The prefab components are then assembled on-site by a constructor. Prefab apartment complexes are popular in the San Francisco region since they are less expensive. Labor expenses are clearly cheaper since building in a factory is far more efficient than building on-site. The complex is then delivered to its location and attached to existing buildings or inserted into the ground. Prefabricated apartments are also used in urban planning as an alternative to site-built construction. In this case, the apartment building is manufactured off-site, bringing the cost down for the developer, and then transported to the site where it is assembled with other pre-fabricated or semi-custom built elements such as parking structures, playgrounds, etc.. This reduces the amount of earth that needs to be moved and therefore the environmental impact of the project. Finally, some developers choose to produce custom-built apartments that cannot be produced in a factory. These apartments tend to be higher quality and sell for more money than their prefabricated counterparts. An example is an apartment built for a single owner who wants maximum customization. They may include special features such as loft spaces, private yards, or even rooftop decks. The choice of whether to use a factory or not will depend on the type of development and the price point involved. There are advantages and disadvantages to both methods.