The Oakland City Council on Friday unanimously passed an ordinance that will require property owners to add seismic retrofits to soft-story apartment buildings in the next six years.

The city law applies to certain types of old, wood-frame residences with at least two stories and five or more units. Soft-story buildings often have commercial retail or parking in the ground floor — vulnerable structural openings that could lead to collapse during a powerful earthquake.

In the coming months, the city will send notices to owners of more than 1,400 buildings in Oakland that may need retrofitting, said David Harlan of the Planning and Building Department. Different types of residences will be on slightly different timelines for completing the work.

The legislation was modeled on similar ordinances in Berkeley and San Francisco, whose mandatory soft-story retrofit programs began several years ago and are now winding down.

“We all know that it’s only a matter of time before a major earthquake ... hits the East Bay,” City Councilman Dan Kalb, who introduced the measure, said at the council meeting. “The potential damage would be devastating, in all seriousness, on multiple levels, in loss of life and massive displacement.”

Under the proposal, up to 70 percent of the costs of the retrofit can be passed on to tenants over a 25-year period. Tenants forced to move out while the work is done would be eligible for relocation payments from owners, who can apply for financial assistance from the city, which received grant money from state and federal programs.

Costs are expected to average $69,000 for buildings under 20 units and $105,000 for buildings of 20 or more units. The monthly increase in rent for tenants would come out to between $12 and $27, according to a city report.

Any retrofit costs covered by insurance or grant money would not be allowed to be passed on to tenants.

Still, some expressed concern at the council meeting about tenants understanding their rights and getting proper notice before construction work begins.

Greg McConnell of the Jobs and Housing Coalition, a lobbying group of Oakland developers and businesses, said property owners already face long wait times to get permits and inspections from the city. The new ordinance, he said, could increase the backlog in the absence of significantly expanded staffing.

Kimberly Veklerov is a San Francisco Chronicle staff writer. Email: kveklerov@sfchronicle.com Twitter: @kveklerov