What Is an ADU? The Complete 2026 Guide

An ADU, short for accessory dwelling unit, is a smaller, independent home built on the same lot as an existing single-family house. It has its own kitchen, its own bathroom, and its own entrance, so whoever lives there doesn’t share space with the main home the way an extra bedroom or a converted den would. In California, state law makes ADUs legal on nearly every single-family lot, with a streamlined approval process that most other types of construction don’t get. This guide covers what an ADU actually is, the laws behind it, the different types, typical size and cost, and how Framework First specifically builds them.

The short version: An ADU is a self-contained second home on a single-family lot, complete with its own kitchen, bathroom, and entrance. California law requires nearly every city to allow one, with fast, ministerial approval instead of a public hearing, and sizes commonly range from about 500 to 1,200 square feet. Cost varies widely by type, roughly $80,000 for a simple conversion up to $500,000-plus for a large custom build, while Framework First’s own factory-built lineup runs $180,000 to $557,000 all-inclusive across 14 models.

What exactly is an ADU?

“ADU” stands for accessory dwelling unit, but the term describes something pretty simple: a second, complete home on the same property as your existing house. To count as an ADU under California law, a unit generally needs:

• Its own kitchen (a full kitchen for a standard ADU, or a smaller efficiency kitchen for a Junior ADU)
• Its own bathroom
• Its own entrance, separate from the main house
• Independent living space, meaning someone could live there full time without ever setting foot in the primary residence

That last point is what separates an ADU from a bonus room, a converted den, or a home addition. An addition becomes part of the main house, sharing its address, its systems, and usually its ownership. An ADU is legally its own dwelling unit. It can be rented out separately, it shows up differently on an appraisal, and depending on the type, it can even be sold separately from the main home under newer state condo laws.

ADUs sit on the same parcel as a single-family, or increasingly a multifamily, home. They’re smaller than the primary residence, generally capped well under the size of a typical single-family house, and they’re accessory to it in a legal sense, meaning the lot still has one primary use (the main house) with the ADU as a secondary use.

The many names for the same thing

If you’ve researched this at all, you’ve probably run into half a dozen different terms, and it’s easy to assume they’re all different things. They’re not.

• Granny flat and in-law unit are older, informal terms, popular before “ADU” became standard legal language, both referring to a second unit meant for a parent or older relative.
• Backyard cottage is a more design-forward term, often used for a detached ADU that looks like a small standalone house.
• Casita, Spanish for “little house,” is common in parts of California and the Southwest for the same idea.
• Secondary unit and accessory unit are the more bureaucratic, planning-department versions of the term.

Every one of these describes the same underlying thing: a smaller, independent dwelling on a lot that already has a primary house. “ADU” won out as the standard term because it’s the language California’s housing statutes actually use, and it’s what your city’s planning department, your lender, and your appraiser will call it no matter which word you grew up using.

California’s ADU laws, in plain English

California has spent the last several years rewriting the rules around ADUs, and the throughline across nearly all of it is the same: state law sets a floor that cities cannot build below, even when local zoning says otherwise.

A few things that make California unusually ADU-friendly compared to most of the country:

• State law overrides local bans. Even if a city’s old zoning code doesn’t mention ADUs or effectively prohibits them through lot-size or setback rules, state law preempts that. Cities must allow ADUs on qualifying lots regardless of what their local code says.
• Approval is ministerial. That’s a specific legal term: it means a compliant ADU application gets checked against objective standards (size, height, setbacks) by planning staff, not voted on at a public hearing where neighbors can object. There’s no discretionary review to derail a project that meets the rules.
• There’s a 60-day clock. Once you submit a complete application, your city has 60 days to approve or deny it. Miss that window, and the application is generally deemed approved.
• Owner-occupancy isn’t required for a standard ADU. You can build one and rent out both the main house and the ADU to separate tenants. (A Junior ADU is the exception, more on that below.)
• Size and setbacks are guaranteed. Every city must allow at least an 800-square-foot ADU at 16 feet tall, with setbacks capped at 4 feet from the side and rear property lines, no matter what the local code used to say.

This doesn’t mean every lot automatically qualifies, or that local rules don’t matter at all. Cities still control things like design standards, exact fee amounts, and how a specific lot’s zoning interacts with the state minimums. But the overall direction of the law is clear: California wants ADUs built, and it has removed most of the tools cities used to previously block them. For the full breakdown, including parking rules, impact fees, and how many units a single lot can hold, see California ADU laws 2026.

