It’s about to get a lot easier to build backyard cottages and mother-in-law apartments in Seattle after the City Council voted unanimously on Monday to ease restrictions on those types of units inside the city. The city estimates the law will bring in more than 4,400 backyard units over the next 10 years, an increase advocates hope will lower Seattle's housing costs.

Council Member Mike O’Brien framed the legislation as a “modest” step towards increasing density in Seattle’s restrictive single-family zoning.

“I believe this will be an opportunity to invite so many people into some of our most exclusive neighborhoods,” O’Brien said Monday.

The vast majority of Seattle’s land is zoned for single-family use—which forbids any structures other than detached single-family homes—yet these areas are actually dropping in population as the city grows overall. That has worsened Seattle's housing crisis, turned many neighborhoods into wealthy enclaves, and reinforced racist historic housing policies in the city, according to a recent report from the Seattle Planning Commission.

O’Brien said the legislation would hopefully increase the number of people living in those neighborhoods.

“I think this is an opportunity to stem that trend, to allow additional units, to allow smaller units that will be more affordable…” O’Brien said. “While it is relatively modest, modest both in terms that I don’t think this is going to solve our housing crisis nor do I think it is radically going to transform our neighborhoods.”

Detached accessory dwelling units (DADUs) and attached dwelling units (ADUs) are already legal in Seattle, but Monday’s bill makes it easier to build those units in a number of key ways:

• Allows two ADUs per unit instead of the current one ADU maximum,
• Removes off-street parking requirement and owner-occupancy requirements,
• Increases the height allowances on DADUs,
• Allows for larger DADUs and ADUs,
• Implements a “McMansion” ban that would limit the size of new single family homes.

The ordinance attracted criticism from groups worried that investment companies would exploit the law to speculate on single family homes and then flip them, as well as concern that the new units could be used overwhelmingly for short-term rentals like Airbnb. O’Brien successfully added an amendment that says the council “intends to impose additional restrictions or a prohibition of short-term rental use in accessory dwelling units,” if the city’s data shows significant amount of short-term rentals in the new units.

The so-called “McMansion” ban will require a new maximum floor area ratio (FAR) of 0.5, meaning a new house on a 6,000 square foot lot could only be a maximum of 3,000 square feet (not including any accessory units). The Seattle Times reported that nearly half of the new houses built in Seattle since 2010 would be blocked by this McMansion ban.

The law is expected to reduce the number of houses that are torn down and increase the number of units in the city. Without the legislation, the city should expect to see 1,970 accessory units and tear down 2,030 houses in single family zones, according to city estimates. The new law is expected to add 4,430 accessory units while seeing 1,580 houses get torn down, nearly 500 fewer than if the new law was not passed.

O'Brien's legislation comes amidst a nationwide conversation over single family zoning in the country's expensive cities. Last year, Minneapolis legalized duplexes and triplexes in all single family zones. Last week, Oregon legalized multi-family units in all single family zones in cities with a population of over 10,000 people.