ADU 101: A Complete Guide to Accessory Dwelling Units

By Raven Vuong

Accessory Dwelling Units — ADUs — are one of the most practical and financially rewarding things a Bay Area homeowner can add to their property. A self-contained living space on the same lot as your primary home, an ADU can generate rental income, house family members, increase your property value, and make better use of land you already own.

This guide covers everything you need to know: what ADUs are, the different types, why homeowners build them, what California law says, how much they cost, and how to get started. Whether you are just exploring the idea or ready to move forward, this is the place to start.

What Is an ADU?

An Accessory Dwelling Unit is a self-contained residential unit located on the same parcel as a primary single-family home. It has its own entrance, kitchen, bathroom, and sleeping area — everything needed for independent living. ADUs go by many names: granny flat, in-law suite, backyard cottage, casita, secondary unit. They are all the same thing under California law.

ADUs are distinct from JADUs (Junior ADUs), which are smaller units created within the existing footprint of a primary home. Both are legal, both require permits, and both offer different advantages depending on your property and goals.

Types of ADUs

• Detached ADU — A fully separate structure in the backyard or side yard. Most privacy, most flexibility, highest cost. Ideal for rental income or independent family living.
• Attached ADU — An addition physically connected to the primary home, sharing at least one wall. More cost-efficient than detached, slightly less privacy.
• Garage Conversion — Converting an existing attached or detached garage into living space. Lower cost since the structure and foundation already exist.
• Basement or Attic Conversion — Repurposing existing underutilized space within the home. Cost varies widely depending on current condition and ceiling height.

Junior ADUs (JADUs)

A JADU is a unit of no more than 500 square feet carved out of the existing floor area of a single-family home. It must include a separate entrance and an efficiency kitchen but may share a bathroom with the primary residence. JADUs are the lowest-cost ADU option and are approved ministerially — meaning the city cannot deny them if code requirements are met. They are ideal for homeowners who want to add a rentable unit without major new construction.

Why Build an ADU?

The reasons homeowners build ADUs vary, but the financial case is almost always compelling in the Bay Area. Here are the four most common motivations.

Rental Income

A one-bedroom detached ADU in San Jose typically rents for $2,200 to $3,800 per month. At those rates, a $280,000 ADU pays itself back in 6 to 10 years — and generates income for decades after. Unlike stocks or index funds, rental income from an ADU is relatively predictable, inflation-adjusted, and backed by a hard asset you already own.

Property Value

Adding a permitted ADU to your property increases its assessed value and market value. In the Bay Area, a well-built ADU typically adds $350,000 to $550,000 in appraised value — often more than its construction cost. Buyers increasingly look for properties with ADUs as a built-in income source, making your home more competitive when you eventually sell.

Multigenerational Living

ADUs are the most practical solution for multigenerational households: aging parents who want independence without moving to assisted living, adult children who cannot afford Bay Area rents, or extended family who want proximity without sharing a home. A detached ADU provides full privacy for both households while keeping family close. Our in-law suite and ADU construction services are designed specifically for this use case.

California Regulations & Permitting

California has made it significantly easier to build ADUs over the past several years. A series of state laws have overridden restrictive local ordinances, reduced permit timelines, and eliminated many of the barriers that previously made ADU construction difficult.

State Law Overview

Under current California law, most single-family homeowners can build one ADU and one JADU on their property by right — meaning local governments cannot deny the application based on neighborhood opposition or subjective design criteria. Key provisions include:

• ADUs up to 800 sq ft cannot be denied solely for exceeding lot coverage limits
• Setback requirements for detached ADUs are a maximum of 4 feet from rear and side property lines
• Parking requirements for ADUs are waived in most cases, particularly near transit
• Owner-occupancy requirements have been suspended for many ADU types
• ADUs within existing structures (garage conversions, basement conversions) must be approved within 60 days

Our team also assists homeowners with SB-9 feasibility assessments for those considering lot splits, which opens up even more development potential on a single parcel.

Permits & Approvals

All ADUs require building permits regardless of type or size. The permitting process typically involves architectural plans, structural engineering (for new construction), Title 24 energy compliance documentation, and coordination with local utilities for connection upgrades. In most Bay Area cities, ADU permit timelines run 8 to 16 weeks for new detached units and 4 to 8 weeks for conversions.

9Builders manages the full permitting process for every project — from initial feasibility review through final inspection sign-off. Homeowners do not need to navigate the planning department on their own.

ADU Cost Breakdown

ADU costs in the Bay Area are driven by four main factors: type of construction, square footage, site conditions, and finish level. The ranges below reflect real project costs in San Jose and the South Bay as of 2026.

Cost by ADU Type

The chart below shows typical low and high cost estimates by ADU type. Detached new construction carries the highest cost but also the highest rental income potential and greatest impact on property value.

ADU Types at a Glance

How to Get Started

Building an ADU involves several stages — feasibility, design, permitting, construction, and final inspection. Here is a practical checklist to guide your early planning.

• Assess your lot. Measure your rear and side yard setbacks and calculate available footprint. Most Bay Area lots can accommodate at least a JADU or garage conversion.
• Check your zoning. Confirm your parcel is zoned for single-family residential use and is not subject to any special overlay restrictions.
• Define your goal. Rental income, family living, or both? Your goal determines the right ADU type, size, and finish level.
• Get a feasibility review. A contractor or architect familiar with local ADU regulations can tell you exactly what is buildable on your lot before you spend money on plans.
• Request itemized bids. Compare at least two or three contractors. Ask for line-item breakdowns, not lump-sum estimates.
• Understand the permit timeline. Factor permitting time into your overall project schedule — it is often the longest phase.
• Plan your financing. Options include HELOCs, home equity loans, construction loans, and California ADU-specific lending programs.
• Hire a contractor experienced in ADUs. ADU permitting and construction has specific requirements. A general remodeler without ADU experience will cost you time and money.

Frequently Asked Questions

Common questions about ADU construction in the Bay Area.