The popular tiny-house movement may get a big boost in the District this fall when significantly looser restrictions regarding “accessory dwelling units” — the technical name for the trendy residences — go into effect.

Under provisions in the city’s new zoning code, from Sept. 6, homeowners will have an easier time getting the necessary approval to build and rent out tiny houses. Given the lack of affordable housing in the District, advocates say they think they’ll see a dramatic increase in the number of rentable tiny homes, micro-homes or two-story carriage houses popping up alongside gardens, tree houses and swing sets behind homes all over the city.

Previously, owners seeking to rent out the units were required to argue their cases before the Board of Zoning Adjustment (BZA) to receive an exception. Under the new rule, the structures will be permitted in some neighborhoods as a matter of right: Once homeowners acquire building permits from the Department of Consumer and Regulatory Affairs (DCRA), they will be free to build and then rent out the units.

The changes are not applicable citywide: They are largely limited to R-1, R-2 and R-3 zones, which cover outer neighborhoods such as Brookland, Chevy Chase and American University Park, where there are larger single-family homes.

Other regulations restrict size and alley access: The tiny houses cannot be more than 35 percent of the gross floor area of the primary home, which must be at minimum 1,200 square feet in most zones and 2,000 square feet in R-1 zones.They must be adjacent to either a 24-foot-wide alley, or a 15-foot-wide alley and at most 300 feet from a main road for fire safety. And the principal dwelling must be owner-occupied.

Still, advocates say they think that the rules are not overly restrictive and that many homeowners will jump at the opportunity to create rental units.

“We are really, really pleased,” said Cheryl Cort, policy director at the Coalition for Smarter Growth. “It’ll lead to the diversification of neighborhoods that are largely single-family homes. This provides an opportunity to provide rental housing and smaller-sized homes. Maybe a single mom and a kid would like to live in a neighborhood where they can walk to school, or empty-nesters could rent a home to young people who could mow the lawn and keep an eye on things.”

Between the height restriction and other physical limitations, the District has been struggling to add housing stock to accommodate a population that is growing by about 1,000 residents per month, with average home prices and rents rising steadily. In addition to accommodating in-laws, an au pair or a grown child, a backyard home could help to broaden rental options in the city.

“The changes open the door for D.C. to become more like Portland, [Ore.],” said Brian Levy, owner of a “micro-home” and a longtime lobbyist for less-restrictive zoning rules for accessory dwellings.

Portland and Seattle have some of the most liberal rules regarding detached accessory dwellings. Companies such as Hammer & Hand and Backyard Box have sprung up to capitalize on the consumer demand for ready-designed backyard accessory dwellings there.

In the District, some architects are preparing for a possible hot market here.

Michael Cross, founder of R. Michael Cross Design Group, says he hopes his team will be among the first local firms to prepare a D.C.-specific product. His team has been analyzing the zoning and building codes and the city’s housing stock, and plans to have a set of prototypes ready when potential clients start inquiring in the fall.

“What we’re trying to make is super functional and, hopefully, affordable,” Cross said. The prototypes can facilitate the affordability, he said, because clients will not have to pay for a custom design.

“We’ve seen these little tiny houses and little jewel boxes that get thrown up on blogs, but they are astronomically expensive compared to their tract-home peer,” Cross said. “The goal here is to be cost-effective by having a product that is low-frills and is likely not even all that sophisticated in the way that it’s manufactured.”

In looking at the details of the code, such as setback requirements, alley widths and average lot sizes, Cross found that his company could build cottages of 450 to 550 square feet, comparable to small apartments around the city.

Early prototypes include a two-bedroom cottage with a loft; a two-car garage with a one-bedroom apartment on top; and a one-car garage with a studio apartment on top.

For potential landlords, the costs are something to consider. Preparing a tiny house to rent could be more expensive than, say, renovating a basement into a legal apartment. A foundation has to be laid. The unit has to be hooked up to city utilities. And the unit has to be built or purchased.

Cross said he isn’t ready to put a price on the prototypes. But a 2013 survey of 200 owners of accessory dwellings conducted by Oregon’s Department of Environmental Quality put the average cost of a tiny house in Portland at $78,000, with many hovering around $100,000 and a few reaching as high as $300,000.

In the District, it remains to be seen what the typical rental income on such a property will be.

Emily Bacher, an architect at Cross’s company who has taken the lead on accessory dwellings, says the designs will maximize privacy between primary home and tiny home; windows will be arranged to avoid direct views into the main house; and some kind of landscape block may be designed.

Although the code has become less restrictive and homeowners operating by matter-of-right don’t need community approval, Bacher says she’s still concerned that there will be a neighborhood backlash when construction of these dwellings begins.

“I’m a little surprised that they passed it,” Bacher said. “I wonder if it’s something people didn’t pay attention to, and then they’ll say, ‘What have we done?’”

Cort, however, says that naysayers were active during the code-revision process and that many of their concerns have been taken into account.

In the code, said Cort, “the secondary doorway has to be discreet, and you can’t have a balcony or a roof deck. The zoning regulations have been designed to address some of the concerns that were raised. I don’t think there will be a backlash.”

Cross and Bacher also say they hope that the new backyard structures will lead to a more vibrant alley life, adding more “eyes on the street,” Bacher said.

“One alley [accessory dwelling] on a block may be seen as a detriment to the community,” Cross said, “but if you have a couple of them, all of a sudden the alleys are activated and clean and thriving with activity.”

The city’s current carriage houses in alleys — many of which were built about 100 years ago and were grandfathered into the last zoning code — are often hot properties, and the changes could lead to a modern addition to alley life.

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