Property Lines is a column by Curbed senior reporter Patrick Sisson that spotlights real estate trends and hot housing markets across the country. Comments, tips, and suggestions on where Property Lines should head next are welcome at patrick@curbed.com .

Despite recessions and demographic shifts, few building types have boomed like self-storage lockers. In fact, they’ve proven to be one of the surest bets in real estate over the last half century, while malls, starter homes, and even luxury commercial space in big cities, once safe and steady investments, have struggled. Behind the combination locks and roll-up doors lies a $38 billion industry.

One in 11 Americans pays an average of $91.14 per month to use self-storage, finding a place for the material overflow of the American dream. According to SpareFoot, a company that tracks the self-storage industry, the United States boasts more than 50,000 facilities and roughly 2.311 billion square feet of rentable space. In other words, the volume of self-storage units in the country could fill the Hoover Dam with old clothing, skis, and keepsakes more than 26 times.

Though the adage “sex sells” is hard to dispute, the decidedly unsexy self-storage industry made $32.7 billion in 2016, according to Bloomberg, nearly three times Hollywood’s box office gross. Self-storage has seen 7.7 percent annual growth since 2012, according to analysts at IBISWorld, and now employs 144,000 nationwide.

The volume of self-storage units in the country could fill the Hoover Dam with old clothing, skis, and keepsakes more than 26 times.

The industry’s boom over the last few decades mirrors larger demographic and real estate trends: Americans relocating from the Midwest and Northeast to Sunbelt cities store old gear in self-storage units. Millennials moving into increasingly crowded, high-demand downtowns require extra space. A wave of downsizing baby boomers needs a place to put a lifetime of accumulated memories. Small businesses want room to store excess inventory.

The confluence of these trends has created a building spree. The last few years have seen record-setting investment in self-storage expansion, including $4 billion alone in 2017. This year alone, planned or existing warehouse expansion will add 40 million square feet, or about 800 facilities, to the market, according to Investing Daily.

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Investors see abandoned malls as a candidate for conversion into self-storage consumer cubby holes, a true full circle of consumerism. Venture capital firms are even betting on high-tech storage startups, such as Clutter, an on-demand service which does all the packing, storing, and moving for consumers, and can even retrieve specific items and deliver them to your door.

The current boom has led some market analysts to predict the previously unthinkable: We may just be inching close to peak storage. Many see a slowdown coming, due to a glut of space in cities like Phoenix and New York City, and in Orange County, California. Sky-high stock valuations for many of the big players, like Public Storage and Extra Space Storage, have tapered off. What happens when Americans, who see expansion and relocation as a birthright, get close to having enough room?

The unstoppable expansion of self-storage

The history of the storage industry has been one of steady growth and remarkable resilience.

Self-storage isn’t new. Many trace the concept back to Bekins, an Omaha, Nebraska, moving company founded in 1891 by brothers Martin and Josh Bekins. The company, which played a significant role in moving Americans to Los Angeles in the early 20th century, established a network of concrete-and-steel warehouses for new arrivals beginning in 1906.

The first business that looked more like contemporary storage units, complete with roll-up doors, was A-1 U-Store-It-U-Lock-It-U-Carry-the-Key, which opened in Odessa, Texas, in 1964. It was marketed to oilmen as a place to store their equipment (the company’s name was an attempt to get listed first in the phone book). Owners Russ Williams and Bob Munn, his stepson, erected a simple structure of cinder blocks and corrugated steel with wooden doors on each unit. Due to oil’s ups and downs in Odessa, the facility fell into disrepair and was closed. But, in what is perhaps a sign of the industry’s recent growth, the facility was purchased, renovated, and reopened as a new self-storage site in 2013.

During the Great Recession, the business thrived as millions downsized, moved, or faced foreclosure. In the ’90s and early aughts, when cities started seeing residential growth, self-storage found a new generation of space-starved consumers moving into neighborhoods filled with warehouse space ripe for repurposing.

A marriage of consumer behavior and rising rents explains the self-storage industry’s rapid recent growth in urban areas. High-end self-storage sites can command two or three times the rent per square foot than commercial or residential uses, and in many major metros, these warehouses are 90 percent occupied.

Once they sign up, renters become captive audiences, according to the Wall Street Journal. Paying for storage space is like a gym membership; consumers join and forget about it. Even better for owners, they’re often willing to accept slight increases in cost, rather than deal with the hassle of moving their possessions across town to a competitor’s warehouse.

While national firms have proliferated, with six of the largest—some of which are billion-dollar firms—cornering 18 percent of the market, 74 percent of the industry is still mom-and-pop shops, according to data from SpareFoot.

Can self-storage outrun our need for more stuff and more space?

Has self-storage hit a saturation point? With growth potentially becoming a glut, some analysts believe builders may have finally caught up with demand. It’s also led to backlash and restrictions in major markets. Critics argue that self-storage spaces are taking valuable buildings off the market; the oversized profits possible in storage mean that warehouses which could have been used for commercial, industrial, and even residential purposes are being transformed into profit-generating piles of boxes.

In New York City, which has roughly 50 million square feet of self-storage spread over 920 locations, Mayor Bill de Blasio signed a bill late last year that restricted new facilities in the city’s Industrial Business Zones, where much of New York’s remaining manufacturing takes place. Both Miami and San Francisco have also passed restrictions that limit where self-storage units can be built.

Investors see abandoned malls as a candidate for conversion into self-storage consumer cubby holes, a true full circle of consumerism.

Demographics may also finally dampen enthusiasm for the storage sector, according to the Wall Street Journal. Baby boomers, set to downsize and shrink their housing footprint, will be a big boost for years to come. But younger adults are, compared to older generations, delaying marriage, parenthood, and homeownership, which means less suburban living and less stuff. In addition, Americans are less mobile, meaning fewer moves and less of a need for temporary storage during resettlement and relocation.

There’s even a technological threat on the horizon: autonomous cars. A recent report by the Urban Land Institute and research firm Green Street Advisors predicts that as mobility-on-demand services decrease car ownership, fewer vehicles will equal more space in garages for storage.

Industry boosters, of course, see these issues as simply speed bumps on the road toward greater growth. New Yorkers who feel like self-storage warehouses have sprung up everywhere haven’t seen anything yet. A report from CBRE says New York City, with 3.5 square feet of self-storage per resident, is actually the “most underserved market” in the U.S. (the national average is 7.2 square feet per person).

But consumers in the United States are far from the only ones navigating small space and conspicuous consumption. Unsurprisingly, China, along with much of Southeast Asia, has emerged as a massive growth market.

With larger trends pointing toward less space, less ownership, and more urban living, self-storage seems primed to find more room for, and profit from, your possessions. As Jason Lopez, chief marketing officer of U.S. Storage Centers, says about the industry’s prospects, “density trumps demographics.”