When the proposal to allow "accessory dwelling units" finally becomes reality this week, nobody is sure just how much construction activity to expect. But city permitting offices, tipped off by phone inquiries of the level of interest, are preparing to be busy.

And in the private sector, there’s some buzz about it, too. For Honolulu architect Philip "Pip" White, inquiries about ADUs have become a common element in the half-dozen or so residential projects going on at any given time.

"All have talked about how they would like an addition," White said. "I don’t know that it’s pent-up demand, because people have been doing this for a long time."

And in addition to the preparatory work by architects expecting requests for ADU blueprints, the lending industry is gearing up for new loan packages, too.

Whether they’re new add-ons to single-family homes around Oahu or illegal units whose owners now can legitimize their add-on, the potential of Oahu to add residential density — and, city officials hope, reduce the shortage of affordable rentals — is about to increase. They’re also aware of public concerns that the units don’t morph into illegal vacation rentals, or otherwise overtax neighborhood utilities and roads.

Mayor Kirk Caldwell plans on Monday to sign Bill 20, legislation the City Council passed to enable ADU construction and regulation.

"This is part of a very much bigger picture," Caldwell said in an interview last week. The other two legs of the affordable-housing "stool" the mayor described are transit-oriented development (TOD) around the rail alignment, in zones that will incentivize affordable housing projects; and regulations to drive more affordable housing projects elsewhere on Oahu.

All of that is down the road a bit — the regulatory piece is still in draft form and will be submitted for Council approval this fall, he said.

So right now the goal is for some progress chipping away at the affordable rental deficit through scattered activity building ADUs. There are some strict rules aimed at mitigating the impact of greater density (see the outline of the law, Page E4).

But it’s essentially being powered by a private-sector demand for the units, he said.

"We believe it is a game-changer," Caldwell said. "We don’t need to spend any taxpayer money. We just need to free up the market to allow a second dwelling or an add-on, as long as it’s not more than 50 percent of the lot."

The current understanding is that Oahu needs 24,000 affordable units built to meet current needs, said Harrison Rue, the city’s TOD administrator.

What part of that ADUs can fill is just an estimate, but the city administration is looking to three decades of experience with the "ohana unit" law that the ADU ordinance — which eliminates the ohana-unit requirement that the rentals be for family members — will now replace.

"I’ve been told that in the heyday, about one-fourth of all the building permits being pulled were for ohana," Caldwell said. But that was before the family-relation rule was adopted; thereafter, the construction of the add-on units cooled.

There were capacity issues, too, said Questor Lau, an architect who last year earned his doctorate through a dissertation that explored the ADU issue. The study is titled, "Black Boxes and Gray Spaces: How Illegal Accessory Dwellings Find Regulatory Loopholes."

When contacted last week about what can be expected of the pending ADU ordinance, Lau cited a 1984 city publication evaluating the ohana-unit program.

"History might give us an idea of what to expect from ADUs," he said. "The report states that in that first year, 2,000 ohana units were approved but only 600 built. One of the main constraints was — and, I believe, still is — sewer capacity."

Lau’s thesis used a calculation based on an examination of building permits issued for structures that were "suspicious" of being converted, or convertible, to a second dwelling. Among the criteria he looked for were separate entries and provisions for cooking.

Based on such design elements, Lau counted 5,683 units as potentially illegal ADUs on Oahu. Many of them are structurally legal as family rooms or guest rooms, but illegal because of their use: They were rented out as a second dwelling to a tenant or in a location that did not qualify for an ohana unit.

Lau is not opposed to ADUs; his paper points out the degree to which residents have found ways of building them, which suggests a high demand. The illegal units, he wrote, "highlight an interesting contradiction in housing policy — while they are officially unwanted, they are desperately needed."

The city plans to give prospective applicants a checklist, screening their site for sewer, water, roadway, fire-safety and other criteria at the beginning of the process before they get too far into investments, said George Atta, director of the city Department of Planning and Permitting.

"All of that is a requirement of building permits right now," Atta said. "If you wanted to do any kind of renovation or addition to an existing house, you have to go through sewer and water hookup fees. If an area does not have sewer capacity, the ADU will have to go through that checklist, like anything else. And if there is no capacity, we won’t issue them a permit."

On the issue of preventing the units from drifting into illegal vacation-rental use, the bill incorporates a long-discussed enforcement strategy: making advertisements of the unit for short-term rentals a violation, said Art Challacombe, DPP deputy director. An inspector could then go on-site to check that the occupant is the person on the lease, which has a minimum term of six months.

There are safety and code requirements for fire walls, windows that can provide an emergency exit and other elements, but aesthetic aspects of design aren’t legislated in this bill or elsewhere in the building regulatory framework, he said. The presumption is that market competition for design aspects keep appearances within bounds as well as any rule.

Some design aspects are dictated by Hawaii climate and culture, said architect White. Weather considerations make sun-blocking awning and roof designs a plus, as well as provisions for cross-ventilation, he said.

White and Lau agree that allowing a legal framework for existing and new ADUs will be a plus for Honolulu. But neither is sure there’s going to be a mass rush to lock in plans now: It may be seen by many as an attractive option for the future, when children are grown, there is an aging elder needing care — or a need for rental income.

Lau’s study showed that house additions, potentially for rental use, peaked during the recession, in 2008. People do need that kind of push to want to have strangers renting an extension of their own home, he said.

"People don’t do it out of choice," he said. "The fact that people are doing it, it’s more of a need. It’s a pretty big decision."