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