A custom home is a one-of-a-kind house that is designed for a specific client and for a particular location. The custom home builder may use plans created by an architect or by a professional home designer. Custom homes afford consumers the opportunity to control layout, lot size, and accessibility, and range in size from small homes, guest houses, ADU's as well as large luxury homes. However, in comparison to prefabrication, custom home construction is considerably less efficient in both the construction process and maintenance. In most cases, custom home builders construct on land the home buyer already owns. Some developers sell fully serviced lots specifically for the construction of custom homes. This makes it easy to build a custom home since the lot is construction-ready and builders can focus purely on the design of the home. On some projects, work on-site will not start until design work is largely complete; on others, some design work may be undertaken concurrently with the early stages of on-site activity (for example, work on a building's foundations may commence while designers are still working on the detailed designs of the building's internal spaces). Some projects may be a hybrid of custom and prefab, which include elements that are designed for off-site construction and are then delivered to the site ready for erection, installation or assembly.