What Is ADU Building? A Complete Guide for San Jose Homeowners
An Accessory Dwelling Unit (ADU) — also known as a granny flat, in-law suite, backyard cottage, or secondary suite — is a self-contained residential unit on the same property as a primary single-family or multi-family home. ADU building refers to the design, permitting, and construction of these secondary units.
ADUs have become one of the most powerful tools for adding housing capacity, generating rental income, and accommodating multi-generational families. Our ADU Building and ADU Construction Service teams handle the entire process — from initial feasibility through final inspection.
Types of ADUs
California recognizes several distinct ADU configurations. The right choice depends on your lot, budget, and goals:
Detached ADU
A fully separate structure in the backyard, not connected to the main house. Maximum privacy for both households and the highest typical rental value.
Attached ADU
Connected to the main home but with a separate entrance and full living facilities. Often more cost-effective due to shared walls and utilities.
Garage Conversion ADU
An existing garage converted into a habitable living space. Among the most affordable options since the structural shell already exists.
Junior ADU (JADU)
Up to 500 sq ft, created within the existing home's footprint. Lower fees and faster permitting, but smaller and shares some systems with the main home.
Above-Garage ADU
A unit built on top of an existing or new garage structure. Maximizes lot usage when ground space is limited.
Basement Conversion ADU
Converting an existing basement into a code-compliant ADU. Requires egress windows, proper ceiling height, and full mechanical systems.
What Makes a Unit an ADU?
To qualify as an ADU under California law, the unit must have all of the following:
- Its own entrance — separate from the main home's primary entry
- A full kitchen — sink, cooking appliance, and refrigerator
- A bathroom — with toilet, sink, and bathing facility
- Sleeping space — typically a designated bedroom or sleeping area
- Code compliance — must meet building codes enforced by the City of San Jose
Without all five of these elements, the structure is considered a non-habitable accessory building (e.g., a guest room without a kitchen is just an extension of the primary home).
ADU Building in California: State Law Summary
California has been the national leader in ADU reform, enacting a series of laws that strip away the local barriers homeowners used to face. Key legislation includes:
- AB 68 (2020): Eliminated single-family zoning restrictions on ADUs statewide. Cities can no longer prohibit ADUs in single-family zones.
- SB 9 (2022): Allows lot splits on single-family parcels, enabling more ADU opportunities and even multi-unit configurations.
- AB 2221 (2023): Clarified size limits and exempted small ADUs from impact fees that had previously made small units financially unworkable.
- Owner-occupancy elimination: California eliminated owner-occupancy requirements for ADUs through January 1, 2025 — meaning landlords and investors could build ADUs, not just owner-occupiers.
The California HCD ADU page maintains the most current statewide guidance.
Why ADUs Are a Powerful Investment
ADUs typically add significant value to your property, generate rental income (often $2,500–$4,000/month in San Jose), and provide flexible space for aging parents, adult children, or guests. See our guide on ADU cost in San Jose and whether you can build an ADU on your property.
The ADU Building Process — Step by Step
Here's what to expect when building an ADU from start to finish:
- Feasibility & site analysis — we check your lot's setbacks, utilities, and zoning to confirm what's possible
- Design & floor planning — architectural drawings tailored to your goals and budget
- Permit submission — full plan package submitted to San Jose's Development Services Department
- Plan check & corrections — we handle any city revision requests
- Permit issuance & pre-construction meeting
- Site work & foundation — grading, footings, slab or crawl space
- Framing, MEP rough-in, inspections
- Insulation, drywall, exterior finish
- Interior finishes, fixtures, appliances
- Final inspection & certificate of occupancy
For most San Jose ADUs, the full timeline runs 8–14 months from contract signing to move-in ready, with construction itself typically taking 5–8 months.