It was a showdown between badly needed housing and precious open space on South of Market’s hardscrabble Sixth Street.

And on Thursday, open space won out — at least for now.

In a rare decision, the Planning and Recreation and Park commissions forced the developer of a proposed 84-unit apartment complex at 301 Sixth St. to redesign the project after residents and community advocates complained that the 82-foot-high building would cast a shadow on the Gene Friend Recreation Center.

While the amount of shadow cast may seem negligible — 0.89 percent of new shadow cast mostly in the morning before the park opens — it’s enough to make a difference for the kids and families along the Sixth Street corridor, many of whom live in residential hotels or cramped studio apartments, according to neighborhood residents.

Misha Olivas, program director of the neighborhood antiviolence youth organization United Playaz, argued that the nearly 1 percent of additional shadow would bring to 49 percent the total shadow cast on Gene Friend, which, along with Victoria Manalo Draves Park two blocks to the southwest, represents the only public recreation space in that part of the South of Market.

“That’s the only grassy spot in the rec center where kids can play catch and roll around in the grass, as kids should,” said Olivas, who frequents the park with her children.

The proposed project is one of a handful of new projects proposed for Sixth Street that violate 1984’s Proposition K — the Sunlight Ordinance — that blocks construction of any building over 40 feet that casts an adverse shadow on Recreation and Park Department property unless the Planning Commission decides the shadow is insignificant. Another development that would cast a shadow on Gene Friend, 980 Folsom St., was originally scheduled for a vote Thursday, but that was postponed.

While the city regularly allows shadow-casting development to go forward by raising the “absolute cumulative limits” for specific parks, Gene Friend is one of eight small open spaces in the densely settled downtown where no new shadow is supposed to be approved, based on a 1989 Recreation and Park Commission interpretation of the sunlight ordinance.

The developers, Ed and Nancy Connor, have owned the car wash and parking lot at 301 Sixth St. since the 1970s. Nancy Connor has been a member of the Friends of Gene Friend Recreation Center and has been active supporting parks citywide. Eleven of the 84 units would be affordable. The project would also have 5,868 square feet of retail and 36 parking spots.

“The Sixth Street corridor is being developed, and we feel it’s time for this property to be put to better use for housing and to provide local businesses with new opportunities,” Ed Connor said.

Most of the new shadow would fall from sunrise through 9:14 in the morning from January through late November. The longest shadows cast would come at 7:57 a.m., when the park is not open. But critics of the project said that it would set a bad precedent as more parcels are developed along Sixth Street.

“We have the lowest amount of open space in the city and the most development going on,” said Chris Durazo of the nonprofit Veterans Equity Center, which opposes the project as currently designed.

In the end, the two commissions unanimously voted to postpone the decision until late July and instructed the Connors to work with their architect and neighbors to come up with a redesign that has less shadow impact.

Recreation and Park Commission President Mark Buell said the developer needs to make a better case for why additional shadow is justified. “The development has merits and the developer has a good reputation, but we have a park here with zero tolerance for new shadow,” said Buell.

But Cory Smith of the San Francisco Housing Action Coalition said the benefits of the additional housing should outweigh the negatives of the shadow.

“In the grand scheme of things, we think housing people should take precedent over shadow impact on a small corner of a park,” said Smith.

At a time when there are a half dozen developments proposed for the area in and around Sixth Street, the city should study the shadow impact of all of them together, according to Planning Commissioner Dennis Richards.

“The real issue for me is shadows on a piece of land that will be there for 100 years,” Richards said. “Once that shadow is there, it’s never going to go away unless the building burns down.”

J.K. Dineen is a San Francisco Chronicle staff writer. Email: jdineen@sfchronicle.com Twitter: @sfjkdineen