planning office; planning department
A section or room within an organization responsible for planning and development.
The new project is being discussed in the planning office.
He was assigned to the planning department right after joining the company.
They are discussing plans for a new product in the planning department.
企画室 and 企画課 both name planning units, but 企画課 is a standard corporate 'section' (課) rank, often smaller than a 室.
企画室 is the planning department (a place/organization), whereas 企画書 is a planning proposal document—easy to mix up because both start with 企画.
Compound of 企画 (kikaku, 'planning') and 室 (shitsu, 'room' or 'office').