How to Build an ADU: 11 Steps From “Can I Do This?” to Move-In Day

Quick ADU Process Overview: The 11 Steps

Before we go deep, here’s the full sequence at a glance. Most guides skip to step 5 or 6. That’s why most projects hit avoidable problems. We start where you should actually start — with your property.

1. 1Confirm your property qualifies
2. 2Define your use case and ADU type
3. 3Set a realistic all-in budget
4. 4Secure financing (or confirm you can self-fund)
5. 5Hire the right team
6. 6Design the ADU and prepare construction documents
7. 7Submit for permits
8. 8Navigate plan review and corrections
9. 9Prepare the site
10. 10Build
11. 11Pass final inspections and move in

Here’s what “qualifies” actually means:

What to pull before you talk to anyone

• Your property's zoning designation (available on your city's GIS/parcel map)
• A current survey or plot plan showing lot dimensions, existing structures, and setbacks
• Your utility connection type (municipal sewer vs. septic, electrical panel amperage)
• Any deed restrictions, easements, or HOA CC&Rs

The 8 questions to ask your city on the first call

1. 1

   What ADU types are allowed on my lot?

2. 2

   What are the setback, height, and size limits?

3. 3

   Do you have pre-approved or standard ADU plans?

4. 4

   Is a separate utility connection required, or can I tie into the existing service?

5. 5

   Are any impact fees, school fees, or connection fees waived for ADUs under a certain size?

6. 6

   What's the current permit review timeline?

7. 7

   Do I need a separate address before I apply?

8. 8

   Are there any overlay zones, historic districts, or special review requirements for my lot?

Write down who you spoke with and the date. Policies change, and you want a record.

For a full breakdown of ADU types, definitions, and how they compare, see our complete guide to ADU types. Your use case drives the decision:

Preconstruction cost categories

Official homeowner guidance supports this order. Washington, DC’s homeowner guide places estimating total project cost before finding a lender. Boston advises homeowners to apply for ADU financing once they’ve begun working with their architect or GC and understand design, scope, and estimated cost. (Sources: DC Front Door program; Boston.gov ADU FAQ. Verified March 2026.)

The main financing paths, organized by which situation they fit:

Your financing path depends on your equity position, cash flow, and risk tolerance. For a deeper comparison of each option, see our ADU financing guide.

Who to hire first

7 questions to ask before you sign anything

1. 1

   How many ADUs have you designed/built in my city specifically?

2. 2

   What's your average timeline from design start to Certificate of Occupancy?

3. 3

   What's your average number of permit correction cycles?

4. 4

   Can I see three recent ADU projects with homeowner contact info?

5. 5

   How do you handle cost overruns — fixed bid, cost-plus, or something else?

6. 6

   Who manages the permit process — you, me, or a third party?

7. 7

   What's included in your quote, and what's explicitly excluded?

Design is where your ADU goes from idea to buildable plan. This phase produces the construction documents your city will review — not just a floor plan, but a complete technical package.

Three design approaches

What goes into a permit-ready plan set

A floor plan alone won’t get you a permit. Depending on your city and project, the full package typically includes:

• Site plan showing ADU placement, setbacks, existing structures, utility routes, and access
• Architectural drawings — floor plans, elevations, sections, details
• Structural engineering — foundation design, framing calculations, load paths
• Energy compliance — Title 24 in California, or your state's equivalent energy code
• MEP plans — mechanical (HVAC), electrical, and plumbing layouts
• Supporting documents — soil report, drainage plan, utility capacity letters as required

DC’s homeowner guide explicitly requires electrical, plumbing, and technical plans in the permit package. Portland requires formal submittal documentation that feeds into their checksheet review system. (Sources: DC Front Door program; Portland.gov residential permitting. Verified March 2026.)

Permitting is the step everyone worries about — and the one most guides oversimplify. Here’s what actually happens.

You (or your designer/contractor) submit your complete plan set and permit application to your city’s building department. In some cities, zoning approval is a separate step that happens before or alongside the building permit. In others, it’s one application.

What you’re submitting

• Completed permit application form
• Full construction document package (see Step 6)
• Applicable fees (plan check fees are typically due at submission; building permit fees at issuance)
• Any supplemental documents your city requires — soil reports, utility letters, HOA approval, stormwater plans

What happens after you submit

How to minimize correction cycles

1. 1

   Submit a complete package the first time. This is the single most effective thing you can do for your timeline.

2. 2

   Work with someone who's permitted in your city before. They know what reviewers look for.

3. 3

   Respond to comments quickly and thoroughly. Each round that sits on your desk adds that same time to your timeline.

4. 4

   Don't change the design during review. Moving a wall or adding a bathroom after submitting essentially restarts review.

The interdepartmental clearance maze

Your permit doesn’t just sit with one reviewer. Depending on your city, it may need sign-off from planning, fire, public works, water utility, sewer/sanitation, electric utility, and historic preservation (if applicable). Each works on its own timeline. Your designer or contractor should track all of these in parallel.

Once all comments are resolved, fees are paid, and clearances are obtained, your permit is issued. Some cities have an additional activation step — Austin, for example, requires permit activation before construction can legally begin. (Source: City of Austin Development Services, verified March 2026.)

How Long Does It Take to Build an ADU?

But here’s what that reality hides: the process is predictable. The steps don’t change. The sequence doesn’t change. What changes is how long each step takes in your specific city, with your specific lot, and your specific team. And it’s getting faster — over the past several years, dozens of states have passed legislation that streamlines permitting, removes barriers, and in many cases sets statutory review timelines.

Permit timelines vary by city — documented examples

Once your permit is issued, the real work begins. Site preparation gets the ground ready for construction.

What site prep involves

Construction follows a regulated sequence — and understanding it helps you know exactly where your project stands.

Each major phase requires a city inspection before work proceeds to the next stage. Failed inspections happen — usually for minor issues — and typically add days if your contractor addresses them promptly.

What trips homeowners up during construction

Final inspection and approval

A city inspector visits the completed ADU to verify everything was built according to approved plans and meets all applicable codes — structural, electrical, plumbing, HVAC, fire safety, and energy compliance. Once you pass, the city issues a Certificate of Occupancy (CO), final permit completion, or the local equivalent.

What happens after final approval

What Slows ADU Projects Down (And How to Prevent It)

Most delays are preventable — if you know where they come from.

What Changes If You Plan to Rent, Move Family In, or Sell

The build process ends at final approval. But your real-world obligations depend on how you use the unit.

What to Do Next, In Order

If you’ve read this far, you’re serious. Here’s your action plan:

Frequently Asked Questions About the ADU Process