Downes suffered blunt force head and neck trauma, including numerous skull fractures, from the altercation with Higinio Soriano Salgado, according to the San Diego County coroner's report.
Salgado was a development manager since 2008 with Blokhaus, a leasing and development firm affiliated with Graham Downes Architecture Inc.
Police found Downes unconscious in the street in front of his home. Salgado, 31, pleaded not guilty to murder charges on April 23; he remains in custody on $3-million bail. A preliminary hearing is set for May 11.
Spokepersons for the police and for the district attorney did not make public a motive for the assault.
Using a credit card, South Africa-born Downes launched an eponymous modernist architecture and planning practice in 1994 that focused on boutique hotels, restaurants, retail and multi-unit housing in Southern California, Nevada and Arizona.
The private firm's best-known work includes the 12-story, 420-room Hard Rock Hotel and 45,000-sq-ft Tower 23, both in San Diego. Downes started the umbrella firm Blokhaus in 2001.
Graham Downes Architecture Inc., with about 40 employees, was among the Top 250 architects ranked by Architectural Record magazine, sister publication to ENR, in 2008 and 2009.
The company will continue without its founder, officials say.
"Graham was a brilliant designer, creative visionary and vivacious, strong and kind leader. We are actively working to continue his vision through the work of Graham Downes Architecture," said Blokhaus CFO Alix Veen in a statement.