If you are an interior decorator based in Mumbai and are providing services on-site to a client in Bengaluru (where you don't have a fixed place of business), then irrespective of your turnover, you will have to register under Goods and Services Tax (GST) in the state of Karnataka.Small businessmen or professionals (such as architects, fashion designers, make-up artists, trainers, musicians, stand-up comedians — et al) providing taxable goods or services may find that they have to register under GST if the term 'casual taxable person' applies to them. This registration will be required even if they fall below the threshold limit for GST levy. Currently, the exemption limit for GST is a turnover of Rs 20 lakh (Rs 10 lakh in NE states).

Source - http://economictimes.indiatimes.com