Your ADU idea
Define what you want, understand the basic options, and develop an early cost and timing expectation.
View this StepThinking about building an ADU? Start here.
Understand what an ADU is, what may be possible on your property, which local rules matter, and what to expect before you hire anyone.
Check local ADU rulesRules depend on the government agency that has jurisdiction over the property.
What is an ADU?
An ADU is an additional home on the same property as an existing or proposed primary residence. It can provide space for family, rental income, caregiving, or a more flexible way to use your property. The design, size, location, approval path, and construction work can vary.
Concept diagrams show how each type relates to the main home—not a site plan.
Detached ADU
A separate structure from the main house. It usually requires new foundations, utility connections, and full exterior construction.
Attached ADU
Connected to the existing home. The connection may affect structure, fire separation, utilities, and how construction reaches the site.
Garage Conversion
Existing garage space converted into living space. Existing foundations and walls may help, but upgrades can still be substantial.
Interior Conversion
Existing space within a home converted into an ADU. Access, privacy, fire separation, and utility layout need careful review.
Junior ADU
A smaller unit contained within a single-family home. Size, kitchen, bathroom, and owner-occupancy rules may differ from a full ADU.
Customized ADU
A more individualized design for a specific site or way of living. More design decisions usually mean more coordination and cost uncertainty.
The type of ADU affects the project
Different ADU types can have different costs and levels of construction complexity. If you are not sure which approach fits your property, ask an experienced ADU Contractor. Search ADU Contractors
ADU rules depend on your location
California law creates a statewide framework, while the City or County responsible for the property may have additional standards, procedures, and special-area requirements. Confirm the jurisdiction before relying on a size limit, setback, fee, or permit timeline.
California statewide rules
Start with the current state framework, definitions, approval protections, and rules that limit what a local agency can require.
City or County rules
Local agencies explain submittal steps, local standards, fees, utility coordination, and special areas. A mailing address does not always identify the responsible agency.
Allowed ADU typesMaximum size
Height and setbacksParking
Owner occupancyRental restrictions
Permit processFees
Fire sprinklersSolar
Utilities and metersSpecial overlay areas
Could an ADU work on your property?
You do not need to solve every technical question yourself. Start by noticing what is already on the property and what might make construction easier or harder. These conditions can change the design, cost, and schedule.
Available spaceLot layout, existing buildings, and the intended ADU location.
Slope and foundationFlat ground, hillside conditions, soil, and structural work.
Construction accessHow workers, equipment, and materials can reach the site.
Existing structuresGarages, additions, sheds, or other work that may remain or be demolished.
Sewer or septicExisting capacity, connections, and required upgrades.
Electric serviceMain-panel capacity, service upgrades, and meter choices.
Water and gasAvailable service, new lines, shared service, or separate meters.
Fire accessAccess conditions, existing sprinklers, and local fire review.
You do not need to answer everything yourself. An experienced ADU Contractor may need to review the property. Search ADU Contractors
What should you expect?
Early numbers are useful when you know what assumptions they include. Look at the whole project—not only the construction price—and leave room for design, permits, utilities, site work, selections, and unknown conditions.
Likely cost
Understand square-foot, lump-sum, and cost-plus pricing. Check allowances, exclusions, design fees, permits, and Owner-purchased items before comparing totals.
Understand ADU pricingLikely timing
Separate design, permit review, bidding, construction, inspections, and closeout. Ask which dates are estimates and what could delay them.
Understand the timelinePossible return
Estimate the full investment, potential income or family value, ongoing costs, and a simple payback range. Test more than one scenario.
Explore the ADU ROI calculationWhat changes the price
Site conditions, utility work, finish level, design, included services, and quote assumptions can make two similar-sized ADUs cost very different amounts.
Review what an ADU bid should includeWho handles design, permits, and construction?
These responsibilities can be packaged together or handled separately. There is no single right setup for every Owner. Understand who is responsible, how information moves between people, and what is included in the price.
Design
You may use a standard plan, hire a designer or Architect separately, or choose a Contractor that includes design.
May work well when
You want more design control or already have plans.
Watch for
Who coordinates design decisions, revisions, and construction pricing.
Permit
The Owner, designer, permit specialist, or Contractor may prepare and manage the permit process.
May work well when
One person clearly owns submittals and agency responses.
Watch for
Permit fees, corrections, utility approvals, and services not included.
Owner-Builder
You take responsibility for hiring and coordinating trades instead of hiring a General Contractor for the full build.
Possible advantage
More direct control over hiring, purchasing, and project decisions.
Main tradeoff
You take on more coordination, paperwork, schedule, payment, and jobsite responsibility.
General Contractor
A licensed General Contractor coordinates the construction, trades, schedule, and day-to-day work.
Possible advantage
One primary party is responsible for coordinating the build.
Main tradeoff
Compare scope, included services, allowances, exclusions, and contract price—not price alone.
Four Steps from idea to closeout
Use the route as a map from the first idea through final records. Each Step explains what usually happens, what decisions matter, what you should be clear on, and which Checklist belongs to that stage.
Find the Contractor
Identify relevant experience, compare bids and scope, and understand the agreement before signing.
View this StepConstruction in progress
Prepare for the work, keep project and payment records, and communicate clearly during construction.
View this StepCloseout
Complete the Punch List, final walkthrough, records, approvals, and final payment review.
View this StepRelated ADU guides
Find Contractors with relevant ADU experience
Project type is already set to ADU. Search by location to see Contractors with documented work records relevant to ADUs. Review the kind of work, location, license information, and services shown before deciding whom to contact.
Not sure where to start? Talk through your ADU project with us and we’ll help you organize the next step.
Talk to UsQuestions that often come up
Short answers help you identify what matters. Detailed explanations link to the relevant ADU Guide or Step.
Can I build an ADU on my property?
The answer depends on the jurisdiction, property, existing buildings, proposed ADU type, and any special-area requirements.
Do I need an Architect?
It depends on the design and service model. Some Contractors offer packaged design and permit services; other projects begin with a separately hired Architect.
Should the ADU have separate utility meters?
Separate, shared, and private submeter arrangements can affect cost, utility coordination, and future billing. Confirm the assumption before comparing quotes.
Does an ADU price per square foot include everything?
Not automatically. Confirm the counted area, finish level, site work, utilities, design, permits, allowances, and exclusions.
Will the main house or ADU need fire sprinklers?
This depends on applicable state requirements, the primary dwelling, the proposed ADU, and local review.
How long does an ADU project take?
Separate the design, permit, bidding, construction, inspection, and closeout periods. Site and utility work can also affect the schedule.