ADU Rules in Campbell: What Homeowners Should Know

Thinking about adding a backyard home to your Campbell property but not sure where to start? You are not alone. ADUs are popular across Santa Clara County, yet the rules can feel technical and the process can take time without a plan. In this guide, you will learn the basics of ADU types, where they are allowed, parking expectations, permitting steps, and practical pros and cons so you can move forward with clarity. Let’s dive in.

ADU basics in California and Campbell

California has strong state laws that make it easier to build accessory dwelling units on many residential lots. These laws set clear guardrails for cities and require staff-level, ministerial review for qualifying ADUs that meet objective standards. Local cities, including Campbell, still apply their own zoning and building rules within those state limits.

Most homeowners look at four ADU types:

• Detached ADU: a separate backyard cottage.
• Attached ADU: an addition connected to your home.
• Conversion ADU: reusing existing space like a garage or basement.
• Junior ADU (JADU): a small unit within the existing home, typically up to 500 sq ft, that often shares systems with the main house.

Your property’s zoning, lot layout, and utilities will shape which type is most feasible. A quick feasibility talk with the City’s planning staff is a smart first step.

Where ADUs are allowed

State law opens the door to ADUs on many single-family and multi-family lots. Campbell applies its own objective standards by zone, and some areas may have additional rules due to overlays or historic considerations.

What to confirm for your address:

• Zoning designation and whether ADUs are permitted on that lot.
• Any special overlays or historic review requirements.
• Lot coverage, open space, and building envelope constraints that could limit size.

If your property sits in a multi-family zone, the rules for quantity and configuration can differ from single-family lots. Ask planning staff how many ADUs are allowed and whether conversion of existing non-livable areas is possible.

Size, height, and setbacks

State law provides upper bounds for ADUs and protects certain size allowances. Local standards still control maximum square footage, height, and setbacks as long as they fit within state law. JADUs are typically capped at 500 sq ft under state rules, while detached and attached ADUs follow local caps that cannot be more restrictive than state allowances.

Before you design, verify for your lot:

• Maximum ADU square footage and height allowed.
• Side and rear setbacks and how they change by ADU type.
• Lot coverage and floor area ratio limits that affect buildable area.

If your property is in a conservation or historic area, you may see additional design or placement requirements. Plan for this early to avoid redraws.

Parking and access

Parking rules are lighter for ADUs than for new homes in many cases. State law restricts when a city can require new parking for an ADU, such as when the property is near transit or when converting existing spaces. In other situations, Campbell may require one additional space for a new ADU.

What to check on your site plan:

• Whether your project qualifies for a state parking exception.
• If parking is required, where you can place it on your lot.
• Driveway access, on-street permit rules, and any alley frontage.

Good site diagrams go a long way. Show the parking location, access path, and lighting for safe entry to the unit.

Permitting steps and timeline

Approvals for qualifying ADUs are ministerial when you meet objective standards. That means staff checks your plans against the code without public hearings. A solid, complete submittal helps keep things moving.

Typical workflow:

1. Pre-application. Speak with planning staff about your concept, zoning, setbacks, height, parking, utilities, and any overlays. Bring a simple sketch and property info.
2. Formal application. Submit site plan, floor plans, elevations, structural details as needed, Title 24 energy documents, and a utility plan. Include parking diagrams and any drainage or grading sheets if relevant.
3. Plan check. Planning reviews zoning and design standards while Building reviews structural, mechanical, electrical, plumbing, energy, and fire-safety compliance. You may get a corrections list.
4. Resubmittal. Address comments and upload revised sheets. Multiple rounds are common for new construction.
5. Permit issuance and inspections. Once approved, you receive permits. Construction moves through required inspections until final sign-off.

Time expectations vary by project complexity and how complete your submittal is. Many homeowners see plan review in the range of several weeks to a few months. Construction can take a few months to a year depending on scope, site work, and contractor availability.

Fees, utilities, and inspections

State law limits certain fees for ADUs and often reduces or exempts impact or capacity fees for smaller units. Many jurisdictions use a 750 sq ft threshold to determine exemptions or proportional fees, though you should confirm the current schedule with the City. You will still pay plan review and building permit fees, and utility connection charges may apply.

Key items to pin down early:

• Current permit and plan review fees for your ADU type and size.
• Whether utility impact or connection fees apply to your project.
• Water, sewer, and power capacity or panel upgrades.

ADUs must meet the California Building Code and related electrical, mechanical, plumbing, and energy standards. Fire-safety requirements apply, including egress and smoke alarms. Some projects may trigger fire department review.

