Yes — in most parts of the country, you can build an apartment in your backyard, usually in the form of an accessory dwelling unit (ADU), sometimes called a granny flat or in-law suite. Whether you actually can depends on four things: your local zoning, any state law that overrides it, your lot’s physical constraints, and your HOA if you have one. As of mid-2025, at least 18 states have passed laws requiring local governments to allow ADU construction, and the number keeps growing.
Is It Allowed on Your Lot
Start with zoning. Every parcel has a designation — something like R-1 (single-family) or R-2 (two-family) — that controls what you can build. Your city or county planning department’s website will show your property’s designation and whether ADUs are permitted there. Some jurisdictions allow ADUs by right, meaning you just need a building permit. Others require a conditional use permit, which involves a hearing and public notice and adds time and uncertainty.
If local zoning says no, check your state law before giving up. Ten states now have strong ADU laws that prevent cities from blocking both attached and detached units, and another eight have weaker but still meaningful protections. These state laws often cap how long cities can take to review applications, eliminate parking requirements near transit, set minimum unit sizes that cities can’t shrink further, and prohibit owner-occupancy mandates.
Zoning isn’t the last word, though. A homeowners’ association can still block you. HOA covenants, conditions, and restrictions (CC&Rs) are private contracts that run with the property, and they commonly restrict outbuildings, secondary kitchens, or rental activity. Some state ADU laws now limit HOA authority to block ADU construction, but this varies. Read your CC&Rs carefully, and if they’re ambiguous, get a written interpretation from the HOA board before spending money on plans.
What Will Actually Fit
Once you’ve confirmed an ADU is legally allowed, the next question is what you can physically build.
Size and Height
Most jurisdictions cap ADU square footage somewhere between 800 and 1,200 square feet, though some tie the limit to a percentage of the primary home’s living area. Height restrictions typically range from 16 to 25 feet, which usually means one story for a detached unit and sometimes two stories if the ADU sits above a garage. State preemption laws often set a floor, for example requiring cities to allow at least 800 square feet regardless of lot size, so check both your local code and state law.
Setbacks
Setback rules define how close to your property lines you can build. Rear and side setbacks for ADUs commonly range from 4 to 5 feet; front setbacks are usually larger. These distances sound small, but on a tight urban lot they can eliminate half your potential building footprint. Easements for utilities or drainage can further restrict placement. A surveyor can flag these issues before you invest in architectural plans.
Parking and Owner-Occupancy
Some local codes still require an additional off-street parking space for an ADU, but this requirement is disappearing quickly. Many state ADU laws now waive parking mandates entirely, especially when the property is near public transit. Owner-occupancy rules, which require you to live in either the main house or the ADU, have followed a similar trend. A growing number of states prohibit cities from imposing this restriction, which matters if you plan to rent both units or don’t live on the property full-time.
Utility Connections
Every ADU needs water, sewer (or septic), and electrical service. Extending these connections from the main house is sometimes straightforward, but the cost can surprise you. Trenching for new water and sewer lines, upgrading your electrical panel to handle the additional load, and paying municipal connection or “tap” fees can run anywhere from $5,000 to $15,000 or more depending on how far the ADU sits from existing lines and what your utility provider charges. Get quotes from contractors and contact your local utility providers early.
What It Costs
Total construction costs for a new detached ADU generally fall between $100,000 and $300,000 across most of the country. That range is wide because it depends on size, finishes, site conditions, and local labor costs. A 400-square-foot studio with basic finishes in a moderate-cost market will land near the low end. A 1,000-square-foot two-bedroom with higher-end materials in an expensive metro area will push toward the top.
Beyond the construction contract itself, several costs are easy to underestimate:
- Permit fees vary enormously by jurisdiction, from a few thousand dollars to over $20,000 for larger projects, and are often calculated as a percentage of estimated construction value.
- Architectural and engineering plans typically cost $5,000 to $15,000, depending on complexity and local requirements.
- Utility connections can add another $5,000 to $15,000.
- Site work — grading, tree removal, retaining walls, or demolishing an existing structure — can add significant costs that are hard to estimate without a site visit from a contractor.
How to Pay for It
Most homeowners don’t pay cash. Several loan products are built for this kind of project.
Fannie Mae treats an ADU the same as any other home improvement, so you can finance it with any standard Fannie Mae loan product, including a HomeStyle Renovation loan for adding an ADU to an existing property or a construction-to-permanent loan if you’re building a new home with an ADU from scratch. To qualify, the ADU must have independent space for living, sleeping, cooking, and a bathroom, and it must be accessible without walking through the primary residence.
The FHA’s 203(k) rehabilitation loan program also lists building an ADU as an eligible improvement, rolling the purchase or refinance amount and renovation costs into a single mortgage with a down payment as low as 3.5%. The property must be your primary residence, a licensed contractor must do the work, and a limited version of the loan is capped at $35,000, which won’t cover most ground-up construction. One important limitation: FHA guidelines generally require the ADU to be attached to or converted from space within the main house, so a fully detached backyard unit may not qualify.
