Kahaunani Mahoe- Thoene was so eager to put up an accessory dwelling unit on her Waimanalo lot that she filled out all the information for her building permit even before Mayor Kirk Caldwell signed Bill 20 on Monday morning.

As a result, Mahoe- Thoene will likely be the first person to use the new law to put up an ADU. The goal is to create a second dwelling for her dad, who now lives in a trailer on the site, she said.

An ADU will allow him to live in more comfortable digs, without the higher costs needed to subdivide her property and build an entirely new single-family dwelling for him, she said. “I would like to keep my dad home with me,” she said. “We always forget about our hidden homeless.”

Mahoe-Thoene estimated the improvements will cost $40,000-$60,000 and that it would take about seven to eight years to recoup that cost if she were to rent it out for $600-$700 a month.

Caldwell and other ADU supporters are hoping Mahoe-Thoene will be just the first of thousands of homeowners who will take advantage of the new law to provide affordable housing in a tight rental market while, at the same time, perhaps earn a little rent money to help pay their mortgages.

Up to 20,000 homeowners could be eligible to put up ADUs, the city estimates, but officials are hoping at least 5,000 units are actually created.

Homeowners with residential and agricultural lots are now able to put up ADUs if they meet certain criteria. An ADU is a second smaller housing unit, either attached or detached, that can be rented out to either family members or outside parties.

An ADU must have a separate entry, kitchen and bathroom from the main dwelling. To qualify, a property must have at least 3,500 square feet, as well as adequate water and sewer facilities. It must also have at least one designated parking stall, a requirement that is waived if the property is within a half-mile of a rail station. The ADU can have a maximum floor area of 400 square feet if the lot is between 3,500 and 5,000 square feet, or up to 800 square feet if the lot is larger than 5,000 square feet. The homeowner must live in one of the two units.

At a news conference Monday, Caldwell applauded Council members for improving the ADU bill by adding provisions aimed at discouraging property owners from using the new tool to create illegal bed-and-breakfast establishments or transient vacation units. The new law requires an ADU to be rented to the same 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 evidence that the unit is being used illegally.

A $300,000 increase in the Department of Planning and Permitting’s budget approved by the Council will allow the agency to hire between three and five more inspectors to guard against abuse of the ADU program, city officials said.

ADUs differ from permitted ohana units, which will also be allowed to continue, in that ohana units can house only family members, have no size limitations and must have two on-site parking spaces.

Councilman Ron Menor, an early proponent of ADUs, predicted that the ADU legislation “may eventually be viewed as landmark legislation” if it produces a significant number of new housing units.

Council Zoning Chairman Ikaika Anderson said that in his own district, families as large as 12 live under the same roof because family members can’t afford to live on their own “sometimes sharing three bedrooms and two bathrooms.”

Both Menor and Anderson said they’ve received numerous inquiries from constituents about when they can start developing ADUs.

Because Council members did not oppose ADUs, the Department of Planning and Permitting, which is administering the program, already had prepared informational materials and an ADU “pre-check form” to help prospective applicants through the process. They can be found at the department’s Building Permit Center and online at www.honoluludpp.org.

Affordable housing advocates said they, too, have already been preparing for the ADU bill to become law by figuring out ways they can help homeowners with financial advice, such as where to get financing or how to make the housing plan work, or other assistance.

Kali Watson of the Hawaiian Community Development Board said a majority of the homeless in Hawaii are Native Hawaiian. As a result, Watson said, his organization is working with the Department of Hawaiian Home Lands and other agencies in hopes of tapping into the inventory of roughly 4,000 homestead units on Oahu to find ADU opportunities.

Many homesteaders are eager to participate to help provide housing for relatives and others in the Native Hawaiian community, said Watson, a former DHHL chairman.

Betty Lou Larson of the homeless advocacy coalition Partners In Care said financial hardship is the only thing preventing about 1 in 4 homeless from finding housing. Boosting the supply of lower-priced rentals would help them, she said.

The ADU concept is one part of the Caldwell administration’s three-pronged approach to tackling Oahu’s affordable housing dilemma. In the next month or two, Caldwell said, he will unveil a proposal to alter affordable housing formulas — which require developers to set aside a percentage of their homes for below-market prices — to provide incentives to homebuilders for putting up more rentals and lower-priced sales units.