housefly vs house fly

housefly and house fly both are nouns.

Word	Noun	Adjective	Verb	Adverb
housefly	Yes	No	No	No
house fly	Yes	No	No	No

As nouns, housefly and house fly are synonyms defined as:

housefly and house fly: common fly that frequents human habitations and spreads many diseases

Other synonyms of housefly include Musca domestica.

housefly (noun)	house fly (noun)
common fly that frequents human habitations and spreads many diseases	common fly that frequents human habitations and spreads many diseases