Northamptonshire Fire and Rescue Service said one crew was called to the scene in Kettering Road, close to Tesco Express in Spinney Hill, at 7.47am.
An NFRS spokesman said: “A crew from Moulton was on the scene shortly afterwards and found smoke billowing from the annex of a detached two-storey property. The crew deployed two firefighters with breathing apparatus and used a hose reel jet and a 45mm jet to try and contain the spread of the fire.
“Northamptonshire Police were requested to attend and manage traffic at the scene, while work began to isolate electrics at the property.
“By 8.30am the fire had been extinguished with thermal imaging cameras being used to identify any remaining hotspots and fans used to clear smoke away from the roof. Hose reel jets helped to dampen down the scene.”
An after-fire inspection is set to be carried out this afternoon by officers, the spokesman said.