your request goes to an independent contractor who may contact you about your project.

Illustration of a house with a detached ADU cottage in the backyard

Types of ADUs

Illustration of a detached ADU cottage in a backyard — Detached ADU

Illustration of an attached ADU built onto the side of a house — Attached ADU

Illustration of a garage converted into an ADU with windows and an entry door — Garage conversion

Cutaway illustration of a house with a junior ADU room inside it — JADU (junior ADU)

Detached ADU. A separate building, often in the backyard. In the City, it can be up to 1,200 square feet, and on lots that allow only single-dwelling homes it can be no more than two stories [1].
Attached ADU. Built onto the main house. The same 1,200 square foot limit applies [1].
Garage or accessory conversion. Turning an existing structure into a home. Conversions of existing space are not held to the 1,200 square foot limit [1].
JADU (junior ADU). A unit of 150 to 500 square feet built entirely inside a single-family home [2].

City rules at a glance

These points come from the City of San Diego's own ADU guidance. Rules can change, so confirm them with the City before you build.

A building permit is required for every ADU or JADU. There are no exemptions [1].
No parking spaces are required for ADUs outside the Coastal Overlay Zone [1].
The owner does not have to live on site, and an ADU may not be rented for less than 31 days in a row [1].
Plans that use a preapproved design get a 30-day review [1].
Most new or existing ADUs can now be sold separately as condominiums [5].

Read the full ADU rules and permits guide for setbacks, height, coastal and fire zone notes, and recent changes.

What an ADU costs

We did not find a published survey of installed ADU prices for San Diego. The ranges below are national, so treat them as a starting point.

HomeGuide lists $100 to $300 per square foot, or $60,000 to $180,000 for a 600 square foot unit (national, 2023) [3].
Block Renovation says a 600 square foot prefab ADU often lands at $200,000 to $300,000 once it is installed and permitted (2026) [4].

See the ADU cost guide for costs by type and City permit fees, or try the ADU cost calculator.

How to compare builders

Ask if the quote includes site work, utility hookups, and City fees. These items can add a lot to a prefab price [4].
Ask about the full timeline. One San Diego builder reports that detached ADUs often take 12 to 24 months from first idea to move-in [6].
Ask if the builder can use a preapproved plan to speed up review [1].
Check the contractor's license with the California Contractors State License Board yourself.

More tips are on our ADU contractors page. Thinking about a factory-built unit? Read our prefab ADU guide.

More ADU guides

Where we serve

We pass along requests for projects in San Diego, Chula Vista, Escondido, and Encinitas. See all service areas.

How to request quotes

Fill out the short quote request form.
We send your request to an independent ADU contractor.
That contractor may call, text, or email you about your project.

A referral is not an endorsement, so check any contractor yourself before you sign.

Request a quote

We're a referral service, not a contractor, and independent contractors may pay us for referrals.

Sources

City of San Diego Development Services, "Information Bulletin 400: Accessory Dwelling Unit/Junior Accessory Dwelling Unit," January 2026. Fetched 2026-11-11. https://www.sandiego.gov/development-services/forms-publications/information-bulletins/400
City of San Diego Development Services, "Accessory Dwelling Unit/Junior Accessory Dwelling Unit," no date shown. Fetched 2026-11-11. https://www.sandiego.gov/development-services/news-programs/programs/companion-junior-units
HomeGuide, "How Much Does It Cost to Build an ADU?," November 13, 2023. Fetched 2026-11-11. https://homeguide.com/costs/adu-cost
Block Renovation, "Prefab ADUs: What is the True Cost Savings," February 4, 2026 (updated June 17, 2026). Fetched 2026-11-11. https://www.blockrenovation.com/guides/prefab-adus-what-is-the-true-cost-savings
City of San Diego Development Services, "Public Notice: Information Bulletin 400 Updates," August 27, 2025. Fetched 2026-11-11. https://www.sandiego.gov/sites/default/files/2025-08/dsd_ib400_adu.pdf
SnapADU, "The State of Detached ADUs in San Diego (2026)," no date shown. Fetched 2026-11-11. https://snapadu.com/lessons-from-100-adu-builds-detached-adu-report-san-diego/