Everything homeowners need to know about building an accessory dwelling unit on Oahu - size limits, setbacks, permits, and how a decade of "ohana unit" and ADU history is colliding with some of the highest housing costs in the country.
Last verified: September 2026
6/10 · Moderately ADU-friendly
1,000 sq ft*
Max ADU size
Varies by zone*
Max height
Not required*
Owner-occupancy
By-right*
Permit type
This page summarizes accessory dwelling unit standards for the City and County of Honolulu (the island of Oahu) under the Land Use Ordinance, as amended by Ordinance 25-2, for general reference - it is not legal advice. Regulations are actively changing to meet a state mandate; always confirm current requirements with the Honolulu Department of Planning and Permitting (DPP) before designing or applying for permits. See sources below.
Zoning & size requirements
What you're allowed to build
Lot size
Max ADU size
Parking
Notes
Under 5,000 sq ft
500 sq ft
1 off-street stall (compact allowed)
Smaller lots became newly eligible under Ordinance 25-2 (effective September 30, 2025), which lowered the old minimum-lot barrier
5,000 sq ft and larger
1,000 sq ft
1 off-street stall (compact allowed)
Parking is waived in parts of the Primary Urban Center and Ewa Development Plan areas within about 800 ft of a bus stop
Allowed zoning districts
R-3.5, R-5, R-7.5, R-10, and R-20 residential districts, plus the Country districts (AG-1/AG-2). Height, setbacks, lot coverage, and Floor Area Ratio all continue to follow the underlying zone's standard dimensional rules - Honolulu does not use a single flat ADU height or setback number the way many mainland cities do, so confirm your specific district's limits with DPP. ADUs are distinct from Honolulu's older "ohana unit" program, which allows a larger second unit (up to half the primary home's floor area) under separate rules and its own parking and lot-size requirements.
Additional standards
Setbacks, parking & other rules
Setbacks
Follow the underlying zone's standard side and rear yard setbacks - there is no separate, reduced ADU-only setback. Typical residential zones require modest side/rear yards; confirm your district's figures with DPP
Parking
1 off-street stall per ADU, compact size allowed; waived within about 800 ft of a bus stop in parts of the Primary Urban Center and Ewa Development Plan areas
Units per lot
1 ADU is the standard allowance. As of Ordinance 25-2 (Phase 1), a lot that already has an ʻohana unit or two principal dwellings can add an ADU on top, reaching two small units on one lot in more situations - but the lot cannot be a Condominium Property Regime (CPR)
Owner-occupancy
Not required under the current covenant terms. Honolulu's original 2015 ordinance required owner-occupancy by recorded covenant; Ordinance 25-2's updated covenant drops that requirement, consistent with Hawaii's 2023 statewide preemption. The covenant does still bar short-term/vacation rental use and require a long-term-lease-only ADU, plus a prohibition on selling the ADU separately (no CPR)
Building code
Must meet the Hawaii State Building Code, based on the International Building/Residential Code as adopted and amended for Hawaii
Rental license
No separate Honolulu rental license beyond the recorded ADU covenant; short-term/vacation rental use of the ADU itself is barred by that covenant
Ordinance history
How Honolulu's ADU rules got here
2015ADUs legalized citywide. The Honolulu City Council passed Bill 7, enacted as Ordinance 15-41, adding accessory dwelling units to the Land Use Ordinance alongside the older, larger "ohana unit" category that had existed for decades. The original ADU cap was smaller than today's and came with a recorded owner-occupancy covenant.
2023Hawaii's first statewide ADU preemption. Act 232 required all four counties to allow ADUs by right on residential lots, limited how restrictive local size and setback rules could be, and barred owner-occupancy requirements statewide, effective July 2023.
2024Act 39 raises the bar further. Signed May 28, 2024 (originally Senate Bill 3202), Act 39 directs all Hawaii counties to allow at least two ADUs on every residentially zoned lot by December 31, 2026. Reporting on the bill's path through the legislature noted that stronger by-right language in early drafts was narrowed before passage, leaving much of the detail (parking, setbacks, exact size caps) up to each county.
2025Honolulu's Phase 1 response: Ordinance 25-2. Adopted January 3, 2025, with most provisions effective September 30, 2025, it raised size caps to 500/1,000 sq ft by lot size, dropped the owner-occupancy covenant requirement, lowered the minimum lot size that qualifies, and opened a path to a second unit on lots that already have an ʻohana unit or a duplex.
Worth knowing: even with a decade of legal ADUs, reporting has put Honolulu's cumulative ADU permit count at only around 1,300 units citywide since 2015 - a small fraction of what officials hoped for. Commentary from housing researchers points to sewer-capacity limits, high construction costs, and a slow multi-agency permitting process as the practical bottlenecks that persist even as the zoning code itself keeps getting more permissive. With Hawaii's Act 39 deadline landing December 31, 2026, expect another round of Honolulu ordinance changes before this page's next update.
Permit process
How Honolulu ADU permitting works
1
Confirm lot eligibility
Verify your zoning district (R-3.5 through R-20, or Country), lot size, and that no restrictive covenant blocks an ADU.
