The militants exploded the grenade at the house of the superintending engineer of Manipur Development Society Salam Rupa Kumar at Tera area on the outskirts of Imphal at around 9 PM, the sources said.
The engineer and his family members were present at the house at the time of the explosion in which a portion of the outhouse was damaged.
The militants escaped under the cover of darkness.
Monetary demand from the official was the reason behind the explosion, the sources said.
State works minister Kh Ratan Kumar told reporters that an inquiry has been ordered to find out the cause of the collapse.
A show cause notice was served to the contractor, he said.