Accessory Dwelling Units (ADUs) are small homes on the same lot as a main house. They can be a backyard cottage, a studio over the garage, or a small one-bedroom for family or a long-term renter. In 2024, the State of Hawaiʻi directed counties to make room for two small homes on residential lots. In response, the City & County of Honolulu adopted Ordinance 25-2 on January 3, 2025, and most changes took effect September 30, 2025. On that date, the Department of Planning & Permitting (DPP) announced the changes and published a dedicated Land Use Ordinance (LUO) update page with links to the new rules.

This article explains the ADU portions of Ordinance 25-2 in clear, simple language so homeowners can understand what’s possible now.

How Honolulu Addressed the State’s “Two ADUs” Direction

Honolulu did not simply allow every lot to build two ADUs. Instead, Phase 1 (Ordinance 25-2) opened a path to achieve two small homes on one lot by allowing one ADU on properties that already have an ʻohana unit or two principal dwellings—as long as the lot is not CPR’d (not a condominium property regime).

• ADUs are not allowed on CPR lots.
• A CPR cannot be created on a zoning lot that contains an ADU.

In short: if your property already has more than one dwelling (for example, a main house and an ʻohana unit), you may now be able to add one ADU—provided you qualify and the lot is not CPR’d.

ADU Size Limits (Now Simpler and More Generous)

Ordinance 25-2 simplified the size table and increased what many lots can build:

• Lots from 0 to 4,999 sq ft: ADU up to 500 sq ft
• Lots 5,000 sq ft and larger: ADU up to 1,000 sq ft

Previously, smaller lots often could not build an ADU at all. Now, even sub-3,500 sq ft lots may qualify for a 500 sq ft ADU if other rules are met.

ADUs on Lots With Multiple Dwellings

• Allowed: An ADU may be added on a property with multiple dwellings (for example, a main house + ʻohana, or two principal dwellings).
• Still prohibited: No ADUs on CPR lots, and you cannot CPR a lot that contains an ADU.

This is the key mechanism Honolulu used to respond to the state law while avoiding CPR + ADU combinations.

Bathroom Limits (Application Remains the Same)

The maximum number of bathrooms per dwelling did not change, and DPP continues to apply bathroom limits the same way as before.

• Example: On a 7,000 sq ft lot with three dwellings (main, ʻohana, and ADU), each dwelling may have up to three (3) bathrooms.
• If the main house and ʻohana have a nonconforming number of bathrooms, the new ADU can still have up to three.
• If all existing dwellings are torn down, then all new dwellings must meet the current maximum bathroom rule.

Parking (One Stall, With Transit Flexibility)

• An ADU requires one (1) off-street parking stall, and the stall may be compact.
• If your lot is in the Primary Urban Center or Ewa Development Plan area, the off-street ADU stall is waived when the property is within 800 feet of a bus stop, provided no parking adjustment has been taken for an ʻohana unit.
• Regardless of bus-stop distance, you may apply for a zoning adjustment to address ADU parking if needed.

Plan parking early. It is often the detail that decides whether the site layout works.

What “Qualify” Means (Pre-Form Agency Sign-Off)

Under Ordinance 25-2, Qualify means the owner submits an ʻOhana/ADU Pre-Form and obtains early sign-offs from the public agencies that handle basic services and safety. The Pre-Form routes to:

• Board of Water Supply (water service capacity)
• Department of Environmental Services – Wastewater Division or the State Department of Health (onsite systems), depending on the situation
• Department of Transportation Services (traffic/driveway, where applicable)
• Honolulu Fire Department (fire access and related needs)

If these agencies confirm capacity and access, you can move forward into design and permitting with fewer surprises. If they flag issues, you can adjust the plan before paying for full drawings.

What Still Applies

• All zoning and building standards continue to apply: setbacks, height, lot coverage/FAR (where applicable), access, and life-safety.
• Utility capacity (water and wastewater, or an approved onsite system) must be verified.
• You must complete normal permit review. The Pre-Form is an early check; it does not replace the permit process.

Simple Scenarios

1) Main House + ʻOhana (not CPR):
You may be able to add an ADU if you qualify. ADU size depends on lot area (500 sq ft under 5,000 sq ft; 1,000 sq ft at 5,000 sq ft and up). Plan one parking stall unless waived by transit proximity rules and ʻohana adjustments.

2) Two Principal Dwellings on One Zoning Lot (not CPR):
You may be able to add an ADU if you qualify and meet all standards.

3) CPR Lot:
No ADU is allowed, and you cannot create a CPR on a lot that includes an ADU.

4) Small Lot Under 3,500 sq ft:
You may now pursue a 500 sq ft ADU if you qualify. This is a major change for many older neighborhoods.

Step-By-Step: How to Start

1. Confirm your lot status
   Check whether your lot is not CPR’d and list the current dwellings on site. Note your lot area to know the ADU size you can target.
2. Submit the ʻOhana/ADU Pre-Form
   Have your architect route the Pre-Form to the Board of Water Supply, Wastewater Division or Department of Health, DTS (as applicable), and HFD for early capacity and access approvals.
3. Right-size the design
   • 0–4,999 sq ft lot → up to 500 sq ft ADU
   • ≥5,000 sq ft lot → up to 1,000 sq ft ADU
     Keep bathroom limits in mind (typically up to 3 bathrooms per dwelling in the example above).
4. Solve parking
   Provide one stall (compact allowed) or document a transit-based waiver if you’re in the Primary Urban Center or Ewa and within 800 ft of a bus stop with no prior ʻohana parking adjustment. If needed, apply for a zoning adjustment.
5. Complete drawings and apply for permits
   After Pre-Form approvals, finish construction documents that meet code and submit to DPP for permit review.

Bottom Line

Ordinance 25-2 (effective September 30, 2025) is Phase 1 of Honolulu’s LUO updates. It makes ADUs possible on more lots, especially where an ʻohana unit or two principal dwellings already exist and the lot is not CPR’d. The size table is simpler (500 sq ft or 1,000 sq ft), bathroom rules are unchanged in how they’re applied, and parking is more flexible near transit. The new Pre-Form step helps confirm water, wastewater/health, traffic (as needed), and fire access before you invest in full drawings.

Thinking about an ADU on your property? We can help you check your lot, complete the Pre-Form, and design an ADU that meets the new rules.

Ready to see costs for your property? Get a fast, free instant estimate now at https://www.architecthonolulu.com/instant-estimate.