PORTLAND, Ore. (KOIN) — To help build more housing, the City of Portland lifted a nearly century-old rule that allowed only single-family homes to be built in many neighborhoods.

But new research from the Sightline Institute shows that’s unlikely to solve the housing crisis anytime soon.

“We are not going to see a rapid transformation of many lots in Portland,” said Michael Andersen, a senior researcher with the institute.

The first city law to require single-family homes was passed in 1924 and is considered part of Portland’s legacy of racial exclusion. More restrictions followed. Neighborhood property owners banded together to prevent new multi-family homes from being built

By 1959, Portland had zoned 77% of residential land for detached, single-family homes, leaving renters and low-income families with even fewer options.

But that’s all changed.

As of August 1, five new types of housing are legal in all of Portland, including duplex accessory dwelling units (ADUs), triplexes, fourplexes and homes on wheels. A single family home can also be converted into multiple units for rent.

Andersen said that an additional option –- sixplexes -– requires that half the units be affordable, and make even less financial sense at this time. He said they’re unlikely to become more common without additional subsidies to bring down the cost of construction.

The hope is to spur more housing where it’s desperately needed.

But researchers at Sightline Institute ran the numbers and found that even with Portland’s rising rents, most of these options cost too much to build for developers to make a profit.

“It’s expensive to build things,” Andersen told KOIN 6 News. “It costs about $200 per square foot to build each square foot of a new building. In parts of town that are less expensive, it also means the rents are lower. They’re less likely to be able to afford a brand new building.”

Andersen found that with current rents and costs the best options now are to add units to a property rather than demolish and rebuild. That means things like putting new duplex apartments or tiny homes in the backyard of an existing home.