Counterterror police are involved in the probe and the fire is being treated as suspicious.
While Starmer is based in Downing Street as prime minister, he still retains his north London residence.
A London Fire Brigade spokesperson said: “Firefighters were called to a small fire outside a property” in Kentish Town Monday morning, adding: “Two fire engines from Kentish Town Fire Station attended the scene.”
A Metropolitan Police statement said: “On Monday, 12 May at 1.35 a.m., police were alerted by the London Fire Brigade to reports of a fire at a residential address.”
Downing Street said the prime minister was grateful to the emergency services for their work but could not comment further due to the live investigation.
Matt Honeycombe-Foster and Emilio Casalicchio contributed to this report.