The blaze started at the detached building in Drivers End in Codicote, near Welwyn, during the morning.
There was another fire at the building 20 years ago, after which fire-proof foil was placed under the thatch, which is protecting the main building.
More than 15 fire engines and support vehicles were called to tackle the blaze.
Roads in the area were closed and traffic diverted.
A major fire has been burning in the thatched roof of a large property in Hertfordshire.