Apart from approval from URA, you also need BCA's approval.
More importantly, you need the approval of the MCST. 
Converting open terraces to enclosed rooms, increase the GFA/Built in area. 
MCST will know if there are any excess GFA for such A&A.
Whether or not there are sufficient GFA, most MCST will not approve such A&A. 
The moment 1 approval is given, others will follow. At some point, the GFA will be exceeded. 
I do not suggest you doing this. There can be legal issue with the MCST. 
Hope my input helps.
Steven Wang
 Edmund Tie & Co
 khim,wang28@gmail.com 
+65 9169 5023 
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