In most parts of the country you can build another house in your backyard, usually in the form of an accessory dwelling unit (ADU), but whether your specific lot qualifies depends on local zoning, any HOA or deed restrictions on your property, and a permit process that runs several months and often costs into six figures. The first calls to make are to your local planning department and, if you have one, your homeowners association. Both can shut the project down before you spend a dollar on design.
Is an ADU Allowed on Your Lot?
Zoning is the gatekeeper. Your local planning or zoning department controls whether an ADU is allowed on your lot, and the answer can change from one neighborhood to the next inside the same city. Look up your property’s zoning designation on your municipality’s website or GIS map, then check whether that zone permits accessory dwelling units. Some jurisdictions allow them outright in all residential zones. Others require a conditional use permit or special exception, which adds time and uncertainty.
At least nine states, including Oregon, California, Colorado, Connecticut, Hawaii, Maine, New Hampshire, Utah, and Washington, have passed laws that override local zoning to some degree and require cities to allow ADUs on residential lots. If you’re in one of these states, your city may have to permit an ADU even if its older zoning code didn’t allow one. State preemption doesn’t erase local rules on size, setbacks, and design, though, so you’re still working inside a local rulebook.
Call your planning department directly before doing anything else. Staff can confirm whether your lot qualifies, flag overlay districts or environmental restrictions, and tell you whether the project needs administrative approval or a public hearing. That conversation is free and can save you thousands in architectural fees for a project that was never going to be approved.
HOA Rules and Deed Restrictions
Clearing zoning doesn’t mean you’re clear. If your property is in a homeowners association, the CC&Rs (covenants, conditions, and restrictions) may prohibit secondary structures entirely or impose rules stricter than your city’s. HOAs are private entities, and in most states their restrictions hold up even when local zoning would allow an ADU.
A few states have addressed this conflict directly. California, for example, prohibits HOAs from banning ADUs or imposing unreasonable restrictions on them. Outside those states, an association’s governing documents can effectively kill the project. Read your CC&Rs before you hire an architect. If you find a restriction, ask the HOA board whether a variance or amendment is realistic. Even properties without an HOA can carry deed restrictions recorded against the title, and a title search or a real estate attorney can surface those early.
Size, Height, and Setback Limits
Once an ADU is allowed on your lot, the next question is how much of one. Local codes cap size, height, placement, and appearance, and those numbers determine what actually fits behind your house.
Most jurisdictions cap a detached ADU somewhere between 800 and 1,200 square feet of living space. An attached ADU is often limited by a different formula, like a percentage of the primary home’s square footage. Some cities also offer a streamlined path for smaller units under 500 square feet built within your existing home’s footprint, sometimes called a junior ADU. Those interior conversions face fewer hurdles and lower costs, but they’re obviously more constrained.
Detached ADUs typically face height limits between 16 and 25 feet, depending on whether a second story is permitted. Side and rear setbacks commonly run four to five feet, and front setbacks are usually larger, often required to match the main dwelling’s setback line. Many jurisdictions exempt smaller ADUs (frequently those under 800 square feet) from lot coverage limits. Parking rules have been relaxed in many places, especially near transit or when the ADU is a conversion of existing space, and some cities have eliminated off-street parking requirements for ADUs entirely.
Design standards vary more but are increasingly common. Your jurisdiction may require the ADU’s exterior materials, roof pitch, or window style to match the main house or the surrounding neighborhood. These rules are aesthetic rather than structural, but they’ll hold up your permit just as surely as a setback violation.
Permits, Inspections, and Fire Sprinklers
Getting from an approved concept to legal construction requires building permits, and the process has more steps than most homeowners expect.
You’ll submit detailed architectural and engineering plans to your local building department. Those plans must show compliance with zoning, structural building codes, energy codes, and fire safety standards. For most homeowners, that means hiring an architect or designer experienced with ADU projects in your area. Plans that don’t address the specific requirements your city cares about will bounce back for revision, costing weeks each time.
Submitted plans go through review by multiple departments, commonly planning, building, fire, and public works. The overall permitting process typically takes three to six months from initial submission to permit issuance, and projects requiring variances, conditional use permits, or neighborhood notification can take longer.
Fire safety review deserves its own attention because it can add real cost. The International Residential Code requires fire sprinkler systems in new dwelling units, and many jurisdictions apply that requirement to ADUs. Some local codes exempt detached ADUs under 1,200 square feet; others enforce sprinklers regardless of size. Ask your local fire department or building code office before you finalize your budget. Sprinkler installation in a small ADU typically adds several thousand dollars.
Once your permit is issued, construction must begin within a set window or the permit expires, commonly 180 days to the first inspection. Overall project completion windows usually run one to two years. Inspectors visit at the foundation, framing, rough plumbing and electrical, insulation, and final stages, and each must pass before the next phase of work proceeds. After the final inspection, you’ll receive a certificate of occupancy that makes the unit legal to inhabit.
What It Will Cost
ADU construction costs vary dramatically with location, size, site conditions, and finishes. As a rough national benchmark, detached ADUs typically cost between $200 and $400 per square foot. That puts a 600-square-foot detached unit somewhere between $120,000 and $240,000, and a 1,000-square-foot unit between $200,000 and $400,000. Garage conversions and attached ADUs tend to cost less per square foot because they use existing structure. Sites with slope issues, difficult utility runs, or high-cost labor markets can push costs well above these ranges.
