The words praghaṇaḥ and praghāṇa are irregularly formed meaning 'a portion of a dwelling house'.,
Thus प्रघणः or प्रघाणः 'a porch before the door of a house, a portico'. This is formed from हन् with प्र, irregularly.
Why do we say 'when meaning a portion of a house'? Observe प्रघातः ।,