Hi Rujib
I am no expert but am currently looking to buy an old cottage so have a bit of interest in the area. As far as I can see the actual extension you wish to build, you have no permission for. Some things occur to me:
1) The council make you pull it down. They can do this but I don't think they do it very often and would probably depend on your county council.
2) If you need to sell your house, there will be problems if it doesn't comply with planning. The buyer would have to be prepared to take on the risk.
Would you consider re-applying for what you would like to build without the sunroom? I would guess that that would have been the most out-of-character part of your original application. If you are lucky to live in a part of the world with an approachable planning dept, it would be worth a try to schedule a meeting, put your cards on the table; show them the costs of the two proposed extensions, tell them you will scrap the sunroom. Might be worth a shot.
Hope it all works out for you.
Good luck.