Installing an individual solar system on an apartment terrace requires permission from both the builder and the society, as the terrace is considered common property. Even if the Deed of Apartment initially grants rights to the builder, the Delhi Apartment Ownership Act states common areas belong to all flat owners. You cannot install equipment without the General Body's approval. If you've already installed a system without permission, seek retrospective approval, which may involve penalties or charges. The society can also demand removal if permission is denied. While solar systems aren't typically basic amenities, arguments can be made for their necessity. Consulting a structural engineer is also recommended.