Later, it was decided to purchase the building, which was occupied by the court until it left Sonoma for the new county seat of Santa Rosa, following a vote in 1854.

[2]: 18–19  The Sonoma courthouse was not well loved; the local Sonoma Bulletin warned in February 1854 "our worthy officers of the law ... run the risk of being crushed beneath a mass of mud and shingles, for we really believe it will cave in the next heavy rain" and a grand jury report called it "an old dilapidated adobe of small dimensions, in part roofless and unfit for a cattle shed".

[2]: 24  The Court of Sessions, presided over by Judge Frank W. Shattuck, held its first meeting in Santa Rosa on October 2, 1854, at the old Masonic Hall.

[2]: 27–28 After assuming his seat on the county board of supervisors in January 1883, T.J. Proctor of Santa Rosa immediately moved to construct a new courthouse in Santa Rosa; Petaluma countered a month later, proposing to donate Hill Plaza Park (now Penry Park) and $100,000 for construction of a new courthouse, provided the county seat be moved there instead.

[4]: 161  The tension between Santa Rosa and Petaluma grew to a point where Petalumans began circulating petitions to secede and form a new county.

The new courthouse was designed by Curtis & Bennett, and the construction contract was let for $80,000 to Carle & Crowley of Sacramento on September 2, 1883, with the condition that it be completed by January 1, 1885.

[4]: 161–163  The cornerstone for the new courthouse was laid on May 7, 1884, made of granite quarried from Rocklin; the ceremony was presided over by General Mariano G. Vallejo, considered to be the first person who settled in Sonoma County.

[9] The 1910 building, which had included a significant amount of reinforcing steel, was so sturdy the contractor hired to demolish it was unable to complete the razing within the 75-day window allotted.