Marty Walsh wants half a billion for new housing. What would all that money buy?

“Rents and home prices are stabilizing,” Walsh said during his address. “But they’re still too high for too many people. There’s much more work to be done.”

While the speech was light on specifics, the mayor’s aides this week outlined a grab bag of programs they envision the $500 million paying for: everything from increased investments in senior housing to financial assistance for first-time home buyers to seed money for a fund — largely raised by big companies and foundations — that would finance development of affordable housing.

“These are great ‘big picture’ ideas,” said Vanessa Calderon-Rosado, CEO of Inquilinos Boricuas en Acción, a nonprofit housing developer in the South End. “Now the devil’s in the details of how this money will flow.”

Since taking office in 2014, Walsh has made housing a priority, pledging to build 69,000 units by 2030 in a bid to ease rents that are among the nation’s highest. Amid a surge of new construction, the city is on track to hit that mark, with more than 30,000 units permitted so far.

But in recent years especially, Walsh has come under fire from critics who say that development has mainly benefited the well-off and does little for lower-income renters who risk being priced out of fast-changing neighborhoods where some have lived for years.

Some of the proposals Walsh offered this week are aimed at answering those concerns.

Perhaps the most concrete one is a pilot program to create rent subsidy vouchers for the homeless, formerly homeless, and low-income elderly and disabled residents — not unlike the federal Section 8 program. The city would dedicate $5 million a year to such vouchers, housing chief Sheila Dillon said in an interview, and aim to launch it quickly.

“We’d like to be up and running on this in a matter of months, not years,” Dillon said.

Some of the money would supplement efforts that are already underway, including partnerships between the Boston Housing Authority and private developers to rebuild aging public housing complexes into mixed-income neighborhoods.

Progress on the largest of those projects — the Bunker Hill complex in Charlestown and the Mary Ellen McCormack community in South Boston — has been slow, partly due to funding challenges. Having city money to work with, especially as federal funding for public housing declines, could make a huge difference, said Gilbert Winn, CEO of WinnCompanies, which is spearheading the $1.6 billion McCormack redevelopment.

“Federal funding has such stringent rules,” Winn said. “City funding can help create something new and different that fits the McCormack site. The old models don’t work.”

Still, some housing advocates said Walsh’s call for action could have been stronger, especially when it comes to protecting vulnerable renters. He recently unveiled a plan to cut evictions by 25 percent and has proposed stricter protections for older renters. But he didn’t mention those topics Tuesday, and his $500 million plan doesn’t do much to directly help renters who are at imminent risk of being forced from their homes. That bothered Mike Leyba, director of development at tenant advocacy group City Life Vida Urbana, which is pushing a proposal that could bring rent control back to Massachusetts for the first time in 25 years.

“What I wish the administration saw is the urgency that walks in our doors every day,” Leyba said. “People are fighting immediate displacement. It’s a crisis.”

But before Walsh can divvy up that $500 million, he has to raise it. And that will be challenging.

The first $100 million — $20 million a year over five years — would come from city operating funds. Walsh proposes roughly doubling the budget for housing and combating homelessness, to $38.7 million, starting next year.

The city also would sell the Lafayette Garage in Downtown Crossing, putting those proceeds toward housing. It’s not clear how much the roughly 1,000-space underground garage might fetch, said Chris Osgood, the city’s top transportation official. But unlike downtown’s Winthrop Square Garage, where Millennium Partners paid about $160 million to secure the site for a skyscraper now under construction, Lafayette Garage is not likely to be a development site any time soon, Osgood said. That could affect its selling price.

Under Walsh’s plan, the biggest portion of the housing funds would come from a proposed tax on real estate sales over $2 million, passed by the City Council last month and sent by Walsh to Beacon Hill for state legislative approval. At a rate of 2 percent, the tax could raise tens of millions of dollars a year, mostly from the sale of large office and apartment buildings, which Walsh would then steer to housing programs.

Powerful real estate interests oppose the so-called transfer tax plan, and Walsh has a mixed record when it comes to getting such home rule petitions passed on Beacon Hill. But city chief financial officer Emme Handy said the administration is optimistic it can get this one through.

“We’re feeling pretty confident it will pass,” she said. “We’re pushing pretty hard for it.”

The Walsh administration may have allies in its effort. Five cities and towns in Eastern Massachusetts — including Somerville, Brookline, and Concord — have filed similar petitions with the state. On Wednesday, some of their elected officials, along with housing advocates, held a State House rally to push for an umbrella bill that would permit transfer taxes on sales that are above the state’s median home price — $400,000 for single-family homes in November, according to the Warren Group, a real estate data firm.

Walsh didn’t attend the rally, though Dillon did. One of the speakers was Boston City Councilor Lydia Edwards, who cosponsored the transfer tax bill and steered it through City Hall to Walsh’s desk.

A former Walsh administration housing staffer, Edwards remembers it wasn’t so long ago that the amount of money required to dramatically address Boston’s housing crunch seemed way out of reach. So she was cheered Tuesday night when the mayor talked about raising the stakes — even if all the details haven’t been figured out.

“To have the city come back and say we’re committed to half a billion dollars in five years means that that number is realistic,” she said. “It means we’re committed to getting something done.”

Milton Valencia of the Globe staff contributed to this report. Tim Logan can be reached at tim.logan@globe.com. Follow him on Twitter at @bytimlogan.