If you’ve got room for an extra house on your Chicago lot or an apartment in your basement, Mayor Lori Lightfoot believes you should be able to add it. Her administration is moving on this issue to increase the supply of affordable housing and to help long-term homeowners stay put if they need rental income.

To Lightfoot’s housing commissioner, Marisa Novara, those are two positive and incremental changes that shouldn’t detract from neighborhood character. “We can gently increase the density in a community without changing the fabric of its built environment greatly,” she said.

This is about legalizing new coach houses and in-law apartments — “granny flats,” to some. Their technical name is accessory dwelling units, or ADUs, although the Department of Housing has adopted the name Additional Dwelling Units, seeing it as more straightforward.

That’s the name used in a proposed ordinance Lightfoot is expected to introduce to the City Council soon. It would remove the ban on ADUs that has been in place since Chicago’s 1957 rewrite of its zoning rules. The coach houses or basement and attic apartments that do exist were either built on the sly or predated 1957 and thus were grandfathered as legal.

“In our view, it’s long overdue,” Novara said of the ordinance. “Coach houses never should have been banned in 1957. This is a pretty archaic way of thinking about how we create units for people with a range of incomes across the city.”

She emphasized the ordinance enables ADUs but doesn’t force them on blocks where they aren’t wanted. “Where this makes sense for people, they can make use of it, and where it doesn’t, they won’t. But at least we are allowing that option,” she said.

The ordinance would include rules to limit abuses many people associate with ADUs. Coach houses — structures separate from the main house — would be limited to 700 square feet. They could not be used for short-term rentals such as Airbnb. And it would require backyards be preserved in accordance with neighborhood standards. City building standards and permit rules still apply.

It would allow owners of larger apartment buildings to add basement units so long as half of those new apartments are affordable under city ordinance. Owners of two- to four-flats would not be covered by that rule.

“Legalizing additional dwelling units marks our latest step in expanding housing access across Chicago’s communities,” Lightfoot said in a statement issued by the Department of Housing. “This innovative measure simultaneously creates thousands of new rental units for our families and residents, while also opening new income streams for building owners —particularly seniors, and strengthens housing safety by allowing existing units operating in the shadows to become a part of Chicago’s mainstream.”

Novara said the ordinance will get a joint hearing by the housing and zoning committees. Ald. Harry Osterman (48th), chairman of the housing committee, is a sponsor and others may be added. Asked about other aldermanic sentiment, Novara said, “We’ll see. We’re in ongoing communications with folks about how this might play out in the neighborhoods they represent.”

The assumption is coach houses or apartments squeezed into properties will rent for less than the neighborhood’s prevailing price, making the housing stock more diverse. Novara said the change would help older residents “age in place” if they want help paying for higher property taxes and other needs. “People that are longtime homeowners and that are the stalwarts of their community — we want them to stay for as long as possible,” she said.

Nobody’s saying this a panacea for the housing crisis. But it incentivizes the creation of cheaper housing the city does not have to subsidize. Daniel Kay Hertz, director of policy at the Department of Housing, said a similar change in Los Angeles yielded 7,000 new units over six years, with most coming later in that period. “One thing unique about Chicago is our housing stock has all of these basements that are suitable for apartments,” Hertz said.

The ordinance contains no requirement for parking to serve the new units. Some neighborhood groups may complain they don’t have enough street parking as it is, but Hertz said occupants of ADUs are more likely to be students or seniors without a car.

The measure was drafted with help from groups such as the Urban Land Institute Chicago, an association of professionals in real estate and other fields.

Around Chicago, there are thousands of ADUs. One estimate by the Chicago Cityscape blog using zoning data and surveys of built structures said there are about 2,400 coach houses in the city.

How that might change is anyone’s guess. But it is interesting the mayor is engaging debate on a measure whose effects won’t be known for years.