The most common form of flexible planning is PUD, which offers options to developers for determining uses, densities, building placement, and other planning and design factors applied to their sites. It allows more flexible site design than ordinary zoning would allow by permitting options or relaxing some requirements. PUD provisions establish overall parameters for development, such as average densities and open space requirements, but allow variable treatment of these factors within a given site. PUDs almost always require special review procedures, including design reviews, to approve these variations from normal requirements. Overlay zoning/districts (See Overlay Zoning/Districts.) can be adopted to provide for special treatment of certain areas such as transit station areas, downtown areas, and historic districts. Source/Reference: Porter, 1997, pp. 26 and pp. 50.

The City of Austin Land Development Code defines a PUD as an appropriate zoning district designation when needed for large or complex developments under unified control planned as a single continuous project, to allow single or multi-use projects within its boundaries and provide greater design flexibility for development proposed within the PUD. Use of a PUD district should result in development superior to that which would occur using conventional zoning regulations. PUD zoning is appropriate if the PUD enhances preservation of the natural environment; encourages high quality and innovative design and ensures adequate public facilities and services for development within the PUD. The minimum size generally considered appropriate for a PUD is ten acres. Absent unique or special topographic constraints or other exceptional circumstances affecting the property, creation of a PUD is not justified for development of tracts of less than ten acres since conventional zoning regulations should provide for adequate development. (See 25-2-411) http://www.ci.austin.tx.us/development/zonginfo1.htm