In most of the United States you can build a casita on your property, but whether you specifically can comes down to four layers of rules stacked on top of each other: your state’s ADU law (if it has one), your city or county zoning code, your homeowners’ association’s CC&Rs, and the physical limits of your lot. A casita is a small, self-contained home on the same lot as your primary residence, and planners call it an accessory dwelling unit, or ADU. The legal picture has shifted quickly. Roughly 18 states now have laws that broadly permit ADUs, and about half of those go far enough to override the local barriers that used to kill these projects outright.
Start With Your State’s ADU Law
Before opening your city’s zoning code, find out whether your state has passed an ADU preemption law. These laws set a floor: your city can be more permissive than the state, never more restrictive. As of mid-2025, about 18 states have enacted such laws, and roughly half are considered strong preemption statutes.
Strong state ADU laws tend to do the same handful of things. They require local governments to allow at least one ADU on any single-family lot. They cap setbacks, often at four to five feet from the rear and side property lines. They prohibit owner-occupancy requirements. They eliminate or reduce parking mandates near transit. And they establish minimum size thresholds, commonly 800 to 1,000 square feet depending on bedroom count, that cities cannot shrink. Some go further and waive development impact fees for smaller units or bar cities from requiring design review.
If your state has one of these laws, the practical answer to the question is almost certainly yes. The remaining questions are how big, where on the lot, and at what cost. If your state hasn’t acted, you’re subject entirely to your local ordinance, which may or may not permit ADUs at all. Check this first. A vote in your state capital two years ago may have quietly unlocked your property.
Local Zoning Decides the Details
Your property sits within a specific zoning district, and that classification controls what you can build. Most single-family residential zones now permit ADUs in some form. The rules that matter for a casita are usually three:
- Minimum lot size. Some jurisdictions require a minimum square footage before you can add a second dwelling. Strong state ADU laws often eliminate or reduce this.
- Setbacks. These dictate how far the casita must sit from property lines. Under preemption laws, rear and side setbacks are often capped at four to five feet. Where local rules control, setbacks can run up to 20 feet, which on a narrow lot can make a detached casita physically impossible.
- Lot coverage. This is the maximum percentage of your lot that structures can cover. If your primary home and garage already use most of your allowance, there may be no room for a casita, unless your state exempts small ADUs from the calculation. Some do, typically for units under 800 square feet.
You can look up your zoning designation on your city or county planning department’s website, usually through an interactive map. A phone call or a visit to the planning counter will normally answer the setback and coverage questions in under an hour, and many departments now run pre-application consultations specifically for ADU projects.
HOA Rules Can Still Say No
If your property is inside a homeowners’ association, the CC&Rs are a separate, private layer of rules that operate independently of city zoning. Historically many HOAs flatly banned secondary structures. That is changing: several states with strong ADU laws have also stripped HOAs of the power to prohibit ADUs outright, leaving them able to regulate design details but not to say no.
In states without that protection, a CC&R prohibition is legally enforceable no matter what your city’s zoning code allows. Where HOAs do permit casitas, they often impose restrictions on exterior materials, paint colors, landscaping, and placement that go beyond city rules. Pull the current CC&Rs and get written approval from the architectural review committee before you spend money on plans or permit fees. Doing it after you’ve started the city process is how homeowners end up eating architect bills.
Building Codes and Design Standards
Zoning decides whether you can build; building codes decide how. Your casita has to meet requirements for structural integrity appropriate to your region’s weather, fire safety measures including smoke detection and fire-rated walls, adequate ventilation and natural light, and proper electrical, plumbing, and mechanical systems.
On top of the safety code, local design standards cap the casita’s size and shape its appearance. Maximum floor area is typically either a fixed cap or a percentage of the primary home, whichever is less, with fixed limits commonly falling between 800 and 1,200 square feet. Height limits apply too. Historic districts and neighborhoods with character guidelines may also require exterior materials, rooflines, or colors that match the main house.
The Permit Process
Building a casita without a permit is illegal in every jurisdiction and causes serious problems when you sell or file an insurance claim. The process itself is predictable even when timelines aren’t.
You submit an application package to your building or planning department. A typical package includes architectural drawings and floor plans, a site plan showing property lines and the casita’s exact placement with setback dimensions, structural engineering calculations, energy compliance documents, the jurisdiction’s application form, and HOA approval if applicable. Your architect or designer normally assembles this. Incomplete submissions get rejected, so getting it right the first time matters.
After you pay plan review fees, the city routes your plans through planning, building, fire, and sometimes public works. Each reviewer checks against their own code. One round of corrections is normal. Three rounds means something fundamental needs rethinking. Once every department signs off, the city issues the building permit and construction can start.
During the build, you’ll schedule inspections at critical stages: foundation before the concrete pour, framing before the walls are closed, the rough electrical, plumbing, and mechanical work while systems are still exposed, insulation before drywall, and a final inspection at the end. After the final inspection passes, the city issues a certificate of occupancy. That document is what makes the casita legal to inhabit. No certificate, no legal occupancy.
What It Costs
ADU budgets split into soft costs (permits, design, fees) and hard costs (construction). Homeowners tend to focus on the construction number and get surprised by the rest.
