Five basic functions that planners can be differentiated in looking at the basic functions of cities are working, residing, nourishing, transporting, and communicating. These functions could be expanded to additional functions such as aesthetics or cultural functions.
The level of urban design and planning ranges from microscopic to macroscopic levels in individual households, neighborhoods, cities, and regions. To address the overall advantages and disadvantages of a city the interrelation between levels should be recognized.  
The question that arises here is what role sustainability plays within the urban design process? 

Seven design concepts of urban forms that follow sustainability design principles contain Density, diversity, mixed land uses, compactness, sustainable transport, passive solar design, and greening. For example, compactness is an element that generally deals with the containment of sprawl. On the other hand, diversity represents the idea of heterogeneous urban patents such as housing types, household sizes, or the age of the inhabitants to attract diverse residents it copes with the social and cultural context of urban form. Diversity is a multi-dimensional factor that joins up the densities and mixed land uses but the mixture and complication of these elements are different and produce a number of notable sustainable urban forms between them. 

According to Gibson (2006) first step in assessing sustainability is defined as a core set of generic criteria related to sustainability that stress the requirements for improvement. These criteria should be location specific and consider inter-dependency among criteria. Therefore they should be comprehensive to have an overall positive effect. As a result, these criteria would support decision-makers by putting rules for processes and trade-offs.