(a) In cases of the conveyance or encumbrance of an apartment which has been planned but whose construction has not yet begun, the share of the grantor or the encumbrancer, as the case may be, in the common elements of the proposed building and his right to have the apartment constructed for him shall be deemed acquired by the new owner, or encumbered, and the new owner or person to whom the encumbrance is made shall be deemed subrogated to the rights and interest of the grantor or encumbrancer for all legal purposes.
(b) In the case of the conveyance or encumbrance of an apartment already under construction, the share to which subsection (a) of this section applies shall be deemed acquired or encumbered, as the case may be, and the part of the apartment already constructed shall be deemed individually acquired or encumbered, the new owner or person to whom the encumbrance is made being also subrogated to the rights and interest of the grantor or encumbrancer for all legal purposes.