Unconventional wisdom from John Kominicki: OK, not in your backyard. But how about your garage and basement?

Imagine a new kind of housing stock that could accommodate both young professionals and older empty nesters, that was easy to zone, quick and affordable to build and came without appreciable environmental impact.

No height or density variances required, thank you, and forget the usual traffic snarls and grousing by the civics. This is living space even a school board can love.

Planners call them Accessory Dwelling Units, a new name for an old idea that’s getting a fresh look in places where housing costs have outpaced salaries. You might know ADUs as granny flats, mother-in-law units or alley apartments. They go by dozens of other names, too, depending on the region. In the Pacific Northwest, you’d rent a laneway unit. In Amish country, look for a dawdy haus.

Traditionally, they’ve been built in basements and over garages, in backyard cottages and carriage houses, and they were a popular way to earn extra income or put up a friend or family member. And they were once cool: The Fonz, you may remember, lived above the Cunningham’s garage.

Though they fell out of favor in the 1970s, accessory units are being reconsidered in places like Silicon Valley, where even workers making six figures struggle to afford the rent because housing development hasn’t kept up with demand.

(California as a whole needs 3.5 million housing units over the next eight years. It’s on pace to build only a third of that.)

Accessory units are also gaining popularity among homeowners who want to continue living on their property but no longer need or want the space and amenities of the main house. In our region, there are thousands more who would love the extra income to help pay for upkeep and ever-climbing property taxes.

Alas, Long Island’s record on accessory housing is dodgy. It’s embraced in many parts of Suffolk County – Babylon, for one – but gravely limited in Nassau and banned completely in Glen Cove and Long Beach, according to a new study by the Long Island Index.

(Enforcement, of course, is something else again. It’s why we have an estimated 100,000 illegal apartments on the Island.)

And it’s not just us and the Valley. Seattle, Austin, Denver and dozens of other spots have started grappling with housing costs and their clear impact on workforce development. Even Maine. On the accessory apartment front, enlightened municipalities have passed new ADU-friendly zoning rules, reduced lot size requirements and eased parking regulations, creating what some are calling “naturally occurring affordable housing.”

“Existing neighborhoods absorb the rental-seeking population like a sponge, while stabilizing finances for tax-strapped homeowners,” the Index report notes. “And having a resident homeowner usually means they are better maintained than rentals with absentee owners.”

As development projects go, very few do such a perfect job of supporting the workforce, boosting neighborhood diversity and helping home owners make their mortgage payment. One Portland study suggests that accessory units even reduce automobile usage.

Again, this is not a new idea, just one that deserves another look. Counting all the converted garages and basements, carriage houses and pool sheds, we already have thousands of Accessory Dwelling Units on Long Island.

Call ’em what you want. We just need several thousand more