After 8 Months, No Clear Cause for Fire That Razed Affordable Housing Construction

Updated 4:45 Thursday

Nearly eight months after a fast-moving fire destroyed an affordable housing building that was under construction near Redwood City, fire officials said they cannot definitively identify a cause.

According to a fire investigation report by the Menlo Park Fire District that labels the cause as undetermined, the fire began on the fifth floor in a room in the northwest corner, and it “most likely” started when the plastic wrapping on a package of insulation ignited.

How the package was sparked is still unclear, though investigators couldn’t rule out that “an open flame device,” such as a torch used to solder pipes, may have been used in the area and caused an ignition.

“We wish we had a more definite answer because we always want to know,” Menlo Park Fire District Marshal Jon Johnston told KQED. Whether the fire was intentionally set or not is also not clear, he said.

The fire was reported to 911 around 10:15 a.m. on June 3 at the affordable housing project site known as Middlefield Junction, being built by Mercy Housing California. The project consists of two buildings totaling 179 apartments, located on San Mateo County-owned property at 2700 Middlefield Rd. in the North Fair Oaks neighborhood south of Redwood City.

One building, which had 104 planned units, burned down to its concrete foundation, while a neighboring 75-unit building that includes a planned child care center was not damaged.

Construction workers at the site tried to put out the blaze with fire extinguishers but were unsuccessful, the fire district said in a Jan. 29 statement. “The building was evacuated safely, 911 was called, and the fire spread quickly consuming the entire structure.”

The district said 36 fire engines and six ladder trucks from around San Mateo and Santa Clara counties were called in to help control the fire and ultimately contained it to the one building. Some fences and vehicles near the property were damaged due to the fire, Johnston said, but no injuries were reported.

The fire’s spread was accelerated because of the exposed wood framing throughout most of the in-progress structure, along with open airflow and other fuel sources, such as construction materials.

The building had some windows installed, but it had no doors, was not weather sealed, and was “open to free ventilation,” contributing to a “total burn,” the report said. Fire sprinkler systems were still being installed but not complete.

Investigators also received varying reports about the “housekeeping of the construction site,” which could have contributed to the rapid fire spread.

The fire district, along with the San Mateo County Fire Investigation Task Force, interviewed witnesses, including site workers and neighbors, and reviewed pictures, videos and other evidence.

Crews also conducted tests of different potential ignition sources, documenting how things like a cigarette, chopping saw and a soldering torch might have started the fire.

“They had no smoking signs. We are told people smoke, but we don’t know where,” Johnston said. “We also had sparks from cutting tools because we were told that there was some metal cutting going on around that location, or there was also potential for some pipe sweating, but maybe not. So we took that again as a possibility,” Johnston said of the testing.

Johnston said the investigation is complete for now but could be reopened if new information is discovered or brought to investigators that warrant follow-up.

In the days after the fire, county officials vowed the housing project would be rebuilt. A county spokesperson did not immediately respond to a request for comment on Thursday.

San Mateo County needs to build more than 1,200 housing units for low- and very-low-income families by 2031 to meet its expected demand, and the damaged complex was among the largest in the county’s development pipeline.

Kelly Hollywood, an associate director of real estate for Mercy Housing, said the organization received approvals from the fire district and county building officials and has moved forward with site cleanup and construction.

“We are on track to open the first building, featuring 75 apartments, this summer, and the second building, with 104 apartments, in early 2026,” Hollywood said in an email. “Mercy Housing California is thrilled to bring these affordable homes for families and a new childcare center to serve the North Fair Oaks and surrounding communities.”