2
Submit an ʻOhana/ADU Pre-Form
DPP routes this early-stage form to the Board of Water Supply, the Department of Environmental Services (sewer capacity), and the Department of Transportation Services for capacity verification before you invest in full design.
3
Prepare architectural drawings and a site plan
Show the ADU's size, setbacks, parking, and its relationship to the primary dwelling.
4
File the building permit application with DPP
Submit through DPP's ePlans portal or in person at 650 South King Street, along with the required agency sign-offs.
5
Record the ADU covenant
Sign and record a covenant with the Bureau of Conveyances agreeing to the current terms: no short-term rental use, long-term lease only, and no separate sale of the ADU (no CPR).
6
Build, inspect, and get a certificate of occupancy
Complete construction, pass required inspections, and obtain final sign-off before occupying or renting the unit.
Reported review timelines for Honolulu residential permits commonly run several months; confirm DPP's current processing times, as they vary with application volume and completeness.
Budgeting
What it costs to build an ADU in Honolulu
Garage conversion
Roughly $120,000-$220,000
New detached ADU
Roughly $220,000-$480,000 or more for an 800-1,000 sq ft unit, depending on finishes and site conditions
Prefab/modular ADU
Roughly $100,000-$300,000 installed, before site work and utility connections
DPP permit fees
Based on construction valuation; confirm the current fee schedule with DPP
These are general ranges from local contractor estimates and regional reporting, not quotes. Oahu's construction costs run well above the mainland national range in this table's other cities, driven by shipped-in materials, labor costs, and site conditions - so budget toward the higher end and get multiple local bids before committing to a design.
Before you apply
Tips for a smooth approval
Check sewer and water capacity before you design. Capacity limits, not zoning, are the most commonly cited practical bottleneck for Oahu ADUs - the Pre-Form process exists specifically to catch this early.
Don't confuse ADU rules with ohana unit rules. They're separate programs with different size caps, parking rules, and lot-size minimums - make sure your contractor and plans are using the right one.
Plan your rental strategy around the covenant. The recorded covenant bars short-term/vacation rental use of the ADU - if income was part of your plan, budget for long-term tenants only.
Get several local bids before locking in a design. Given how far construction costs run above the mainland, an early budget check can save a costly mid-project redesign.
Watch for another rule change before the end of 2026. Hawaii's Act 39 requires counties to allow two ADUs per lot by December 31, 2026, so Honolulu is expected to update its ordinance again - a project timed after that update may have more options.
FAQ
Common questions
Do I need to live on the property to build an ADU in Honolulu?
No, not under the current covenant terms. Honolulu's 2015 ordinance originally required owner-occupancy, but Ordinance 25-2 (2025) dropped that requirement, consistent with Hawaii's 2023 statewide preemption of local owner-occupancy mandates.
How many ADUs can I build on one lot?
The standard allowance is 1 ADU per lot. As of Ordinance 25-2, lots that already have an ʻohana unit or two principal dwellings can add an ADU as well, provided the lot isn't a Condominium Property Regime. Hawaii's Act 39 requires counties to allow 2 ADUs on every residential lot by December 31, 2026, so this is expected to expand further.
Does Hawaii have a statewide ADU law, or is this purely a Honolulu rule?
Both. Hawaii's Act 232 (2023) and Act 39 (2024) set statewide floors that all four counties must meet - by-right approval, no owner-occupancy mandate, and (by the end of 2026) at least two ADUs per residential lot. Honolulu's own Land Use Ordinance, most recently amended by Ordinance 25-2, supplies the county-level details: exact size caps, parking, and setbacks.
Do I need extra parking for an ADU?
Generally yes: 1 off-street stall (compact size allowed). It's waived within about 800 ft of a bus stop in parts of the Primary Urban Center and Ewa Development Plan areas.
What's the maximum size for an ADU in Honolulu?
500 sq ft on lots under 5,000 sq ft, and 1,000 sq ft on lots of 5,000 sq ft or larger, under Ordinance 25-2. This is separate from the older, larger ohana unit program.
How tall can a Honolulu ADU be, and what are the setbacks?
There's no separate flat ADU height or setback number - both follow the underlying zone's standard dimensional rules (commonly in the roughly 25-30 ft range for height in typical residential zones, though this varies by district). Confirm your specific zone's figures with DPP.
Can I rent my Honolulu ADU out on Airbnb or a short-term rental platform?
No. The recorded ADU covenant requires long-term lease use and specifically bars short-term or vacation rental use of the unit.
How much does an ADU cost to build in Honolulu?
Garage conversions tend to run roughly $120,000-$220,000, and new detached units roughly $220,000-$480,000 or more, based on local contractor estimates and regional reporting - noticeably higher than mainland cities due to Oahu's elevated construction costs.
Why does Honolulu ADU policy matter so much right now?
Oahu has some of the highest housing costs in the country - the single-family median sale price has been running above $1.2 million in 2026, among the highest in the nation after California. ADUs are one of the few tools that can add housing on already-built lots without new land costs, which is why state lawmakers have twice preempted local rules to push counties toward allowing more of them.
