Home sale prices are approaching all-time highs. The number of sales has been growing steadily upward since 2011, but the inventory of single-family homes and condos for sale is about half of what it was in 2012, frustrating real estate agents across the commonwealth, who all seem to have more buyers than sellers.

In January 2012, there were 24,372 single-family homes and 8,205 condominiums on the market, according to the Massachusetts Association of Realtors (MAR). After four years of steady decline, MAR reported last month, there were 14,291 single-family homes and 3,808 condos on the market.

“The lack of inventory is certainly our biggest issue in the real estate business right now,” said Annie Blatz, president of MAR.

Recent increases in home prices are supposed to incentive people to sell their homes and move up or move on, at least in theory. Most homeowners who bought at the peak of the market and were underwater for years have regained equity and could now sell their homes. So when are all those listings going to come online?

A Top-Down Market

It’s a trickle-down effect that begins at the high end of the market, said Larry Rideout, co-owner and CEO of Gibson Sotheby’s Cambridge.

“It’s Real Estate 101,” he said. “When you look back at the market, new homes are the catalyst. Your empty-nesters will move into new, high-end condos and sell their house to families moving up, who sell their house to someone moving up or just getting into the market. When the empty-nesters move, everyone has a chance to move up. The catalyst is having some place to go, and for the last several years, nothing was being built.”

Rideout said inventory is getting marginally better in the Boston/Cambridge market. One of the reasons inventory looks low is that homes that are priced right get snapped up immediately, he said, adding that the increased number of real estate-owned (REO) properties coming onto the market will be a small and insignificant help.

“Money is so cheap right now and demand is so enormous,” Rideout said. “REOs are going to be bought. It will be a blip on the radar.”

Rideout said too much inventory is also problematic because it puts downward pressure on prices, and that’s not good, either.

“We’re experiencing good growing pains, Rideout said. “There are buyers and sellers across all price points. Eventually inventory will outstrip demand, but it will take some time. The bottom line is if you’re thinking about selling, this is a great time. For sellers who are waiting for the right moment, this is the right moment.”

Recovery Will Take Years

The main reason for the low inventory is that it’s notoriously difficult to get construction projects approved in both the city and the suburbs, so there’s relatively little new construction in the Bay State, said Albert Saiz, associate professor and director of MIT’s Center for Real Estate.

“It’s a classic pattern in real estate,” Saiz said. “It’s something we forget each time. There’s nothing wrong with it, it’s just the way things happen. As the economy picks up, you start seeing the market pick up and new construction always trails demand.”

Saiz said there has clearly been more development in Boston since the late 2000s. He said when that trend extends into building new developments in the suburbs, that’s when inventory will really grow.

“Developers need to plan a project three or four years in advance, especially in the suburbs, where any changes in zoning have to go through Town Meeting,” Saiz said. “It’s starting, but it’s going to take a while. In Massachusetts it’s worse because of the slow approval process. It’s going to take a while and then we’ll see a lot of construction.”

As prices continue to rise, Saiz said he expects inventory to improve.

“Inventory is basically about the fundamentals of demand and supply,” Saiz said. “Historically there’s a positive correlation between prices and volumes.”

The overall lack of middle-income housing in Greater Boston is a large and complex problem that will take years to solve, said Barry Bluestone, professor of public policy at Northeastern University.

“We’ve got the population of Greater Boston growing again and we’re attracting lots of young people and some seniors,” he said. “Rental housing and condos are in short supply. As a result there’s tremendous pressure on the housing stock.”

Bluestone said it’s still profitable for developers to build high-end luxury condos and subsidized low-income housing, but it’s not cost-effective to build housing for working families, largely because of the expense of getting approval for those projects.

“The big gap is for everybody in between. The reason that gap is not being filled is it is virtually impossible to build housing for working families given costs,” he said.

At the current rate of construction, 42,000 housing units will be built in Greater Boston over the next five years. In order for supply to meet demand, an additional 36,000 units would need to be built during that time, according to Bluestone’s research.

“It’s going to take a comprehensive approach which has to do with new zoning to reduce onerous regulations,” Bluestone said. “It’s going to take all kinds of political and economic changes.”