Whenever you buy options, there is no margin (SPAN + Exposure Margin) levied, since the premium paid to buy options forms part of the upfront margin. Moreover, the loss is limited to the premium paid for long option holders. Hence, no margin is required on naked buy options positions.
However, after you buy options, if you intend to hedge your long options position through either a short options trade or a futures trade, your margin requirement increases, since the maximum loss goes up, on having an additional short position as compared to a naked long option.
Hence, even though you have the funds necessary to pay premium on long options trade, you will need to bring in additional margins before you decide to hedge the long position via futures / short options trade, otherwise your second leg will not go through.
Alternatively, if you execute a futures trade or sell options before buying options to hedge your position, you will have to maintain full margin while trading in futures or selling options first, and then your margin requirement will go down after you buy options to hedge your short options position.