Behave/act as if you own the place
Definition 1 of 2
Pro Tip 1/3Cultural Perception
Using this phrase can suggest arrogance, inappropriate in formal or unfamiliar settings. 
His first day at work, and he's already acting as if he owns the place.
Pro Tip 2/3Assertive Tone
It can express admiration for someone's confidence when used positively. 
She walked in and acted as if she owned the place, impressing the judges.
Pro Tip 3/3Metaphorical Use
Not literal ownership but a metaphor for displaying dominant behavior. 
He's not the boss, but he behaves as if he owns the place.