If you have significant equity, a home equity line of credit (HELOC) or cash-out refinance can fund the project without the complexity of a renovation loan. The trade-off is that you’re borrowing against your home with no guarantee the ADU’s appraised value will match what you spent building it. HELOC rates are typically variable, which adds risk on a project that might take six months to a year.
Permits, Inspections, and Sign-Off
A building permit application requires a completed form from your local building department, detailed architectural plans showing floor layouts and exterior elevations, and a site plan drawn to scale showing the ADU’s location relative to property lines, existing structures, and utility lines. You’ll also need your property’s parcel identification number, an estimated project cost (since permit fees are often based on valuation), and your contractor’s license information if you’re not acting as owner-builder.
After you submit, city or county reviewers check your plans against zoning codes, building codes, fire safety requirements, and sometimes utility capacity. Plan review is usually the slowest part of the process, taking anywhere from a few weeks in jurisdictions with streamlined ADU processes to several months where staffing is tight. Getting a correction letter back is normal. Your designer revises, you resubmit, and the cycle repeats until the plans pass.
Once the permit is issued, construction can begin, but inspections happen at set stages: underground utilities before they’re buried, foundation before concrete is poured, framing, mechanical/electrical/plumbing while walls are still open, insulation before drywall, and a final inspection at the end. The golden rule is simple: don’t cover anything up until an inspector has seen it.
After the final inspection, you’ll receive a certificate of occupancy or equivalent sign-off. This document confirms the ADU meets all applicable codes and is legally habitable. You cannot move tenants in — or in most places use the unit as a dwelling at all — until this certificate is issued. Skipping this step doesn’t just create a code violation; it can void your insurance coverage and create serious liability if something goes wrong.
What Changes After It’s Built
Property Taxes
Building an ADU will raise your property taxes, but probably not as dramatically as you’d expect. In most jurisdictions, the assessor doesn’t reassess your entire property when you add an ADU. Instead, they add only the value of the new improvement to your existing assessed value, typically based on construction cost rather than a market appraisal. If your home is currently assessed at $400,000 and you build a $150,000 ADU, your new assessed value would be roughly $550,000, and your tax bill would increase proportionally. Specifics vary — some areas cap annual assessment increases, others use different valuation methods — but the core principle holds broadly.
Insurance
A standard homeowner’s policy typically covers detached structures under its “other structures” provision, but that coverage is usually limited to around 10% of your dwelling coverage amount. For a home insured at $400,000, that’s only $40,000 toward your ADU — nowhere near enough to rebuild a $150,000 unit. Contact your insurance company before construction begins to either increase your other-structures coverage or add a separate policy for the ADU.
If you plan to rent the unit, the gap widens. Standard homeowner’s policies aren’t designed to cover tenant-related risks. You’ll likely need a landlord or rental dwelling policy covering property damage, liability from tenant injuries, and lost rental income if the unit becomes uninhabitable due to a covered event. A single liability claim from a tenant can easily exceed the cost of the ADU itself.
If You Plan to Rent It Out
Tenants in an ADU have the same legal protections as tenants in any other rental property. Your lease must comply with your state’s landlord-tenant laws, covering security deposit limits, notice requirements for entry (typically 24 to 48 hours except in emergencies), maintenance obligations, and eviction procedures. Tenants have a right to a habitable unit — working plumbing, heat, hot water, and freedom from serious pest or safety hazards — and you’re responsible for maintaining those conditions throughout the tenancy.
Federal fair housing law prohibits discrimination based on race, color, religion, sex, national origin, disability, and familial status. There is a narrow exemption, sometimes called the “Mrs. Murphy exemption,” for owner-occupied buildings with four or fewer units, which would cover most ADU situations where you live on the property. But that exemption only applies to the federal Fair Housing Act. Many state and local fair housing laws have no such exemption or define it more narrowly, so living on-site doesn’t necessarily give you a free hand in choosing tenants. Discriminatory advertising is never exempt, even under federal law.
Short-term rentals are a separate question. Many cities that allow ADUs still restrict or prohibit short-term rentals in them, and some state ADU laws explicitly limit ADU rentals to terms of 30 days or longer. Where short-term rentals are permitted, you may need a separate license, must collect and remit local occupancy taxes, and could face occupancy caps, parking requirements, and noise regulations that don’t apply to long-term rentals.
Rental income from your ADU is taxable and gets reported on Schedule E (Form 1040). The upside is that you can deduct a wide range of expenses against that income: mortgage interest allocated to the ADU, property taxes, insurance premiums, repairs and maintenance, property management fees, and depreciation. Depreciation is particularly valuable — it lets you deduct the cost of the ADU structure (not the land) over 27.5 years, which often creates a paper loss that shelters some or all of your rental income from taxes even when you’re cash-flow positive. You cannot deduct the value of your own labor or the cost of capital improvements, though improvements get added to your depreciable basis. Keep meticulous records of every expense from the day construction begins, and consider a tax professional familiar with rental property to set up your depreciation schedule correctly.