The different types of ADUs

Not every ADU looks the same, and the type you build changes cost, disruption, and what fits your lot.

• A detached ADU is a freestanding structure in the yard, fully independent of the main house.
• An attached ADU shares a wall with, or connects directly to, the primary residence.
• A JADU (Junior ADU) is a small unit, up to 500 square feet, created inside the existing home’s walls.
• A garage conversion turns an existing garage into living space.
• An above-garage or two-story ADU stacks living space over a garage instead of spreading across the yard.
• A basement conversion turns existing basement space into a self-contained unit.

Each has real tradeoffs in cost, privacy, disruption during construction, and how much of your yard it uses. We cover all six in detail, including which one fits which situation, in The 6 Types of ADUs in California, Explained. For most homeowners building for rental income or family housing, a detached ADU, a fully separate, standalone home, ends up being the right call, which is also the only type Framework First builds. More on why in the detached ADU guide.

How big can an ADU be?

California sets both a floor and a ceiling on ADU size. Every city must allow at least an 800-square-foot detached ADU at 16 feet tall, regardless of local zoning, that’s the floor no jurisdiction can build below. On the other end, state law caps a detached ADU at up to 1,200 square feet, though some cities set a lower local maximum (never below that 800-square-foot floor). An attached ADU can run up to 50% of the primary home’s size, with a minimum of 800 square feet allowed. A JADU tops out at 500 square feet since it has to fit inside the existing house.

In practice, most detached ADUs built today land somewhere between a compact studio and a full three-bedroom home, generally 400 to 1,200 square feet. Framework First’s own lineup spans that entire practical range: 14 models from 405 to 1,200 square feet, so a homeowner can pick a size that matches their actual goal, a small rental studio versus a full family-sized unit, instead of building bigger or smaller than they need. See the full model lineup for size-by-size specifics.

What does an ADU cost?

Cost is the question with the widest range of honest answers, because it depends heavily on type, size, whether you’re converting existing space or building new, and where you live.

On the low end, a small Junior ADU or a simple garage conversion on a structurally sound existing garage can run well under $150,000, since it reuses walls, roofing, and sometimes plumbing that already exist. See our garage conversion cost breakdown for real numbers on that path. On the high end, a large, fully custom, site-built detached ADU with high-end finishes can run past $500,000 once permits, foundation, and months of on-site labor are added up.

As one honest, apples-to-apples example: Framework First builds 14 factory-built models, from 405 to 1,200 square feet, priced $180,000 to $557,000 all-inclusive. That price is built around a permitting budget, along with plans, the foundation, the factory build, delivery, utility hookups, appliances, and final inspection, with exact permit costs confirmed for your address in the feasibility study. That’s one builder’s real, all-in pricing, not a generic estimate, but it’s worth knowing that a different builder, a different type of ADU, or a fully custom design can land meaningfully above or below that range. For a deeper, broader look at what drives ADU cost in California generally, see how much does an ADU cost in California.

The one number we’d steer you away from trusting: a price-per-square-foot figure. ADU pricing swings so much based on site conditions, permitting, and what’s actually included that a flat per-square-foot number rarely holds up once you get a real quote for your property.

What people actually use ADUs for

The reason ADUs have become so popular isn’t abstract, it’s that they solve very concrete, very common household problems.

• Rental income. A separate unit with its own kitchen and bath rents close to a comparable apartment in the area, turning unused yard space into a second income stream. See ADU rental income in California.
• Housing aging parents. A private, single-story unit lets a parent stay close to family while keeping their own space and independence, without the household absorbing a full move-in. See ADUs for aging parents.
• Housing adult children. The reverse situation works just as well: a young adult gets a real, independent home instead of a childhood bedroom, and parents get to keep family close during an expensive stretch of early adulthood. See ADUs for adult children.
• A dedicated home office or studio. Not every ADU is meant to be lived in full time; some homeowners build one purely to get a separate, quiet workspace off the main structure.
• Downsizing without leaving your property. Some homeowners move into the ADU themselves and rent out or hand off the main house, staying on the same land while shrinking their own footprint and expenses.

