The short answer
Yes, most homeowners in Fremont, Newark and Union City can build an ADU. State law guarantees at least an 800-square-foot unit with 4-foot side and rear setbacks, and all three cities allow detached ADUs, require no added parking and must decide on a complete application within 60 days. Keep the unit at 750 square feet or less to skip impact fees. Expect about $120,000 to $150,000 for a garage conversion and $250,000 to $500,000 for a new detached unit.What is an ADU?
An accessory dwelling unit (ADU) is a second, smaller home on the same lot as a house, with its own kitchen, bathroom and entrance. People also call it a granny flat, in-law unit, backyard cottage or casita.There are four common types:- Detached ADU: a separate small house in the backyard.
- Attached ADU: an addition joined to the main house, with its own entrance.
- Conversion ADU: an existing garage, basement or part of the house turned into a separate unit.
- Junior ADU (JADU): a unit of up to 500 square feet carved out of the main house, often a bedroom with its own entrance and a small kitchen. It can share a bathroom with the house.
What does California law guarantee for ADUs?
State law sets a floor that every California city, including Fremont, Newark and Union City, must allow. Cities can be more generous but not stricter (HCD ADU Handbook, March 2026).- Size: at least an 800-square-foot ADU must be allowed, even where lot coverage limits would block it.
- Setbacks and height: no more than 4 feet from side and rear lines; at least 16 feet tall for a detached unit.
- Approval: a decision within 60 days of a complete application.
- Fees: no impact fees at 750 square feet or less; no school fees under 500.
- Owner occupancy: not required for an ADU.
- Rentals: long-term rentals are fine. A JADU can never be a short-term rental, and cities may ban ADU stays under 30 days.
- Selling: an ADU can't be sold separately unless the city adopts a condo-style ordinance.
What are the ADU rules in Fremont?
Fremont allows a detached ADU of up to 1,200 square feet and an attached ADU of up to 1,000 square feet, with no added parking. It also offers preapproved plans that cut plan review to 7 business days.- Size: detached up to 1,200 square feet; attached up to 1,000 square feet; JADU up to 500 square feet (Fremont Municipal Code §18.190.005, via GoCodebook).
- Height: 16 feet for a detached ADU, or 18 feet near transit, plus 2 feet for a pitched roof. An attached ADU can be up to two stories.
- Setbacks: 4 feet from side and rear property lines.
- Parking: none required, and none to replace when you convert a garage.
- Fees: no impact fees for an ADU of 750 square feet or less.
- How many: one ADU plus one JADU on a single-family lot.
Fremont's preapproved ADU plans
Fremont keeps a library of detached ADU designs the city has already reviewed, from a 496-square-foot one-bedroom to a 1,000-square-foot three-bedroom (City of Fremont).- Review time: 7 business days, compared with 15 for a custom design.
- Plan review fee: a flat $1,000; construction and inspection fees still apply.
- How it works: you hire the designer who owns the plan. Site-specific drawings and engineering are still required.
- Questions: Fremont Planning Division, 510-494-4455.
What are the ADU rules in Newark?
Newark allows a detached ADU of up to 1,200 square feet and an attached ADU of up to 1,000 square feet, with no off-street parking required. Rentals shorter than 30 days are not allowed (Newark Municipal Code §17.26.040).- Size: detached up to 1,200 square feet; attached up to 1,000 square feet; JADU up to 500 square feet. A garage or room converted to an ADU has no size cap.
- Height: 16 feet for a detached ADU, or 18 feet near major transit. An attached ADU follows the main home's height limit, up to two stories. A second-story addition taller than 16 feet needs design review (City of Newark).
- Setbacks: at least 4 feet from side and rear property lines, including the street side of a corner lot. A conversion in the same footprint needs no setback.
- Parking: none required, and none to replace when you convert a garage or carport.
- Owner occupancy: not required for an ADU. For a JADU that shares a bathroom with the house, the owner must live in the house or the JADU.
- Rentals: no rentals shorter than 30 days for an ADU or JADU.
- Fees: no impact fees for an ADU under 750 square feet.
- Preapproved plans: Newark has a Pre-Approved ADU Plan Program. Ask the Planning Division for the current plan list.
What are the ADU rules in Union City?
Union City allows up to two ADUs on a single-family lot, one new and one converted, plus a JADU, with no parking required. Its rules were updated in March 2025 (Ordinance 2025-002), and it runs a preapproved ADU plan program (Union City ADU rules, via GoCodebook).- Size: an attached ADU can be up to half the size of the main house, but the city must always allow at least 850 square feet, or 1,000 square feet with two or more bedrooms. A JADU can be up to 500 square feet. Confirm the detached size limit with Planning; the state default is 1,200 square feet.
- Height: 16 feet for a detached ADU, 18 feet near transit, and up to 20 feet to match the main roof's pitch. An attached ADU can be up to 30 feet or the zone's limit, and no more than two stories.
- Setbacks: 4 feet from side and rear property lines. No front setback is required for an ADU of 800 square feet or less.
- Parking: none required for an ADU or JADU, and none to replace when you convert a garage.
- Owner occupancy: required for a JADU only.
- Fees: no impact fees for an ADU of 750 square feet or less. Larger units pay Union City's capital facilities, park and traffic signal fees (Union City preapproved ADU checklist).
