The downtown Detroit market for big blocks of office space is tightening, pushing large prospective tenants to locations farther outside the city's core to find suitable space.

Last week's news that Adient Ltd., the Johnson Controls Inc. seatmaker spinoff, plans to bring about 500 employees to the Marquette Building at 243 W. Congress St. follows a series of large blocks of office space getting scooped up by big tenants, further increasing the difficulty others have finding space downtown.

While space certainly remains available with a 14.2 percent third-quarter vacancy rate, according to Newmark Grubb Knight Frank, finding precious configurations of contiguous space in prime locations is getting tougher.

Among the new tenants, just within the past 18 months: WeWork LLC, which is taking 80,000 square feet divided evenly between two Dan Gilbert-owned buildings at 1001 Woodward Ave. and 1449 Woodward; his Rock Connections LLC, a strategic marketing company that has filled up the 108,000-square-foot 1900 Saint Antoine Building; Quicken Loans Inc., occupying most of the former Detroit Media Partnership building along with Molina Healthcare of Michigan; and now Adient, which sources said last week is expected to occupy the 164,000-square-foot Marquette Building, owned by Mexican billionaire Carlos Slim Helu.

WeWork is a New York City-based co-working space company that provides space to startup companies and others.

Because of this, companies in search of large blocks of space might have to look outside downtown in places like the New Center area and Midtown, said AJ Weiner, managing director in the Royal Oak office of JLL (formerly Jones Lang LaSalle). That's made more possible because of the upcoming completion of the QLine streetcar, which will make commuting between the New Center area to the north and the central business district employment bases more convenient.

"The vacant steel dinosaurs lurking over downtown have effectively passed us by," he said. "What I see happening is, as developers look to add residential to the marketplace, which they are actively doing, if you're going to build a 10-story building, add two floors of office to it, 40,000 feet, because you can justify that."

But a 14.2 percent vacancy rate of the market's 14.35 million square feet also means there is availability — just perhaps not in the easy-to-accommodate contiguous blocks that large users prefer and generally need, or in buildings that have things like convenient parking and street visibility, said Matt Farrell, CEO and co-founder of Bingham Farms-based real estate firm Core Partners LLC. He said a number of buildings — admittedly the "good, the bad and the ugly" — are available with floor plates ranging from 8,000 to 15,000 to 20,000 square feet or more.

"Thirty-seven buildings have availability of 8,000 feet, and those have a lot more spaces that can accommodate it," he said.

There is no need for too much concern just yet, Jim Ketai, CEO of Gilbert's Bedrock Real Estate Services LLC, said in a statement.

"Occupancies are climbing rapidly, but there is still plenty of available office space in downtown Detroit, although it needs investment, as well as ample opportunity to create more," he said. "We are actively seeking properties that could accommodate the increasing demand to be downtown as well as opportunities for new ground-up development. WeWork, which is opening early next year, is going to be a phenomenal option for office space as well. We are always willing to work with potential tenants to find them the right space."

Among those new ground-up developments are on Gilbert-controlled property — specifically, the 2-acre site of the former J.L. Hudson's department store on Woodward, where a high-rise is planned, perhaps as high as 60 stories; and the Monroe Block, bounded by Monroe, Farmer and Bates streets, Woodward and Cadillac Square east of the former Compuware Corp. building that Gilbert now owns with Detroit-based Meridian Health.

And experts agreed that the average downtown rents now don't support construction of new office high-rises because they aren't high enough to service debt on construction loans. Rather than the average of $23.23 per square foot currently being asked in the prime Class A buildings downtown, according to a JLL report, new construction would have to command at least $30 per square foot, if not even more.

Some of the large vacant office buildings are the former Detroit Free Press headquarters building at 321 W. Lafayette Blvd., which is 302,000 square feet and which sources have said Gilbert is buying; and the 250,000-square-foot Old Wayne County Building, which was purchased from the county two years ago by New York investors for $13.4 million. Other vacant office buildings, such as the Philip Neudeck Building and the Gabriel Richard Building on Michigan Avenue, are currently vacant but are slated for multifamily construction by developer Joe Barbat.

But those without a planned use in the works are tricky because of their layouts and the amount of work needed to make them usable. Generally, they are seen as buildings for single users, not multiple tenants, and tenants of 250,000 square feet and 300,000 square feet are not easy to come by.

"It's not like we are turning away six-figure users on a regular basis," said Weiner.

According to a report from NGKF, a national real estate company that has offices in Southfield and Farmington Hills, the two largest contiguous blocks of space downtown are both in the Renaissance Center: one of 300,000 to 500,000 square feet, and another of 60,000 to 69,999 square feet. The rest are 30,000 square feet or smaller.

Steve Morris, principal of Farmington Hills-based tenant representation firm Axis Advisors LLC, also said there is 427,000 square feet in the Penobscot Building, and about 100,000 square feet in the Buhl Building, but none of the floors are contiguous. Don't forget the Cadillac Square office building, managed by Southfield-based Farbman Group, and the 211 W. Fort and the 150 W. Jefferson buildings, as well, Farrell said.

The central business district is the region's second-largest office market, behind Southfield, which has 17.22 million square feet. Troy is the third-largest, with 13.26 million square feet, according to NGKF.

Downtown Detroit's third-quarter vacancy rate was 14.2 percent, compared to Southfield's 21.9 percent and Troy's 21.3 percent, according to NGKF. Five years ago, the downtown market was 31 percent vacant, according to NGKF, while Southfield was 31.5 percent vacant and Troy was 31.7 percent vacant.