What an attic finish involves
A finished attic usually needs insulation, drywall, flooring, electrical, lighting, and a way to heat and cool the space. Adding a bathroom or an egress window for a legal bedroom raises the cost.
Check structure and headroom first
Before finishing, confirm the floor joists can carry living loads and that you have enough headroom (commonly 7 feet over at least half the area). Reinforcing the floor or adding a dormer adds significant cost.