This year, however, we're finding that the capital markets are more cautious on buildings with more than 15 percent of their space dedicated to flex. That should loosen up over time as flex space further establishes itself as a permanent, integral part of the office market.

Are there situations in which lenders and prospective landlords don't have as much caution about certain office buildings with flex space that are on the market, or perhaps some scenarios that they even like?

We see the sweet spot at that 15%-or-less ratio. That amount of flex space is small enough to allow a building's strong local-market fundamentals and overall tenant mix to prevail over any perceived challenges with the presence of flex.

That ratio is common today anyway. At the end of last year, slightly more than half of buildings with flex space in the U.S. and Canada had less than 15% of their square footage dedicated to it.

There are still investors who are willing to take big bets on buildings with higher ratios of flex space. But, institutional investors favor buildings with small amounts of flex space versus those with larger portions. In the first quarter, they and REITs accounted for the vast majority of acquisitions of buildings with flex space of 20% or less.

What if a building's flex operator simply fails – or breaks its lease?

In this rapidly rising rent environment, there is a high likelihood the landlord can re-lease the space at higher rates under traditional models.

CBRE's analysis found that slightly less than half of leases signed by flex operators started prior to 2017. Rents have risen a lot since then, including a gain of 5.2% last year. If landlords are forced to look for new occupiers for that space, they might ultimately come out better financially.

If the building was profitable, and the operator failed for another reason, the landlord might even explore taking over the flex operation.

It seems there is a lot of discussion between landlords and operators around management agreements over traditional leases. CBRE's new report highlights this 'asset light' approach as preferable for landlords. Why focus on that?

I think there is realization that there are flaws in a traditional lease model. Many flex operators are taking steps towards an asset-light, management-for-a-fee model. It allows the landlord more control over the design and operations of the space and direct influence over the membership mix. It also, most importantly, provides the landlord a higher net operating income, once stabilized.

Lenders and investors are now discovering how to value the asset-light model because we're early in the process. With time and experience, this model will likely lead to a new way to value real estate with a flexible office component.