Permits, Design, and Impact Fees
Permit and plan review fees range widely, from a few hundred dollars for a small conversion to over $20,000 in high-cost metro areas. Some jurisdictions waive or reduce fees for ADUs below a certain size. On top of city fees, expect to pay for architectural or design services, structural engineering, a soil report if required, and a property survey. Development impact fees for schools, parks, and transportation are another variable cost, though these are commonly waived for smaller ADUs.
Construction and Utility Connections
Hard construction costs for a detached, new-build casita typically fall between $150,000 and $300,000 depending on size, finishes, and the local labor market. Per-square-foot costs run roughly $200 to over $400. Garage conversions and interior conversions cost significantly less because the shell already exists.
Utility connections are the line item that catches people off guard. A detached casita needs its own water, sewer, and electrical service. Some cities require a separate electric meter. If the lot needs long utility runs from the street or main house, costs climb quickly, often billed per linear foot for trenching and piping. If you’re not on municipal sewer, a new septic system can add tens of thousands of dollars. Get quotes on utility work before you finalize the design.
Property Taxes
Building a casita increases your property taxes. The new construction triggers a reassessment of the ADU itself, not your entire property. The assessed value of the casita is added to your existing assessment. A $250,000 casita in a jurisdiction with a 1% tax rate would add roughly $2,500 per year. The assessor uses standardized construction cost data, which may not match what you actually spent.
Update Your Insurance Before You Break Ground
Your homeowners policy needs updating before construction starts, not after. A standard policy covers detached structures under the “other structures” portion, typically set at 10% of your dwelling coverage. For most casitas that’s not enough to rebuild. If your dwelling coverage is $400,000, other structures defaults to $40,000, well short of casita replacement cost.
Contact your insurer to increase other structures coverage or add a rider specific to the ADU. If the casita has its own utility connections or a separate mailing address, some insurers classify it as a standalone structure requiring its own policy. And if you plan to rent it, standard homeowners insurance almost certainly won’t cover you. Rental use typically requires a landlord policy or a rental dwelling endorsement covering property damage, liability, and lost rental income.
Renting the Casita
Rental income is one of the main reasons people build casitas. The rules around renting vary more than the rules around building.
Owner-Occupancy
Some jurisdictions require the owner to live on-site, in either the main house or the ADU, as a condition of having a rental. Several states with strong ADU laws have eliminated this rule, which means you could rent both units to different tenants. Where owner-occupancy still applies, violating it can trigger fines or loss of the ADU permit. Check your local ordinance before assuming you can move out and rent both.
Short-Term Rentals
Even where long-term rentals are freely allowed, using the casita as a short-term rental through platforms like Airbnb or VRBO may be restricted or banned. Some state ADU laws specifically prohibit short-term rentals of ADUs. Others leave it to local governments, many of which require a separate short-term rental permit, cap the number of rental nights per year, or prohibit short-term rentals in residential zones entirely. Violations can carry steep fines and jeopardize the casita’s legal status.
Rental Income and Taxes
Rental income from a casita is taxable and gets reported to the IRS on Schedule E of Form 1040. You can deduct mortgage interest allocated to the rental unit, property taxes, insurance premiums, maintenance and repairs, utilities, and depreciation of the structure. Depreciation is particularly valuable because it lets you deduct a portion of the building’s cost each year without additional out-of-pocket spending. You may also qualify for a 20% deduction on qualified business income from the rental under current rules, though that provision has specific requirements and is scheduled to expire after 2025 unless Congress extends it.1Internal Revenue Service. Topic No. 414, Rental Income and Expenses
How Homeowners Finance the Build
Few homeowners pay cash. Lenders offer several products for ADU projects, and the right one depends on how much equity you have and whether you want to touch your existing mortgage.
- Home equity line of credit (HELOC). A revolving credit line based on your home equity. Useful because you draw funds as construction progresses instead of borrowing a lump sum. Lenders typically cap borrowing at 80% to 85% of your home’s current value minus your mortgage balance. Terms are often around 10 years, and closing costs may be minimal or waived.
- Home equity loan. A fixed-amount second mortgage with a set interest rate, higher than your first mortgage but predictable. You get the full amount at closing and repay immediately. Closing costs are typical mortgage costs.
- Cash-out refinance. Replaces your existing mortgage with a larger one and gives you the difference in cash. Makes sense when current rates are close to your existing rate; expensive if rates have risen since you locked in.
- Construction loan. Based on the projected value of the property after the ADU is done, which helps homeowners with less than 20% equity. Many lenders offer construction-to-permanent loans that convert to a standard 30-year mortgage on completion, saving a second round of closing costs.
- FHA 203(k) loan. A federal program that lets you finance both the purchase or refinance of a home and the cost of building an eligible ADU in a single mortgage. A strong option for buyers purchasing with the intent to add a casita.2U.S. Department of Housing and Urban Development. Section 203(k) Consumer Fact Sheet
Get financing pre-approved before finalizing your design. Knowing the actual budget prevents the painful exercise of redesigning permitted plans because the construction bids came in higher than expected.