ADU rules in
the County.

First, check that you're under the County

The County's rules cover the unincorporated areas, such as Live Oak, Pleasure Point, Seacliff, Aptos, La Selva and Davenport. The cities of Santa Cruz, Capitola, Scotts Valley and Watsonville each have their own ADU rules. The quickest way to be sure is the County's ADU Eligibility Tool: enter your address or parcel number and it shows whether your parcel qualifies and the largest ADU it allows.

How many units you can add

On a lot zoned for a single-family home, the County allows up to four ADUs:

• one ADU that meets the County's own ADU regulations and state law,
• one conversion ADU, made from part of the house, a garage or another existing structure,
• one junior ADU, and
• one detached ADU of up to 800 square feet that meets the state's short list of standards.

On a lot with an existing apartment or condominium building, the County allows up to eight detached ADUs, never more than the number of existing units, plus conversions of space that isn't lived in, up to 25 percent of the existing units.

The three kinds of ADU

• Conversion. Part of the house, a garage or another existing structure becomes the ADU. It needs a full bathroom, a full kitchen, its own entrance, its own heat source, and sound and fire separation from the main house.
• New construction, attached. An addition to the house or an attached garage.
• New construction, detached. A free-standing unit, or an addition to a detached garage or other accessory building.

How big it can be

• Junior ADU: 150 to 500 square feet, entirely inside the walls of the house.
• Conversion ADU: at least 150 square feet, with no maximum.
• New attached ADU: 850 square feet for a studio or one-bedroom, 1,000 for two or more bedrooms, or half of the house's living area, whichever is larger.
• New detached ADU: on a parcel under one acre, 850 square feet for a studio or one-bedroom and 1,000 for two or more bedrooms. On a parcel of one acre or more, 1,200 square feet.

State law guarantees that an ADU of at least 800 square feet is allowed, and the County allows an ADU of up to 800 square feet even if the lot is already at its limit for floor area or lot coverage.

Height, and the Urban Services Line

How tall a detached ADU can be depends on where your lot sits relative to the County's Urban Services Line.

• Inside the line: 16 feet, or 21 feet with Zoning Administrator approval.
• Outside the line: the normal height for the zone, usually 28 feet.
• Above a detached garage, inside the line: 24 feet at the roof peak and 20 feet at the top of the wall.
• Near transit: within half a mile walking distance of a major transit stop or high-quality transit corridor, 18 feet, or 20 feet if the roof pitch matches the house.
• Attached ADUs: the normal height for the zone, usually 28 feet.

Pleasure Point and Seascape Beach Estates have their own standards.

Setbacks

A new ADU needs at least 4 feet from the side and rear property lines. The front setback is the zone's normal one, but the County can't use it to block an ADU of up to 800 square feet that meets the 4 foot side and rear setbacks. The space between the ADU and other buildings is set by the building and fire codes.

Parking

Conversion ADUs and junior ADUs need no parking, and converting a garage doesn't require replacing the spaces. A new-construction ADU needs one new space, except in historic districts, within one block of car share parking, or within half a mile walking distance of a transit stop.

Those exceptions don't apply in the County's designated coastal areas: Live Oak; Seacliff, Aptos and La Selva; Davenport and Swanton; and Opal Cliff Drive between 41st Avenue and Capitola. There, a new-construction ADU always needs a space, and parking removed for an ADU has to be replaced.

Septic

If your house is on septic, adding an ADU means the system has to meet the County's upgrade standards. County Environmental Health's process starts with a qualified septic professional who checks:

1. whether the tank is big enough and in working order for the added use,
2. whether the leach field is the right size, working, and has room to expand, and
3. whether the system meets today's setbacks.

That leads to a site evaluation and then a sewage disposal permit for the upgrade. The County suggests placing the ADU near the system, on the uphill side, and talking to Environmental Health early.

Coastal permits

Most ADUs in the Coastal Zone are excluded or exempt from a Coastal Permit. Where coastal findings are needed, they are usually handled as part of the building permit review.

Fees and waivers

The County reduces or waives several fees for ADUs:

• reduced building permit fees for conversion ADUs,
• no sewer or water connection fees for junior ADUs and conversion ADUs,
• no Planning Department permit fees for ADUs of 750 square feet or less,
• no impact fees or affordable housing impact fee for ADUs under 750 square feet, and
• no transportation and roads fees when the ADU needs no parking.

Water districts, sanitation districts and fire districts set their own requirements and fees, which the County notes can make building more expensive in some neighborhoods.

Free pre-designed plans

The County offers three free pre-designed ADU plans, each in three exterior styles:

Plan	Size	County's estimate of permit fees
Plan 1	425 sq ft studio or one-bedroom	about $5,000
Plan 2	637 sq ft one-bedroom	about $6,000
Plan 3	998 sq ft two-bedroom	about $20,000 plus impact fees

School district and fire department review fees are paid separately. The plans can't be changed, they assume a standard slab foundation, and you still need a site plan, stormwater information and, on septic, possibly an upgraded system. The County is updating them to the 2025 code cycle.

Renting it out

An ADU in the County can't be used as any kind of short-term rental. Rentals must be longer than 30 days, and a property with an active Vacation Rental or Hosted Rental permit has to give it up before an ADU permit can be issued. If a junior ADU shares a bathroom with the house, the owner or a relative has to live on the property.

Legalizing an older unit

The County has two paths for units built without permits before 2020.

• AB 2533, the state amnesty law in effect since January 1, 2025, covers ADUs and junior ADUs built or converted before January 1, 2020. You document the construction date, a licensed contractor completes the County's substandard housing checklist, and the unit is legalized at final inspection.
• The County's SB 13 program is for units with a code violation. A qualifying owner gets five years to get the permits, inspections and final sign-off.

Questions homeowners ask

How many ADUs can I build on a single-family lot in the County?

Up to four: one under the County's own ADU rules, plus a conversion ADU, a junior ADU, and a detached ADU of up to 800 square feet with 4 foot side and rear setbacks.

How tall can a detached ADU be in the County?

Inside the Urban Services Line, 16 feet, or 21 feet with Zoning Administrator approval. Outside the line, the zone's normal height, usually 28 feet.

Can I build an ADU if my house is on septic?

Yes, but adding an ADU means the septic system has to meet the County's upgrade standards. A qualified septic professional checks the tank, the leach field and the setbacks first.

Can I rent my ADU as a vacation rental?

No. ADUs in the County can't be short-term rentals of any kind, and a property with an active vacation rental permit has to give it up before an ADU permit is issued.

Sources

1. ADU Basic Requirements (units, sizes, heights, setbacks, parking, fees), County of Santa Cruz
2. Types of ADUs, County of Santa Cruz
3. ADU Frequently Asked Questions (rentals, coastal, special districts, junior ADUs), County of Santa Cruz
4. Plan Your Financing (conversions, septic placement), County of Santa Cruz
5. Pre-Designed ADU Plans, County of Santa Cruz
6. ADU Eligibility Tool, County of Santa Cruz
7. OWTS Upgrade, County of Santa Cruz Environmental Health
8. Assembly Bill 2533, County of Santa Cruz
9. Senate Bill 13, County of Santa Cruz

Rules change. We checked every source on October 27, 2026. Call the City or County planning counter before you design.