Middle-class homes affordable to build, buy and own: That’s the gist of what Boston Mayor Marty Walsh's Housing Innovation Lab is aiming for as the city's population swells toward 700,000.

"Solving the middle income housing challenge is among the biggest challenges we face today," Walsh said in a press release Tuesday. "The Housing Innovation Lab is bringing a new way of thinking about these issues to the city."

Middle income is defined by the city as households earning $50,000 to $125,000, or 60 percent to 150 percent of area median income. Affordable means a household spends no more than 30 percent of its income on housing.

In a speech Tuesday, the mayor announced four pilot programs designed by the Housing Innovation Lab to encourage the construction of smaller, more densely built homes that are affordable to middle-income families.

Density Bonus Policy: This would allow developers to build more densely — to increase a building's footprint relative to the land it sits on -- in exchange for constructing more affordable units. The lab is working with the Boston Redevelopment Authority planning areas in Jamaica Plain/Roxbury and South Boston to carry this out. Compact Living: The lab wants to team up with architects, developers and contractors for ideas on how to affordably build compact living apartments. This includes a housing innovation competition and a three-month "road show" so people can walk inside compact living units. Community Land Trusts: The idea here is to ensure the affordability of housing over the long term. Working with Greater Boston Community Land Trust Network, the lab looks to develop a technical assistance program for the creation of CLTs. Home Buying Portal: The lab will launch a website where first-time homebuyers can connect with the neighborhoods, houses and loan programs that suit them. A location guide will introduce people to different parts of Boston in order to disperse growth across the city.

The Housing Innovation Lab says the successes or failures of the pilots will determine whether they are appropriate solutions to the housing crunch and should be expanded across the city.

"We’re trying to determine how many potential affordable units could be created through these policies,” said the lab's Marcy Ostberg, who is co-manager of the program.

The lab's Sabrina Dorsainvil said members of the group spent time interviewing residents in Boston neighborhoods to determine what projects to pursue.

"It was really interesting to walk into someone's home and ask them to tell me about the space, and have someone with excitement say, 'This is my think tank... this is where all the magic happens,' " she said.

The Housing Innovation Lab stems from the mayor’s Boston 2030 housing plan released in 2014. Based on population growth projections, the plan calls for the production of 20,000 middle-income units (and 53,000 total units) by 2030.

The city says it's on track to meet the overall goal of the Boston 2030 plan, though starts for middle-income units fell short. According to the third quarter report released in October, the pace of building in the city was 122 percent of what is needed for 53,000 units by 2030. But the production of middle-income units was at 85 percent.

The Housing Innovation Lab is funded by a three-year, $1.35 million Bloomberg Philanthropies innovation grant.