Should I opt for a Bachelor's degree or certification courses in interior designing after B.Com?

You already have a graduation degree so no need to go for it again. Instead, you can do certification course or a Master's degree.

Hi Kriti, A student can apply for the programme for M.Des in Fashion, interior, product design, and more, Students coming from any 10+2 or graduation discipline from a recognised university, or board with a minimum aggregate of 50%, can appear for any entrance or admission process or any design cour