Beyond construction, budget for permit fees, which run from a few hundred dollars to several thousand depending on the jurisdiction and project scope. Impact fees are a separate charge some cities levy to cover the strain new housing puts on roads, parks, and schools. They can add thousands, though many jurisdictions waive or reduce impact fees for smaller ADUs. Ask your building department for a fee estimate before plans are drawn so you’re not blindsided later.
How to Pay for It
Few homeowners pay for an ADU entirely out of pocket. The right financing depends on your equity position, credit profile, and whether you’re building new or buying a property that already has one.
Home Equity Loans and HELOCs
The most common route is borrowing against existing home equity. A home equity loan gives you a lump sum at a fixed rate; a home equity line of credit lets you draw funds as construction progresses at a variable rate. Both leave your existing first mortgage untouched, which matters if you locked in a low rate in prior years. Lenders typically require you to retain at least 15 to 20 percent equity after the loan.
Fannie Mae Conventional Loans
If you’re purchasing a property with an existing ADU or refinancing after building one, Fannie Mae will finance it under specific guidelines. The ADU must be smaller than the primary dwelling and have its own entrance, kitchen with a stove or stove hookup, sleeping area, and bathroom.1Fannie Mae. Special Property Eligibility Considerations Under current rules, only one ADU is permitted per parcel of a primary one-unit dwelling. Beginning March 31, 2026, Fannie Mae is expanding eligibility to allow single-unit properties with up to three ADUs and two- to three-unit properties to include ADUs so long as the total unit count doesn’t exceed four.2Fannie Mae. Selling Guide Announcement SEL-2025-10 Borrowers who qualify for the HomeReady program can include rental income from an existing ADU to help qualify for the mortgage.3Fannie Mae. Accessory Dwelling Units
FHA 203(k) Renovation Loans
FHA 203(k) loans let you roll ADU construction costs into your mortgage when buying or refinancing. The standard 203(k) has no maximum renovation cost limit; the limited version caps non-structural work at $75,000.4U.S. Department of Housing and Urban Development. Section 203(k) Consumer Fact Sheet Most full ADU builds will exceed that $75,000 cap and require the standard loan, which also requires an FHA-approved consultant to oversee the project.
Taxes, Rental Income, and Insurance
Building an ADU creates ongoing financial obligations beyond the mortgage.
Adding an ADU increases your property’s assessed value and your property taxes with it. In most jurisdictions, only the added value is reassessed rather than your entire property. If the ADU adds $150,000 in value and your local tax rate is one percent, expect roughly $1,500 more per year. Garage conversions and interior conversions usually add less assessed value than new detached construction. Your county assessor’s office can estimate the change before you break ground.
If you rent the ADU, that income is taxable and reported on Schedule E of your federal return. You can deduct a wide range of expenses against it, including depreciation on the structure, repairs, insurance premiums, property management fees, and the portion of property taxes attributable to the rental unit. You may also qualify for a 20 percent qualified business income deduction on net rental income if you meet safe harbor requirements.5Internal Revenue Service. Topic No. 414, Rental Income and Expenses Those deductions can shrink the tax bill, but only if you keep decent records from day one.
Your existing homeowners policy probably won’t adequately cover a new ADU. Detached structures are typically covered under the “other structures” portion of a standard policy, often capped at around 10 percent of your dwelling coverage. On a home insured for $400,000, that’s only $40,000 for all detached structures combined, which won’t rebuild even a modest ADU. Call your insurer before construction begins. You’ll likely need to raise your other-structures coverage or add the ADU explicitly to the policy. If you plan to rent the unit, standard homeowners insurance generally won’t cover tenant-related liability claims or lost rental income, so you may need a landlord policy or rental dwelling endorsement. Short-term rentals through platforms like Airbnb often require a separate business or home-sharing policy. An umbrella liability policy is worth considering for any ADU you rent out.
Utility Hookups
Every ADU needs water, sewer, electricity, and often gas. How those utilities connect affects both your upfront cost and your relationship with tenants.
The three basic approaches are separate meters, submeters, and shared service. A separate meter means the utility company sets up a distinct account for the ADU and the tenant pays their own bills. It’s the cleanest arrangement for long-term rentals but costs the most to install because it requires dedicated service lines and meter panels that meet your utility’s specs. Some cities also tie a separate meter to a unique address for the ADU. Submetering keeps one main utility account in your name but installs a secondary meter on the ADU’s branch lines so you can measure its usage; you read the submeter and bill the tenant or fold the cost into rent. Shared service is the simplest: the ADU runs off your existing connections with no separate tracking, and you build estimated utility costs into rent. Your utility provider or building department can tell you if any of these options are required or prohibited locally.
Prefab and Modular as an Alternative
You don’t have to build from scratch on-site. Modular ADUs are built in a factory to the same International Residential Code standards as site-built homes, delivered in sections, placed on a permanent foundation, and classified as real property. From a code and financing standpoint they’re treated essentially the same as conventional construction. Manufactured homes built to the federal HUD Code are handled differently but can still work as ADUs in many jurisdictions. Fannie Mae will finance a HUD Code manufactured home used as an ADU only if the primary dwelling is site-built or modular, the manufactured unit is on a permanent foundation, and HUD compliance documentation is available.1Fannie Mae. Special Property Eligibility Considerations
Prefab can shorten timelines because factory work happens while your site is being prepared. Cost savings are less predictable. The unit itself may cost less than equivalent site-built construction, but delivery, crane placement, foundation work, and utility hookups can narrow the gap. Get a complete installed price, not just the factory price, before comparing options.