Quick Answer: An interior designer plans how your rooms are used, draws the layout, chooses materials and finishes, coordinates the carpentry, electrical and paint work, and oversees the job until handover. You do not always need one. A single wardrobe or a simple repaint can be done without. For a kitchen, several rooms or a whole home, a designer usually saves rework, delays and money. Key Facts: A designer's work covers planning, design, material choice, coordination and s