Everything homeowners ask about building an ADU in Honolulu — answered with 2026 data.
Yes, ADUs are allowed in the Country (C) district and every residential district (R-3.5, R-5, R-7.5, R-10 and R-20) as an accessory use with standards ("Ac*" in Table 21-5.1); not permitted in the AG-1 and AG-2 agricultural districts, the apartment (A-1 to A-3) or mixed-use districts, or on landlocked lots or lots in planned development housing or cluster housing (ROH § 21-5.50-3(a)(1)(A)) in Honolulu, HI. The maximum size for a detached ADU is 1,000 sq ft of floor area on zoning lots of 5,000 sq ft or more; 500 sq ft on lots up to 4,999 sq ft (ROH § 21-5.50-3(a)(1)(B), Ordinance 25-2, effective September 30, 2025). A legally built accessory structure that predates September 14, 2015 may be converted and exceed the cap by zoning adjustment if the director finds the floor area cannot practicably be reduced (§ 21-2.140-1(n)). Attached ADUs can be up to Same 1,000 sq ft / 500 sq ft caps by lot size. An ADU may be new construction or a conversion, attached. Honolulu has an ADU-friendliness score of 7/10. Check our full Honolulu ADU rules page for complete zoning details.
ADU construction in Honolulu typically costs $150,000-$450,000. The cost per square foot averages $250-$550. Garage conversions are more affordable at $150,000-$225,000, while prefab ADUs cost $100,000-$240,000. Permit fees add $0 for the ADU's plan review and building permit: ROH § 18-6.5 waives both fees for the creation of an accessory dwelling unit, extended through June 30, 2030 by Ordinance 25-12 (no waiver if work started without a permit). For scale, ROH 18-6.2 would otherwise charge $700 plus $10 per $1,000 of value above $100,000 (about $2,200 on a $250,000 job) plus a 20 percent plan-review fee. Grading, grubbing and stockpiling permit fees processed with an ADU permit are waived too (§ 18A-2.4). A zoning adjustment, if needed, costs $200 initial review plus $400 processing (DPP, November 2025) to your budget. See our detailed Honolulu ADU cost guide.
Detached ADUs in Honolulu can be up to 1,000 sq ft of floor area on zoning lots of 5,000 sq ft or more; 500 sq ft on lots up to 4,999 sq ft. Attached ADUs are limited to Same 1,000 sq ft / 500 sq ft caps by lot size. An ADU may be new construction or a conversion, attached. Junior ADUs (JADUs) have a cap of No junior-ADU category in Honolulu. The other accessory unit is the ohana unit (§ 21-5.50-3(c)): no floor-area cap since Ordinance 06-15, but its occupants must be related by blood, adoption, guardianship or marriage to the family in the main house, and its parking is the same as the main dwelling's. A lot may have both an ohana unit and one ADU. Maximum height is 25-30 ft building envelope in the residential districts: a plane 25 ft above the high point of the buildable-area boundary, or 30 ft parallel to grade where the two do not meet; any part of a structure above 15 ft must step back 1 ft for every 2 ft of extra height from the side and rear buildable-area lines, and above 20 ft from the front (Table 21-3.2, § 21-3.70-1(c)(1)-(2)). Country district: 15-30 ft (Table 21-3.1). You are allowed up to One ADU per zoning lot, including lots that already hold two principal dwellings; an ohana unit does not count toward the one-ADU limit, so a lot can carry a main house, an ohana unit and an ADU if every development standard is met on a qualifying lot.
Rear setback: 5 ft in R-3.5 through R-20 for a single-unit, two-unit or duplex dwelling and the ADU that shares its lot (Table 21-3.2); 8 ft in R-3.5 and 11 ft in R-5 to R-20 when the lot's floor area ratio exceeds 0.6 (§ 21-3.70-1(c)(3)(H)); 10 ft in the Country district (Table 21-3.1). Side setback: 5 ft in all residential districts (Table 21-3.2), rising to 8 ft in R-3.5 and 11 ft in R-5 to R-20 when floor area ratio exceeds 0.6; 10 ft in the Country district. Eaves and similar projections may extend 30 in. into a yard of 10 ft or less; lanais, balconies and covered walkways may not (§ 21-4.30(b)). Front setback: 10 ft in every residential district for a dwelling and its ADU (Table 21-3.2), with any part of the structure above 20 ft stepped back a further 1 ft per 2 ft of height; 15 ft in the Country district (Table 21-3.1). No parking may sit in a required yard (§ 21-4.30(d)). Note that garage conversions may be exempt from setback requirements if the existing structure is already within the setback area.
