The declarant and his duly authorized agents, representatives, and employees may maintain sales offices or model units, or both, on the submitted land if and only if the condominium instruments provide for the same and specify the rights of the declarant with regard to the number, size, location and relocation thereof. Any such sales office or model unit which is not designated a unit by the condominium instruments shall become a common area as soon as the declarant ceases to be a unit owner, and the declarant shall cease to have any rights with regard thereto unless such sales office or model unit is removed forthwith from the submitted land in accordance with a right reserved in the condominium instruments to make such removal.