In San Diego, detached Accessory Dwelling Units (ADUs) can typically be built up to 1,200 square feet or the size of the primary home, whichever is smaller, without needing a special variance. Units sized between 150 and 800 square feet qualify for streamlined reviews and additional fee waivers under state regulations.

On a single-family lot in San Diego, homeowners can typically build up to three ADUs: one detached ADU up to 1,200 square feet, one attached or converted ADU created from existing structures, and one Junior ADU (JADU) within the primary home, limited to 500 square feet. Multifamily properties have different rules, allowing for multiple units based on the number of existing dwellings.

In most cases, parking is not required for an ADU in San Diego. Typically, one parking space per new bedroom is mandated only if your property is located more than half a mile from public transit, is outside a historic district, the ADU isn't created within an existing building, and street parking permits are available. Most properties meet one of these exemptions, so no additional parking is necessary.

The permit process for ADUs in San Diego usually takes about 3½ to 4 months for a standard review. However, projects in coastal zones or those with complicated site conditions might extend the timeline to around 5 to 6 months. Including design, bidding, and construction phases, the total project timeline typically ranges from 10 to 14 months.

You do not have to live on the property if you're adding a regular ADU, as California has suspended owner-occupancy requirements until at least 2025. However, if you're building a Junior ADU (JADU), you must occupy either the main house or the JADU itself.

Short-term rentals, such as Airbnb, are generally prohibited for ADUs in the City of San Diego unless the property held a valid short-term rental license prior to 2017. The minimum rental period for new ADUs is typically 30 days or longer. Regulations can vary slightly by city, so it's advisable to verify local rules.

Impact fees for ADUs in San Diego kick in once your unit exceeds 750 square feet. Under state law, ADUs of 750 square feet or smaller are exempt from these fees. Larger units incur scaled development-impact fees (DIF), school fees, and facility benefit fees, typically adding between $10 and $20 per square foot, depending on your location and community planning area.

Fire sprinklers are required in your ADU only if your main home already has them installed. Regarding solar panels, California’s Title 24 energy code mandates rooftop solar installations on newly constructed detached ADUs. However, ADU conversions and attached ADUs are exempt if the primary residence doesn't have solar panels.

Generally, an ADU cannot be sold separately from the primary home. The exceptions involve subdividing your property under California’s SB 9 to create two primary homes, each with its own ADU, or converting your property into a condominium map, which is uncommon and costly. San Diego is exploring further options for individual sale of ADUs but hasn't implemented any changes yet.

Several financing programs are available to assist with ADU construction in San Diego. The CalHFA ADU Grant provides up to $40,000 toward pre-development costs during periodic funding cycles. The San Diego Housing Commission (SDHC) offers an ADU Finance Program with construction-to-permanent loans up to $250,000 at around 1% interest during construction for owner-occupied properties. Homeowners can also consider home-equity loans or lines of credit, FHA 203(k) renovation loans, Fannie Mae Homestyle loans, and other conventional construction or renovation loans. Additionally, recent policy updates allow lenders to include projected ADU rental income in conventional refinancing calculations, improving qualification prospects for many homeowners.