When a Bay Area family needs more space, the choice usually comes down to two options: build an ADU (accessory dwelling unit) or add on to the house. Both add square footage, but they are permitted differently, use the lot differently and serve different goals.
The main difference
An addition enlarges your existing home. It shares the kitchen, entry and utility meters and is one dwelling. An ADU is a separate, complete home with its own kitchen, bathroom and entrance, either detached, attached or inside a converted garage.
Zoning and permits
- ADUs get special treatment under state law: 4-foot side and rear setbacks, a guaranteed minimum size of 800 sq ft, and a 60-day review deadline once the application is complete. ADUs under 750 sq ft are also exempt from impact fees.
- Additions follow your normal zoning: front, side and rear setbacks, lot coverage, floor area ratio (FAR) and height limits. In some cities a large addition or a second story triggers design review and neighbor notice.
On a tight lot where an addition would exceed FAR or setbacks, an ADU is often the only way to add space.
How you will use the space
- Choose an addition for a bigger kitchen, a primary suite, a family room or more bedrooms for your own household.
- Choose an ADU for privacy: parents, adult children, caregivers, guests, a home office, or rental income.
Construction impact
An addition opens up the existing house, so living there during construction can be harder. A detached ADU is built mostly in the backyard, and the main house stays usable. A garage conversion is often the fastest path because the structure already exists.
Value and flexibility
An addition grows the main home and its appraised square footage. An ADU adds a second unit that can earn rent and gives flexibility as needs change. In most Bay Area cities, an ADU cannot be sold separately from the main house.
Questions to ask before deciding
- Who will use the space in five and ten years?
- Does your lot have room within normal setbacks and FAR for an addition?
- Do you want rental income?
- Can you live in the house during construction?
- Are there easements, protected trees or slopes in the backyard?
We build both, so we can sketch each option on your lot and walk through the trade-offs. See our ADU builders and home additions pages for more.
Talk to a Licensed Bay Area Builder
Tell us about your lot and what you want to build. We will check your city’s rules and schedule a free consultation.
