The building in Tagene on Hisingen is used by Västra Götaland police to store seized vehicles.
The fire was extinguished by around 8pm and the area was cordoned off and the fire is to be investigated.
“But it is far too early to say anything about how it was caused. We have to talk with the emergency services first,” said police spokesperson Ulf Edberg to news agency TT.
Edberg confirmed that he was not aware of any prior special surveillance of the property.
No personnel were in the building at the time of the explosion, Västra Götaland police. Neither is anyone reported to have been injured.
Tagene is area along the E6 motorway with various businesses within retail, but few residences. It however lies close to the troubled neighbourhood of Backa.