One off-street stall for the ADU, in addition to the main house's required parking; it may be a compact stall of 7 ft 6 in by 16 ft (§ 21-6.30(b), § 21-6.50(a)(2)-(3)). No ADU stall is required within 0.5 mile of a rail station, and the stall is waived on lots in the Primary Urban Center or ʻEwa development-plan areas within 800 ft of a city bus stop unless an ohana unit on the same lot already used that waiver. A pre-2015 accessory structure being converted can also seek a parking waiver by zoning adjustment (§ 21-2.140-1(n)). Many cities and states have been relaxing ADU parking requirements in recent years. In California, no additional parking is required for ADUs near transit.
Dropped from the code in 2025, but confirm the covenant form before recording: Ordinance 25-2's minimum covenant terms (§ 21-5.50-3(a)(2)(A)) require only long-term residential occupancy, no bed-and-breakfast or transient vacation use, no condominium (CPR) separation of the ADU, and lapse on removal; the 2015 ordinance's owner-occupancy clause is gone. DPP's ADU Declaration of Restrictive Covenants still posted on DocuShare is the 2017 version and contains an owner, family or designated-representative occupancy clause and a six-month minimum term, so ask DPP which form applies. Note that owner-occupancy rules vary by state law — California and several other states have eliminated this requirement for standard ADUs.
The ADU permit review timeline in Honolulu is typically 5-11 weeks for DPP's decision on a residential permit in 2026 (DPP reports 32.5 days on average for applications pre-checked with CivCheck and 73 days without; no ADU-specific target is published). From July 1, 2026, state Act 295 (2025) lets an applicant whose complete single-family application has waited 60 business days without full approval request an expedited permit backed by the licensed professional's certification and insurance. A zoning adjustment (parking or floor-area relief) is decided within 45 calendar days of acceptance or is deemed approved. Plan check: Prescreen for plan-format completeness, then routing to every reviewing agency with comments returned through ProjectDox; DPP's One Time Review (OTR-60) checklist applies to residential work. The plan-review fee (normally 20 percent of the permit fee, ROH 18-6.1) is waived for an ADU. Pre-checking the set in CivCheck cut corrections per permit from 23.5 to 7.7 on average (DPP). Pre-approved plans availability: No pre-approved plans. DPP has no pre-approved or stock ADU plan program (checked September 2026); every ADU needs its own plan set, stamped by a Hawaii-licensed architect or engineer once the work exceeds $40,000 for a one-story or $35,000 for a two-story structure.
Average ADU rental income in Honolulu: 1-bedroom units rent for $2,000-$2,800/month, 2-bedroom units for $2,600-$3,900/month. Short-term rentals: No. The recorded ADU covenant limits the unit to long-term residential occupancy and bars bed-and-breakfast or transient vacation use (§ 21-5.50-3(a)(2)(A)(i)). Expected ROI: Roughly 7-13% gross rental yield: a 500 sq ft one-bedroom built for $150,000-$250,000 and rented at $2,000-$2,800/mo returns about 10-16% gross, an 800-1,000 sq ft two-bedroom at $350,000-$450,000 and $2,600-$3,900/mo ab.
The minimum lot size for an ADU in Honolulu is No minimum lot size since September 30, 2025: any residential or Country lot, including those under the old 3,500 sq ft floor, qualifies for a 500 sq ft ADU, and lots of 5,000 sq ft or more for 1,000 sq ft (§ 21-5.50-3(a)(1)(B)). The lot must still have adequate sewer or an approved septic system, water, fire access and first-responder capacity (§ 21-5.50-3(a)(1)(G)), and DPP's Pre-Check form asks for direct access to a street with a 20-ft paved roadway. This applies to detached ADUs — garage conversions and JADUs may have different or no lot size requirements.
Garage conversions are one of the most popular and affordable ADU options. In Honolulu, a garage conversion typically costs $150,000-$225,000. The key advantages are lower cost (no foundation work), potential setback exemptions, and faster permitting. Read our complete garage conversion guide.
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