Complete Guide to Building an ADU in 2026

Step 1: Check Your Zoning

Before spending a dollar, find out if an ADU is allowed on your property.

Visit your city or county's planning department website and look up your parcel's zoning designation. Most cities now have online GIS portals where you can enter your address. Look for:

• ADU allowed by right — you can proceed without a discretionary review
• Conditional use permit required — allowed, but you'll need additional approval
• Not permitted — your city may still be updating its ordinance; check state law

If you're in California, Oregon, Washington, or Colorado, state law likely overrides local restrictions. Check our state guides to see what applies to you.

Step 2: Understand What You Can Build

Once you've confirmed ADUs are allowed, find out what your lot supports:

Detached ADU — separate structure in the backyard. Most expensive but most valuable and most private.

Attached ADU — addition to your existing home. Less expensive, shares a wall.

Garage conversion — convert an existing garage into living space. Often the most cost-effective option.

Junior ADU (JADU) — created within your existing home, up to 500 sq ft. Requires a separate entrance.

Key constraints to research:

• Maximum square footage (typically 800–1,200 sq ft for detached)
• Setback requirements (how close to property lines you can build)
• Height limits
• Owner-occupancy requirements (being phased out in most states)
• Parking requirements (often waived near transit)

Step 3: Hire a Designer or Architect

Unless you're doing a simple garage conversion, you'll need permitted drawings. Your options:

Full architect — most expensive ($15,000–$40,000), most custom. Worth it for complex sites or premium finishes.

ADU design firm — specialized studios that offer flat-fee design packages ($6,000–$15,000). Faster and less expensive for standard ADU types.

Pre-approved plans — some cities offer pre-approved ADU plan sets that skip plan check entirely. Los Angeles, San Jose, and Seattle all have programs. Ask your planning department.

Design-build firm — handles everything from design through construction. Easier to manage but less flexibility on contractor choice.

Step 4: Navigate Permits

The permit process is often the most frustrating part of an ADU project. Here's what to expect:

Plan Check

You submit your drawings to the city's building department for review. The city checks that your plans comply with zoning, building code, and local ordinances.

Timeline: Most states now require cities to respond within 60 days. In practice, expect 60–120 days for a first submission.

Corrections: Plan check almost always comes back with correction requests. Your designer addresses them and resubmits.

Utility Connections

You'll need permits for electrical, plumbing, and mechanical work. These are typically pulled by your licensed contractors.

Some cities require a separate utility connection (meter) for ADUs — others allow a sub-panel from the main house. Ask your city early, as a separate connection adds significant cost.

Impact Fees

Some cities charge impact fees for new dwelling units — water, sewer, school, traffic. California waives impact fees for ADUs under 750 sq ft. Other states vary widely.

Ask about fees before finalizing your budget. They can range from $0 to $30,000+.

Step 5: Get Contractor Bids

Once you have permitted plans (or are close), get bids from at least 3 licensed general contractors.

What to look for:

• General contractor license in your state
• ADU experience specifically (not just general residential)
• References from recent ADU projects
• Insurance: general liability + workers' comp

What to include in your bid request:

• Full set of permitted drawings
• Scope of work (what's included and excluded)
• Timeline and payment schedule
• Allowances for finishes (or specify exactly what you want)

Use our directory to find ADU contractors in your area.

Step 6: Finance Your Project

Most homeowners finance ADUs with:

HELOC (Home Equity Line of Credit) — borrow against your existing equity. Variable rate, flexible draw schedule.

Cash-out refinance — refinance your mortgage for more than you owe and take the difference in cash. Fixed rate, but resets your mortgage.

ADU construction loan — specialized products available in California and Colorado. Often lend against the after-renovation value.

Personal loan — higher rates, no home equity required. Best for smaller conversions under $100,000.

Use our cost calculator and ROI calculator to model what makes financial sense for your project.

Step 7: Manage Construction

A typical ADU build takes 4–9 months from permit approval to certificate of occupancy.

Site work — foundation, grading, utility connections (2–4 weeks)

Framing — structure goes up quickly, often in 1–2 weeks

MEP rough-in — mechanical, electrical, plumbing rough work before walls close (2–3 weeks)

Inspections — city inspects at key milestones. Don't skip or rush these.

Insulation + drywall — closes up the walls (1–2 weeks)

Finishes — cabinets, flooring, fixtures, painting (3–6 weeks)

Final inspection + certificate of occupancy — city signs off and you can legally occupy

Step 8: Rent It Out

Once you have your certificate of occupancy:

1. Set your rent — research comparable rentals in your area on Zillow, Craigslist, and local Facebook groups
2. Screen tenants — run a credit and background check; use a lease agreement specific to your state
3. Understand landlord-tenant law — each state has different rules on security deposits, notice periods, and habitability requirements
4. Check short-term rental rules — if you want to Airbnb your ADU, check your city's STR ordinance first

See our guide on ADU rental income for detailed income projections.

Common Mistakes to Avoid

Starting construction before permits — unpermitted ADUs create serious problems at sale and can require demolition. Never skip permits.

Under-budgeting — add a 15–20% contingency to whatever number your contractor gives you.

Choosing the lowest bid — the lowest bidder is often the highest risk. Check references carefully.

Ignoring utility costs — a separate electrical meter, new water service, and sewer lateral can add $15,000–$40,000 that isn't in your construction bid.

Not checking CC&Rs — if you have an HOA, check your CC&Rs. Some HOAs restrict ADUs even where state law allows them (though this is increasingly being challenged).