What Is an Accessory Dwelling Unit?
An ADU is a second, fully independent home on the same lot as your house – kitchen, bathroom, its own front door. California law now protects your right to build one. This guide covers the types, the rules, the costs, and how to actually get one approved and built.
- Free on-site estimate
- No obligation, no pressure
- We handle the permits
- One contract – design through final inspection
An ADU by any other name
People call them different things depending on who is talking. A granny flat, an in-law unit, a backyard cottage, a casita, a guest house, a garage apartment. Under California law they are all the same thing.
What makes it an ADU rather than a shed or a converted spare room is independence. It has to be a complete, self-contained place to live: cooking facilities, a bathroom, somewhere to sleep, and its own way in and out. That is the line between a legal second dwelling and an unpermitted conversion that causes real trouble when you sell.
The four types of ADU
Which one is right depends on your lot, your budget and what you want the unit to do. Here is the honest trade-off for each.
Detached ADU
A brand-new standalone building in the yard. Most privacy, strongest rents, best appraisal treatment – and the highest build cost. This is what most people picture.
Detached ADU construction →
Garage conversion
An existing garage turned into a legal dwelling. The slab, walls and roof already exist, so you skip the most expensive phase of construction.
Garage conversion ADU →
Junior ADU (JADU)
Up to 500 sq ft carved out of the existing house, with an efficiency kitchen and its own entrance. Lowest cost of all – and many lots can have both a JADU and a full ADU.
Junior ADU explained →
Attached ADU
Built onto the side or rear of the existing house, sharing a wall. A middle path when the yard is tight but you want more than a JADU.
Design and build →
California ADU law: what actually changed
For decades local rules made ADUs practically impossible – minimum lot sizes, parking mandates, owner-occupancy covenants, and discretionary hearings where one angry neighbour could kill a project. A series of state laws dismantled most of that. Here is the floor cities and counties cannot go below.
- ADUs are allowed on single-family lots statewide. Local agencies cannot prohibit them outright.
- Ministerial approval. A compliant application is reviewed administratively – no public hearing, no neighbour veto.
- No owner-occupancy requirement for standard ADUs under current state law.
- Parking replacement is limited. In many garage-conversion situations cities cannot force you to rebuild the lost space.
- Junior ADUs up to 500 sq ft can be created inside the existing home, and many lots may have both a JADU and an ADU.
- HOAs cannot flatly ban ADUs on single-family lots, though reasonable design standards still apply.
- Statutory review timelines require agencies to act within a set period rather than sitting on an application indefinitely.
What still varies locally: setbacks, height limits, impact and utility fees, design standards, and whether a city or a county reviews your plans. That local layer decides whether a permit moves smoothly or stalls. See how we handle ADU permits and zoning →
This is a plain-English summary of statewide law, not legal advice. Requirements change and local rules differ – we verify the current rules for your specific parcel before design.
What does an ADU cost?
Anyone who quotes a price before seeing your property is guessing. Cost is driven by things specific to your lot.
Type and size
A garage conversion reuses an existing shell. New detached construction builds everything from the footing up. That is the single biggest difference.
Site conditions
Slope, rock, drainage and access. A flat suburban pad is far cheaper to build on than a steep foothill parcel.
Utility runs
Every foot of trenching for water, sewer, gas and power costs money. Distance from the main house matters more than people expect.
Service upgrades
Older homes often need an electrical panel upgrade, and septic systems may need capacity work for added bedrooms.
Finish level
The same footprint can be built simply or beautifully. Finishes are where budgets quietly expand.
Local fees
Impact fees and utility connection charges vary by jurisdiction and belong in the budget from day one.
See the full breakdown of ADU cost and financing → or try the 3D planner for ballpark ranges by size and type.
What people actually use ADUs for
Rental income
A permitted second unit on land you already own. You are not buying the dirt – only the building.
Multigenerational living
Parents or adult children close by with their own front door. Independence for everyone.
Home office or studio
Real separation between work and home, without commuting or renting commercial space.
How to get one built
The process is the same whether you are in Sacramento or 150 miles out. What changes is which counter reviews your plans.
Free site visit
We walk your lot, confirm what fits, and give real numbers. Costs nothing.
Design and engineering
Drawings, structural engineering and Title 24 – built to clear plan check first time.
Permits
We submit and work every correction until the permit is issued.
Build
One crew, one schedule, through final inspection and keys.
Common questions about accessory dwelling units
What is an accessory dwelling unit (ADU)?
An accessory dwelling unit is a second, self-contained home on the same lot as a single-family house. It has its own kitchen, bathroom, sleeping area and separate entrance. You may also hear them called granny flats, in-law units, backyard cottages, casitas or secondary suites – in California law they are all ADUs.
What types of ADU are there?
Four common types: a detached ADU (a new standalone building in the yard), an attached ADU (built onto the existing house), a garage conversion (an existing garage turned into a dwelling), and a Junior ADU or JADU (up to 500 sq ft created inside the existing home).
Are ADUs legal everywhere in California?
California law allows ADUs on single-family lots statewide, and local agencies cannot ban them outright. Compliant applications get ministerial review, meaning no discretionary hearing and no neighbour veto. Local rules still control setbacks, height, fees and design standards, which is why specifics vary by city.
Do I have to live on the property to build one?
Under current state law, owner-occupancy cannot be required for standard ADUs. Rules differ for Junior ADUs, and the law changes over time – we confirm what applies to your parcel before design.
How much does an ADU cost to build?
It depends on size, type, site conditions and finishes far more than on any per-square-foot rule of thumb. Garage conversions are usually cheapest because the shell already exists; detached new construction costs the most and returns the most. Our 3D planner shows ballpark ranges and we give exact pricing after a free site visit.
Can an ADU be rented out?
Yes – a permitted ADU is a legal dwelling and can be rented long term. Short-term rental rules are set locally and vary a lot, so check your city before counting on nightly rental income.
Will an ADU increase my property taxes?
The new unit is assessed and added to your assessment. Your existing home is not reassessed just because you added an ADU. Talk to your county assessor or CPA about your own numbers.
How long does it take to build an ADU?
Design and engineering come first, then plan check, then construction. Timelines vary by jurisdiction and by how complete the submittal is. We give you a realistic schedule for your city at the site visit rather than a marketing number.
Where we build
Sacramento is home base and we build up to roughly 200 miles out, across 171 Northern California cities and towns. Each city page carries local permit information and ZIP codes.
Get your free ADU estimate
Four quick questions. We come out, look at your property, and give you honest numbers – free, with no obligation.
Free ADU Estimate
Tell us about your lot and we will call you with real numbers.
Find out what your backyard could be worth
Free on-site visit. Real numbers. And if an ADU is not right for your property, we will tell you that too.