A method of constructing a virtual construction model, comprising: displaying a plurality of construction elements corresponding to respective types of construction elements, allowing a user to select one or more instances of each of the types of construction elements; maintaining a counter of available elements for each type of construction elements; decrementing said counter of available elements of a type of construction elements each time a user selects a construction element of said type; providing an indication to the user when the available number of construction elements of one of the types of construction elements has been used in the virtual construction model.