Multi-family properties

If you own a duplex or larger multi-family building, ADU rules differ from single-family lots. You may be able to convert existing non-livable areas into new units and add detached ADUs within certain limits.

Ask planning staff:

• How many ADUs are allowed on your parcel.
• Which areas qualify for conversion and what structural changes are required.
• Applicable parking and access rules for multi-family sites.

Owner-occupancy and renting your ADU

State legislation has curtailed many owner-occupancy requirements for new ADUs built after certain dates. Check the current rules to see whether you must live on-site.

If you plan to rent the ADU, review local rules for short-term and long-term rentals. Update your insurance and discuss landlord requirements with your insurance provider and a qualified advisor.

Pros and cons for Campbell homeowners

Building an ADU is a major project. It can be a smart move, but it helps to think through both sides.

Pros:

• Rental income potential. A well-designed ADU can generate steady rent that helps offset your mortgage or other costs.
• Multigenerational living. Create privacy and independence for parents, adult children, or caregivers while staying close.
• Property value. Many ADUs increase overall property value, depending on design, quality, and the neighborhood market.
• Flexible use. Use as a long-term rental, guest suite, home office, or future caregiver space.

Cons and considerations:

• Upfront investment. Bay Area construction costs are significant. Garage conversions are often cheaper than new detached units. Get multiple bids.
• Time and complexity. Even with streamlined rules, design, plan check, and construction take coordination and patience.
• Taxes and assessments. Adding an ADU can change assessed value. Contact the county assessor to understand potential impacts.
• Ongoing responsibilities. Maintenance, insurance updates, and property management if you rent the unit.

Budgeting and payback thought-starters

Build a simple model before you start design. Your numbers will vary, but a framework helps you evaluate feasibility.

• Estimate monthly rent for a comparable ADU in your area.
• Subtract loan payments if you plan to finance, plus insurance, utilities, and a maintenance reserve.
• Compare the net to your total project cost to see a rough payback period.

Consider financing options such as a cash-out refinance, renovation loan, or construction loan. Choose lenders who understand ADU projects and local fees.

Design tips that speed approval

A clear, code-conscious plan reduces corrections and saves time.

• Keep your footprint and height within local objective standards.
• Match materials and rooflines to the primary home where required by design standards.
• Place windows and entries to protect neighbor privacy.
• Provide simple, safe access paths with adequate lighting.
• Show accurate setbacks, lot lines, and parking on the site plan.

How to get started: a quick checklist

• Confirm zoning and any overlays for your parcel.
• Pick your ADU type: detached, attached, conversion, or JADU.
• Verify size, height, setbacks, and lot coverage limits for your lot.
• Clarify parking requirements and any state parking exceptions that apply.
• Review current fee schedules and possible small-ADU fee reductions.
• Talk with utility providers about water, sewer, and power needs.
• Ask about fire-safety and access requirements early.
• Gather documents: site plan, floor plans, elevations, structural details, Title 24, and utility plan.
• Get multiple contractor bids and check experience with Campbell ADUs.
• Plan your financing and insurance updates.

A local partner for your ADU goals

An ADU can support family needs, create rental income, and add value, but the process works best when you move step by step. If you want a second set of eyes on feasibility, neighborhood rent potential, or how an ADU could affect resale, reach out. As a long-time South Bay broker, I can help you think through value, timeline, and local contractors so you can move forward with confidence.

Ready to talk through your property and goals? Connect with Louis Ponce for a friendly, no-pressure consultation.

FAQs

Can I build an ADU on a single-family lot in Campbell?

• Many single-family lots are eligible, but zoning, setbacks, and any overlay districts matter, so confirm what applies to your exact property before you design.

What sizes are allowed for ADUs and JADUs?

• State law generally caps JADUs at 500 sq ft, while Campbell sets objective size and height limits for other ADUs that must fit within state allowances.

Do I have to live on-site to rent out my ADU?

• Recent state laws reduced many owner-occupancy rules for new ADUs; check current requirements to see if any on-site residency rules apply to your project.

Will I need to add a parking space for my ADU?

• It depends on your location and ADU type; state law creates parking exceptions in several cases, but Campbell may require one space when no exception applies.

How long does the ADU permitting process take?

• Timelines vary by project scope and plan completeness; plan review can take weeks to a few months, while construction can range from several months to a year.

What fees should I expect for an ADU in Campbell?

• You will pay plan review and building permit fees, and possibly utility or impact fees; smaller ADUs often qualify for reduced or exempted impact fees under state rules.