Supervisor Scott Wiener will introduce legislation Tuesday to allow affordable housing projects to bypass the city’s lengthy conditional use authorization, a change that could shave months — in some cases years — from San Francisco’s time-consuming approval process.

The bill would apply to projects that are 100 percent affordable to families earning less than 120 percent of area median income, about $122,000 for a family of four.

“It takes an ungodly amount of time and expense to move any affordable housing project through the process,” Wiener said. “We are in this housing crisis and we all agree that we need more affordable housing for low-income and middle-income residents. And yet it takes years and years to build one of these projects.”

Conditional use authorization is frequently required for projects that are consistent with height and bulk zoning limits but need special exemptions from other zoning controls — things like the allowable number of units, unit size or ground-floor uses. Under the legislation, projects in need of conditional use would be handled as an “as-of-right” project, subject to design review by the Planning Department but not special conditional use hearings by the Planning Commission.

Wiener said residents who oppose projects could still request a discretionary review from the Planning Commission.

The legislation comes too late for the Booker T. Washington Community Service Center and the John Stewart Co., which will break ground this week on a new 50-unit affordable housing complex at 800 Presidio Ave. after an eight-year fight to win approvals.

“It was a nightmare,” said Pat Scott, executive director of the Booker T. Washington center. “We are trying to build 50 units of all affordable housing. The neighbors were up in arms. They held us up in court for several years. It’s hard to explain how awful it was. They thought they would wear us down, but I was determined to build this.”

The legislation would also exempt affordable housing developments built on public land from rezoning. That could help speed up projects like 2070 Folsom St. in the Mission, a city-owned site that was just awarded to the Mission Economic Development Agency and Chinatown Community Development Center for a 101-unit development.

Also Tuesday, Mayor Ed Lee and Supervisor Mark Farrell will introduce a plan to reform the city’s inclusionary housing rules to speed the production of affordable housing. It will include a provision allowing developers to increase income levels allowed in affordable units in exchange for building more units.

It will also allow them more flexibility in terms of where they can build affordable housing and when it must be completed in conjunction with market-rate units that they produce.

“We need to implement policies that provide the right incentives and opportunities to build more housing — especially affordable housing — as quickly as we can, and this package will certainly help that vision materialize,” Farrell said.

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