Guest house vs ADU in California: what's allowed and what it costs

What a guest house is in California

“Guest house” isn't a legal category in California. When you apply for a permit, the building will be one of two things.

• An accessory dwelling unit. State law defines an ADU as a unit with complete independent living facilities, including permanent provisions for living, sleeping, eating, cooking and sanitation. If your guest house has a kitchen, it's an ADU.
• Guest quarters, or a habitable accessory building. The City of San Diego's municipal code (§ 141.0307) describes these as living quarters that “do not provide complete, independent living facilities,” used only by the people who live in the main house, their guests or their employees. Other cities use similar terms, such as “guest living quarters” or “accessory habitable structure.”

Names like casita, backyard cottage and granny flat usually describe an ADU. Our guide to what an ADU is covers the basics, and granny flat vs ADU sorts out the other names.

Guest house vs ADU

The guest quarters column comes from San Diego Municipal Code § 141.0307; the ADU column from § 141.0302 in the same division and the state's ADU handbook. For the full state picture, see our guide to California ADU laws.

Guest house rules in San Diego

City of San Diego

Under § 141.0307 of the municipal code, guest quarters are allowed as a limited use next to a single dwelling unit, and the main house has to exist or be built at the same time. The rules that matter most:

• No kitchen. The building “shall not contain a kitchen or facilities for the storage and preparation of food.” A bar sink and a miniature refrigerator may be permitted.
• Size cap. Guest quarters may take up no more than 25% of the allowable gross floor area of the lot, and their floor area counts toward the lot's floor area ratio.
• Height over a garage. Guest quarters above a garage or other accessory building can reach 21 feet with a flat roof, or 30 feet with a roof pitch of at least 3:12.
• No renting. They “shall not be rented, leased, or sold as a separate dwelling unit.”
• A recorded agreement. Before the building permit is issued, the owner signs an agreement with the city that the house and guest quarters won't be sold separately, and the city records it with the County Recorder. The form is the Habitable Accessory Structure Agreement.

The city's ADU bulletin says guest quarters are allowed in addition to ADUs, so you can have both on one lot if the floor area works.

Unincorporated San Diego County

The county is stricter. Its guest living quarters rules bar kitchens, wet bars and laundry, and the quarters can't be rented or used as a separate dwelling. Snap ADU's summary, which it says a county planning manager confirmed in March 2026, adds that guests may stay no more than 30 days a year, the quarters are capped at 30% of the main home's size up to 600 sq ft (up to 50% with an administrative permit), and the house and quarters share one electric meter.

Why most owners build an ADU instead

Since state law made ADUs easy to permit, a no-kitchen guest house rarely wins. The reasons:

• You can rent it. In the City of San Diego an ADU can be rented for 31 days or more. Guest quarters can't be rented at all. See ADU rental income in San Diego.
• The city has to say yes. An ADU that meets the rules is approved ministerially; the city must approve or deny within 60 days of a complete application.
• The state guarantees the size. Every city must allow an 800 sq ft detached ADU with 4-foot side and rear setbacks, whatever its local floor area rules say. Guest quarters have no such guarantee and count against your floor area.
• Lower fees. No impact fees for an ADU of 750 sq ft or less, and the City of San Diego exempts your first two ADUs from development impact fees regardless of size.
• No one has to live on site. The state bars owner-occupancy requirements for ADUs.
• More to come. Starting January 1, 2027, AB 956 allows two detached ADUs on single-family lots.

A guest house without a kitchen still makes sense in a few cases: you truly want a guest suite or home office that will never be rented, or your lot has floor area to spare and you want to keep your ADU allowance for something else.

Guest house permits in California

Either way you need a building permit, because both are habitable buildings. The paths differ.

The ADU deadlines come from Government Code § 66317; the guest quarters process from San Diego's municipal code. In the City of San Diego, ADU applications are submitted online through Development Services, an ADU needs no parking within half a mile of transit or when it's a conversion, and the final inspection is your approval to occupy. Our San Diego ADU permit guide walks through each step.

How much a guest house costs

Snap ADU puts a detached guest house in San Diego at $300,000 to $500,000 all-in, or roughly $375 to $600 per square foot, as of March 2026, with construction making up 80–85% of the total. Dropping the kitchen saves the cabinets and appliances, but the foundation, framing, bathroom, utilities and permits stay, so a no-kitchen guest house usually costs only modestly less than an ADU of the same size.

Figures from Snap ADU's published prices, Lars Remodel's cost guide and Snap ADU's fee table. Snap's base prices include design, permit management and about $35,000 of level-lot site work, but not city fees, demolition or retaining walls. For the full breakdown see what an ADU costs in San Diego, or price your own project with the ADU cost calculator.

Property tax goes up only for the new building. The San Diego County Assessor adds the value of new construction without reassessing your existing home.

Can a guest house be an ADU?

Yes. State law requires cities to approve an ADU built inside the existing space of an existing accessory structure on a single-family lot, which can be expanded by up to 150 sq ft for entry and exit. It needs exterior access and setbacks sufficient for fire and safety. No new setback can be required for an existing structure converted to an ADU, or for one rebuilt in the same location and to the same dimensions. In the City of San Diego, conversion ADUs have no maximum size.

What the work usually involves:

• A kitchen, with the plumbing, venting and electrical circuits to support it.
• Code upgrades to bring the building up to the standards for a dwelling unit, which may include insulation, egress windows and smoke alarms.
• Utilities. A separate SDG&E meter is optional, but if you want one, budget $10,500 to $15,000, per Snap ADU.
• The recorded agreement. If the building was permitted as guest quarters, ask Development Services how the agreement is handled once it becomes a permitted ADU.

An unpermitted guest house with a kitchen

Many older guest houses got a kitchen without a permit. Under AB 2533, now Government Code § 66311.7, a city can't deny a permit for an unpermitted ADU built before January 1, 2020 just because it breaks building standards, unless fixing the problem is needed to protect health and safety, and it generally can't charge impact or connection fees to legalize it. The City of San Diego explains its process in Information Bulletin 242. Have a licensed contractor look at the building first; a structure that was never built as a home can need real work.

Guest house plans and prefab units

You don't have to start from a blank page. The County of San Diego offers free ADU plans in 600, 800, 1,000 and 1,200 sq ft versions, and the City of San Diego accepts county pre-approved plans with a 30-day review. Those plans include a kitchen, so they produce an ADU, not guest quarters. See pre-approved ADU plans in San Diego, ADU floor plans and prefab ADUs in California for factory-built options.

Whoever builds it should hold an active B license. Check a contractor's license, read how to hire an ADU contractor, and compare verified ADU builders in San Diego.

Common questions