Permit type guide · ranked #8 nationally
ADU / in-law suite permits — complete guide (2026)
Accessory dwelling units are the fastest-growing permit category in the US. California, Oregon, Washington, and Arizona have the most streamlined ADU permitting laws in the country.
National avg: $400–$1,500
Approval: 1–8 weeks
All 50 states
#8 by volume
~200KIssued annually
$400–$1,500National avg cost
1–8 weeksTypical approval
2–3%Share of all permits
ADU permitting rules have changed dramatically since 2020. Many states have pre-empted local restrictions. Even if your city previously denied ADUs, state law may now require them to approve your application. Always check current state law, not just local ordinances.
When you need a ADU permit
| Work type | Permit required? | Notes |
|---|---|---|
| Detached ADU (new construction) | Always | Full building + trade permits |
| Attached ADU (home addition) | Always | Same as room addition |
| Garage conversion to ADU | Always | Building + electrical + plumbing |
| Junior ADU (room in house) | Always | Often streamlined in CA/OR/WA |
| ADU on existing foundation | Always | Foundation inspection required |
| Pre-approved ADU plan program | Simplified | CA, OR, WA offer pre-approved plans |
How to get a ADU permit — step by step
1
Check state ADU laws first
Your state may have pre-empted local restrictions. California, Oregon, Washington, Arizona, and Montana have strong statewide ADU rights. Check your state's housing agency website.
2
Verify local zoning
Even in ADU-friendly states, setbacks, height limits, and lot coverage rules still apply. Most jurisdictions require a minimum lot size.
3
Choose your ADU type
Detached ADUs have the most flexibility but cost more to build. Garage conversions are the cheapest path. Junior ADUs (within existing home) have the simplest permitting.
4
Submit plans and application
Full ADU construction requires architectural plans. Many cities now offer pre-approved ADU plans that skip plan review entirely — ask your building department.
5
Inspection phases
Same as new construction: foundation, framing, rough trades, insulation, and final. Expect 4–8 inspections total.
Cost by project type
| Project | Permit fee range | Typical timeline |
|---|---|---|
| Junior ADU (room conversion) | $400–$800 | 2–4 weeks |
| Garage conversion ADU | $600–$1,200 | 3–6 weeks |
| Attached ADU addition | $800–$1,800 | 4–8 weeks |
| Detached ADU (new build) | $1,000–$3,000 | 4–12 weeks |
| Pre-approved plan (CA/OR) | $300–$700 | 1–3 weeks |
Frequently asked questions
What is the difference between an ADU and a JADU?
A Junior ADU (JADU) is created within the existing walls of a primary residence and is limited to 500 sq ft. A regular ADU can be up to 1,200 sq ft in most jurisdictions and can be detached.
Does my city have to approve my ADU?
In California, Oregon, Washington, and several other states, cities cannot deny a by-right ADU application if it meets state minimums. Check your state's current law.
Can I rent out my ADU on Airbnb?
Short-term rentals of ADUs are governed by local ordinances, not state ADU laws. Many cities that allow ADUs restrict or prohibit short-term rental. Check your local short-term rental ordinance.
Does an ADU require a separate utility connection?
Not always. Junior ADUs and garage conversions often share utilities with the primary home. Detached ADUs may require separate meters depending on local utility requirements.
Data sources: Shovels.ai national permit aggregator · US Census Bureau BPS · IRC/IBC 2024