Prefab ADUs in Santa Clara County: Sizes, Prices, Permits

10/13/2026

If you own a house in Santa Clara, San Jose, Sunnyvale, Mountain View or Cupertino, a backyard accessory dwelling unit (ADU) lets you add a second home to a lot you already own. This guide from Green Galaxy Prefab in Santa Clara compares the smaller factory-built homes on our site, from 560 to 960 square feet, as backyard ADUs. It covers their listed prices and the California rules that affect how big an ADU can be, what fees apply and how quickly your city has to decide on a permit.

Can a factory-built home be an ADU?

Yes. California's ADU Handbook from the Department of Housing and Community Development (HCD) says that by definition an ADU includes manufactured homes. A factory-built home is a legitimate route to a backyard unit under state law, not a workaround. It still has to meet local standards and get a permit like any other ADU.

The smaller homes, side by side

These are featured homes on our site, with the price ranges listed there:

• CM-1482D: 2 bed / 1 bath, 560 sq ft, 11'8" x 48'. Listed from about $84K–$93K.
• CM-2562L: 2 bed / 1 bath, 747 sq ft, 13'4" x 56'. Listed from about $94K–$104K.
• CM-2602B: 2 bed / 2 bath, 800 sq ft, 13'4" x 60'. Listed from about $103K–$113K.
• CM-6563B: 3 bed / 2 bath, 803 sq ft, 14'4" x 56'. Listed from about $102K–$112K. This was our most-viewed home this month.
• CM-6622L: 3 bed / 2 bath, 889 sq ft, 14'4" x 62'. Listed from about $104K–$115K.
• CM-8402D: 2 bed / 2 bath, 800 sq ft, 23' x 40'. Listed from about $145K–$161K.
• CM-8483D: 3 bed / 2 bath, 960 sq ft, 21'8" x 48'. Listed from about $157K–$173K.

You can view the CM-1482D, the CM-6563B and the CM-8483D online to compare the smallest, most popular and largest of the group.

The listed ranges are for the home. They don't spell out site preparation, foundation, utility connections or permit fees, so ask us for a complete project quote for your lot. Our building process runs in two phases. Phase one covers site preparation and foundations, installation of the structure, and rough electrical and plumbing. Phase two covers finishes. You approve phase one before phase two begins.

Size: what the state says your city must allow

According to the HCD ADU Handbook (March 2026), a city may set minimum and maximum ADU sizes. Its maximum must still allow at least 850 square feet of interior livable space, or 1,000 square feet for an ADU with more than one bedroom. Where a city has no compliant ADU ordinance, the maximum for a new detached ADU is 1,200 square feet.

Every home in the list above is 960 square feet or less. Each three-bedroom model is also under the 1,000-square-foot level the state protects for multi-bedroom ADUs. Local rules may still differ, so confirm the size limit for your lot with your city.

Setbacks and height

The handbook says a city can require a setback of no more than four feet from the side and rear lot lines for a detached ADU. Rules about lot coverage, floor area ratio and front setbacks cannot block an ADU of at least 800 square feet with four-foot side and rear setbacks.

For a detached ADU on a lot with a single-family home, a city cannot set a height limit below 16 feet. The minimum rises to 18 feet near a major transit stop or high-quality transit corridor. Your site plan will show your home's actual finished height.

Fees: why 750 square feet matters

Under state law, an ADU with 750 square feet of interior livable space or less is exempt from impact fees charged by local agencies, special districts and water corporations. Above 750 square feet, impact fees are charged in proportion to the ADU's size compared with the main house. School districts may charge fees on ADUs larger than 500 square feet.

That puts the CM-1482D (560 sq ft) and CM-2562L (747 sq ft) under the 750-square-foot line, based on their listed square footage. Ask your city how it measures interior livable space before you count on the exemption.

Permits, parking and owner-occupancy

When there is already a house on the lot, the HCD handbook says the permitting agency must approve or deny a complete ADU application within 60 days. A city cannot require you to live on the property, except for ADUs sold separately under a specific state provision. Parking may not exceed one space per unit or per bedroom, whichever is less. No parking can be required for an ADU within one-half mile walking distance of public transit.

Cities apply these rules in their own ways. San Jose, Santa Clara, Sunnyvale, Mountain View and Cupertino each have their own ADU ordinance. Check with your city's planning department or ask us, and we'll look into your lot.

How Green Galaxy Prefab helps

Our ADU page says we handle permitting, code compliance and project management from start to finish. We start with a free, no-obligation consultation. The company describes itself as having more than 20 years of building experience in the area. During design we arrange site tests such as soil and drainage where they're needed.

Talk to us about your backyard

Visit Green Galaxy Prefab at Parkway Plaza, 4655 Old Ironsides Dr, Santa Clara, CA 95054, or call (408) 741-9008. You can also contact us online to book a consultation and find out which home fits your lot.