Guide · ADU & Granny Flats
What Is a Granny Flat? San Diego Homeowner's Guide
Granny flat, ADU, in-law suite — these terms get used interchangeably. Here's exactly what each one means, what you can build in San Diego, and how much it costs.
Granny Flat vs. ADU: What's the Difference?
In casual conversation, 'granny flat' and 'ADU' (Accessory Dwelling Unit) mean the same thing: a self-contained living unit on the same property as the primary home. 'ADU' is the legal term used in California law. 'Granny flat' is the informal term that's been around for decades.
Both refer to a structure with its own kitchen, bathroom, and private entrance — fully separate from the main house. The name 'granny flat' originally referred to spaces built for aging parents, but today they're just as often used as rentals or for adult children.
| Term | Legal Name | What It Means |
|---|---|---|
| Granny flat | Detached ADU | Separate structure on your lot — own foundation, utilities, entrance |
| Mother-in-law suite | Attached ADU | Addition to primary home with private entrance |
| In-law suite | Attached ADU or JADU | Same as mother-in-law suite; sometimes interior to existing home |
| Junior ADU (JADU) | Junior ADU | Up to 500 sq ft carved from existing home footprint |
| Garage conversion | ADU conversion | Existing garage converted to living space |
What Makes a Granny Flat Different from a Rental Unit?
A granny flat is on the same parcel as the primary residence — it's not a separate legal lot. The owner of the main home also owns the granny flat. This is distinct from a duplex or multi-family property, which involves separate legal lots or a multi-unit building.
California state law classifies granny flats as ADUs and governs them separately from traditional rentals or multi-family development. This classification makes permitting faster and approval more predictable.
San Diego's ADU Boom
The City of San Diego issues more ADU permits per capita than almost any city in California. State law has significantly streamlined the process — permits must be processed within 60 days, and most lots qualify for at least one ADU.
What Does a Granny Flat Include?
A granny flat (detached ADU) is a fully self-contained unit. California law requires a granny flat to have its own:
- Kitchen with a cooking appliance (full kitchen, not just a kitchenette)
- Bathroom with a shower or tub
- Private exterior entrance — separate from the main home
- Independent utility connections or shared connections from the primary home
- Permanent foundation
This distinguishes it from a Junior ADU (which can be inside the existing home and share an entrance) and from a simple bonus room or in-law suite.
What Does a Granny Flat Cost in San Diego?
Granny flat construction in San Diego typically ranges from $150,000 to $300,000 for a 400–700 sq ft detached unit. The range is wide because cost depends heavily on site conditions, utility connection requirements, and finish level.
| Granny Flat Type | Typical Cost | Key Variables |
|---|---|---|
| Garage conversion (ADU) | $120,000–$180,000 | No new foundation; utility upgrades vary |
| Detached granny flat (400–500 sq ft) | $150,000–$220,000 | Site access, utility connections |
| Detached granny flat (600–800 sq ft) | $200,000–$300,000 | Size, finish level, features |
| Premium / coastal granny flat | $280,000–$400,000+ | Coastal permits, high-end finishes |
Frequently Asked Questions
Answers to what San Diego homeowners ask most.
Yes. California state law (AB 3182) eliminated the owner-occupancy requirement for ADUs. You are not required to live on the property to build or rent out a granny flat. You can build a granny flat, rent both the main home and the granny flat, and live elsewhere.
State law allows detached ADUs up to 1,200 sq ft (or up to 50% of the primary home's square footage if the primary home is smaller). Most granny flats in San Diego are built between 400 and 700 sq ft — larger units cost more and have diminishing rental return per dollar spent.
Yes. All granny flat (ADU) construction requires building permits. The City of San Diego must process ADU permit applications within 60 days under state law. We handle the full permit application, plan submittal, and inspector coordination.
Have a question not covered here? Call (831) 261-7329 or send us a message. We answer the phone.
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