D.C. Mayor Muriel Bowser said Thursday that she is implementing a decade-old law that allows the city to buy residential buildings that are put up for sale in order to ensure that they remain affordable for tenants.

The District Opportunity to Purchase Act was approved by the D.C. Council in late 2008, but regulations were never written to actually implement it. Much like the existing Tenant Opportunity to Purchase Act (TOPA), DOPA was meant as a last resort to prevent residential buildings from being sold and converted into pricier apartments or condos. TOPA allows tenants the first shot at buying their building when it hits the market, and DOPA gives the city itself a chance to purchase if the tenants cannot.

An affordable housing taskforce created by Bowser recommended in late 2016 that the city finally implement the law, calling it an important tool in helping prevent the displacement of low-income tenants when their residential buildings are put on the market. In the years since DOPA became law, dozens of buildings could have qualified for DOPA — from 2013 to 2016, city records indicate that there were 121 notices filed by building owners looking to sell, with the largest number in Wards 1 and 4.

“DOPA is a crucial affordable housing preservation tool, but without regulations to make it enforceable, the law had no teeth,” said Bowser in a statement. “We will finally be able to use DOPA to preserve more affordable housing units for Washingtonians, and will be aggressively identifying properties beginning in the new year.”

Under the rules written by the Bowser administration, D.C. will be able to invoke DOPA on any residential building of more than five units as long as at least a quarter of the existing units are deemed affordable. “Affordable” is defined as rent that doesn’t exceed 30 percent of the income of a tenant making half the area median income, which is $110,000 a year for a family of four. (For a single person making $38,000 a year, “affordable” rent would be about $1,000 a month). It also includes rental units where the tenant is receiving city assistance.

Tenants will still retain their opportunity to buy their building first under TOPA; if they choose not to or fail to do so, D.C. can step in and invoke DOPA. The rules also allow the city to assign their purchasing rights to a pre-qualified developer, which will then be required to preserve a certain number of the units as affordable housing. Whether D.C. or a developer completes the purchase, the rules say that the rent for tenants in affordable units cannot increase under the new ownership. When the units turn over, the rent can increase — but only by set amounts.

Since she took office in 2015, Bowser has increased annual spending on affordable housing, directing $100 million a year to the Housing Production Trust Fund, the city’s main tool for preserving and building affordable housing. The city says that during Bowser’s term, 6,000 units of affordable housing have been delivered.

But critics say that’s been a drop in the bucket in a city where rent and sales prices have increased dramatically in recent years, leaving many residents struggling or unable to afford to stay in D.C. And last year, the D.C. Auditor said that parts of the Housing Production Trust Fund had been mismanaged.

“We just have a huge imbalance, a huge mismatch in our economy, between where housing prices are because of the overall demand, and where incomes are,” said Ed Lazere, the director of the D.C. Fiscal Policy Institute, to WAMU last month. “If we really are going to take it as a serious civic responsibility to create more affordable housing and make sure that everyone can have a decent and safe place to live, we need to think about housing the way we think about schools and public safety — as core government functions that take a lot of resources.”

This story originally appeared on WAMU.