We have a lot of resources on site, including many ambulances, Sören Gustavsson at SOS Alarm's press service says.
The fire is believed to have started on a balcony.
It has since spread to the roof. It is unclear whether there are people in the house, Sören Gustavsson says.
The fire alarm was raised at 3 p.m. on Midsummer's Eve. According to Upsala Nya Tidning, the house is burning with open flames and black smoke is billowing from the building.
The forecast is that it will take at least three hours to put out the fire, Sören Gustavsson says.