- Preapproved plans: use a plan the city has already reviewed, without changes, to speed up approval. Any site-specific change takes the project out of the program.
- Questions: Union City Planning, (510) 675-5379, planning@unioncity.org.
How much does an ADU cost in Fremont, Newark and Union City?
A finished, permitted ADU in the Bay Area typically costs $120,000 to $150,000 for a garage conversion and $250,000 to $500,000 or more for a new detached unit (Maxable, 2026 figures). Builder estimates vary, so get at least three bids.Typical Bay Area build costs (2026)- Garage conversion: $120,000 to $150,000
- Attached ADU: $150,000 to $350,000
- Detached ADU: $250,000 to $500,000 or more; most land around $300,000 to $450,000
- Utility connections: $15,000 to $30,000 for a detached unit, with the sewer line often the biggest piece.
- Contingency: set aside 10 to 15 percent for surprises.
- Plan review: a flat $1,000 in Fremont when you use a preapproved plan.
Can I build an ADU for an aging parent?
Yes. An ADU is one of the most common ways Tri-City families keep a parent close while giving everyone privacy. Build it to aging-in-place standards from the start, because adding them later costs more.Design it for the years ahead- One level: bedroom, bathroom, kitchen and laundry on a single floor.
- No-step entry: a level path to the door, or a ramp of no more than 1 inch of rise per 12 inches.
- Wide doors and halls: 36-inch doors and halls at least 36 inches wide (48 is better).
- Curbless shower: with a bench, a handheld showerhead and blocking for grab bars.
- Lever handles and good light: lever handles, rocker switches and night lights on the way to the bathroom.
Does an ADU add value to a home?
Often, yes. In California, homes with ADUs had a median appraised value of $1,064,000 in 2023, compared with $715,000 for homes without one, and their values grew faster from 2013 to 2023: 9.34 percent a year versus 7.65 percent (Federal Housing Finance Agency).That gap isn't all from the ADU: FHFA compared values, not cause and effect, and says the trend needs more study. What an ADU adds to your home depends on:- Permits: a permitted ADU counts as living space in an appraisal; an unpermitted one may not.
- Quality: a separate entrance, full kitchen and private outdoor space appeal to renters and buyers.
- Rental income: a long-term rental can help cover the mortgage or the build cost.
Frequently asked questions
How big can an ADU be in Fremont?
Up to 1,200 square feet for a detached ADU and 1,000 square feet for an attached one. A junior ADU inside the house can be up to 500 square feet.How big can an ADU be in Newark?
Up to 1,200 square feet detached and 1,000 square feet attached. A converted garage or room has no size cap.How many ADUs can I build in Union City?
On a single-family lot, up to two ADUs, one new and one converted from existing space, plus one junior ADU.Do I need to add parking for an ADU?
No. Fremont, Newark and Union City require no off-street parking for an ADU, and none to replace a converted garage.Do I have to live on the property to build an ADU?
No. California bans owner-occupancy requirements for ADUs. A junior ADU that shares a bathroom with the house is the exception.How long does it take to get an ADU permit?
The city must approve or deny a complete application within 60 days. Fremont reviews preapproved ADU plans in 7 business days.Will an ADU raise my property taxes?
Only by the value of the ADU itself. Your main house keeps its existing Prop 13 assessment.Can I rent out my ADU on Airbnb?
Not for stays under 30 days in Newark, and never for a junior ADU anywhere in California. Long-term rentals are allowed.About the author
Travis VantVoort is a REALTOR® with Coldwell Banker Realty in Fremont, California (DRE #02206082). He helps homeowners in Fremont, Newark and Union City weigh what an ADU could add to their home, and helps buyers find properties with room for one. Reach him at 510.737.2220 or travis@vantvoort.com.This guide is general information, not legal, tax or construction advice. ADU rules change often; confirm current requirements with your city's planning division before you design.Sources
- Accessory Dwelling Unit Handbook, March 2026, California Department of Housing and Community Development
- Preapproved Accessory Dwelling Units, City of Fremont
- Can I build an ADU in Fremont?, GoCodebook summary of Fremont Municipal Code §18.190.005
- Accessory Dwelling Units, City of Newark
- Newark Municipal Code chapter 17.26, via GoCodebook
- Union City ADU rules, GoCodebook summary of Ordinance 2025-002
- Pre-Approved ADU Program submittal checklist, City of Union City
- Building an ADU in the Bay Area, Maxable, updated July 2026
- Building ADU cost in the Bay Area, Autonomous, February 2025
- Will my property taxes go up if I build a granny flat?, Howard Jarvis Taxpayers Association
- Granny units and accessory dwelling units, Santa Clara County Assessor
- Median appraised value for properties with ADUs in California, Federal Housing Finance Agency
Travis VantVoort
REALTOR®
Area served: Union City, Alameda County, CA, Newark, Alameda County, CA, Fremont, Alameda County, CA, Milpitas, Santa Clara County, CA, Pleasanton, Alameda County, CA, Dublin, Alameda County, CA, Livermore, Alameda County, CA, Hayward, Alameda County, CA, Castro Valley, Alameda County, CA, Sunol, Alameda County, CA
Expertise: Downsizing, First-Time Buyer, Fixer-Uppers, 1031 Exchanges, Historic Homes