This year, the group hired a city planner, Gabriel Metcalf, to lead a team of architects and designers. Mr. Metcalf is well known in Bay Area housing circles for the two decades he spent leading the San Francisco Bay Area Planning and Urban Research Association, a think tank that among other things advocates more and denser housing in the region. For years, he has been an outspoken critic of San Francisco planning. A spokesman for Flannery said Mr. Metcalf could not be reached for comment.

Under Mr. Metcalf, Flannery has expanded from a skeleton operation to roughly three dozen people, mostly in Northern California, working on areas like design, engineering and urban planning, according to the people familiar with the work.

As of August, the company had acquired more than 50,000 acres. County maps that show scattershot holdings for Flannery don’t paint a full picture, one person said, because the company has also struck arrangements with some landowners that would not have to be reported to the county.

“The vast majority of landowners in this area concluded that Flannery’s offers were simply too good to pass up and negotiated sales,” the company said in court documents.

Some offers have been summarily turned down. Mr. McConnell, the mayor of Vallejo, said the group had tried twice to persuade the board of the Solano County Water Agency, of which he is a member, to sell a large parcel that the agency had acquired for environmental remediation.