Society asking to demolish compound wall.

Hi, we purchased a ground floor parking flat in 2006. The builder, while passing the plan, showed the garden as scooter parking (not belonging to anyone), but in our agreement, the area is shown as garden. Also, there is a map attached to the agreement which shows it as garden. The garden is at the back side of the building, and we also have a door access to the garden. The parking and common area is on front side of building. Ours is the only flat on ground floor, and there are 12 more flats in the building. Before the society was formed, and the while giving us possession, the builder built compound wall for the garden, seperating the area that is common to society and the garden. Now, we want to sell this flat, and the society is willing to give NoC under the condition that we will demolish the compound wall. They say that the wall is not seen in the plan. They say that the new owner may not access them to the garden part, where there is drainage. This wall was built by builder before the society was formed. My questions are 1) Is their claim valid? 2) I read on internet that bye laws allow sale of flat without NoC? Is this correct? Thanks.