Homeowners meeting certain qualifications can now add a rental unit to their homes under a bill signed into law by Mayor Kirk Caldwell Monday. 

Caldwell is hoping that accessory dwelling units, or ADUs, will help create more affordable rentals in a housing-starved Oahu market. The City Council voted unanimously to approve Bill 20 on Sept. 2. 

To be eligible for an ADU permit, a residential or agricultural lot must be at least 3,500 square feet. and have adequate water and sewer facilities. The second unit can be either attached or detached from a main dwelling. 

It must have a separate entryway, kitchen and bathroom from the main dwelling. It must have at least one off-street parking stall, but that requirement is waived if the property is within half a mile of a rail station. The ADU can have a floor area of no more than 400 square feet if the lot is between 3,500 and 5,000 square feet. The new addition can be up to 800 square feet if the lot is more than 5,000 square feet. 

To address concerns that the new law might create illegal bed-and-breakfast establishments or transient vacation units, the new law requires an ADU to be rented to one party for a minimum of six months. A second clause says an advertisement for an ADU used as either a B&B or TVU can be used as prima facie evidence that the unit is being used illegally. 

The homeowner must live in one of the two units. 

Applications and information are available at the city Department of Planning and Permitting  office, as well as on the DPP website.

Caldwell and other proponents of ADUs believe the concept presents the quickest way to create more affordable housing rentals and envisions up to 20,000 new rentals entering the market.