Most households end up combining two of these: renting the space out today with the flexibility to move a parent in, or an adult child in, later without touching the structure again. That flexibility, more than any one use case, is why ADUs have become such a common addition to single-family lots across California.

How Framework First builds prefab ADUs

Everything above describes ADUs generally, the legal category, the types, the range of sizes and costs you’ll find across different builders and cities. What follows is specific to how Framework First does it, not a universal fact about every ADU.

Framework First builds detached, factory-built ADUs. Roughly 97% of each home, framing, electrical, plumbing, insulation, drywall, cabinetry, flooring, and appliances, is built inside our Salinas factory, under one roof, rather than assembled on your property one trade at a time. While that’s happening, a separate crew is on your lot pouring the foundation, so the two biggest phases of the project run in parallel instead of one waiting on the other. When the home is ready, it’s trucked to your property and set onto that foundation by crane, typically in a single day. For a full walkthrough of that process, see how is an ADU built and what is a prefab ADU.

That factory-first approach is also why our homes use 2x6 exterior stud walls instead of the industry-standard 2x4, James Hardie fiber-cement lap siding, mini-split HVAC systems instead of ducted forced air, and Trex composite decking, materials chosen for durability and low maintenance rather than the cheapest thing that clears inspection. If you’re weighing this build method against a traditional site-built ADU, prefab vs. site-built ADU covers the honest tradeoffs, including the cases where site-built is genuinely the better call.

On pricing, we quote one number, not a base price with the real costs added later. Across our 14 models, from 405 to 1,200 square feet, that number runs $180,000 to $557,000 all-inclusive, built around a permitting budget along with plans, the foundation, the factory build, delivery, utility hookups, appliances, and final inspection. Your exact permit costs are property-specific and confirmed for your address before you sign anything. You can see current pricing on the pricing page or browse the full lineup on the models page.

We’re a family-owned builder (CSLB #1047146, MFG #1595931) working out of our own factory in Salinas, which means the same company that designs your home also builds it, delivers it, and stands behind it under one contract, instead of you coordinating a separate architect, general contractor, and subcontractors yourself.

The first step, before any of that, is a feasibility study: we pull your property records, confirm what’s actually buildable on your lot under your city’s specific rules, and give you real numbers instead of a general estimate. It’s $497 for a standard lot (7 business days) or $997 for an empty lot (14 business days), and it’s refundable if your lot can’t host an ADU. Start a feasibility study to find out what fits your property.

Frequently asked questions

What does ADU stand for?

ADU stands for accessory dwelling unit, a smaller, independent home built on the same lot as an existing single-family house. It has its own kitchen, bathroom, and entrance, and can be rented out or lived in separately from the main home.

Is it legal to build an ADU in California?

Yes, on the vast majority of single-family and many multifamily lots. California state law requires cities to allow ADUs, sets a size floor of 800 square feet at 16 feet tall that no local zoning can go below, and requires a fast, ministerial approval process instead of a discretionary public hearing. See California ADU laws 2026 for the full rundown.

What’s the difference between an ADU and a JADU?

A standard ADU can be detached, runs up to 1,200 square feet, and doesn’t require you to live on the property. A JADU (Junior ADU) is smaller, up to 500 square feet, must be built within the existing home’s walls, and requires owner-occupancy. Many lots can have one of each. See JADU vs. ADU for the full comparison.

How much does an ADU cost in California?

It varies widely by type and size. A simple conversion can run under $150,000, while a large, fully custom, detached unit can exceed $500,000. Framework First’s factory-built lineup runs $180,000 to $557,000 all-inclusive across 14 models, with exact permit costs confirmed in a feasibility study. See how much does an ADU cost in California for the fuller breakdown.

Do I have to live on my property to build an ADU?

No, not for a standard ADU. California removed the owner-occupancy requirement, so you can build an ADU and rent out both it and the main house. Owner-occupancy is still required for a JADU specifically.

What is a prefab ADU, and is it different from a “regular” ADU?

A prefab ADU isn’t a different legal category, it’s a construction method. It’s built largely inside a factory rather than entirely on-site, then delivered to your property and set on a permanent foundation. It still has to meet the same building code as a site-built home. See what is a prefab ADU for the full explanation.

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Ready to see what an ADU could look like on your own lot? Start a free feasibility study to find out what’s buildable, or browse the full model lineup to see sizes, layouts, and pricing.