In the last twenty years, computer tools have progressively enabled the modeling of buildings and cities in lesser time and cost, along with an increase in the results quality. A city modeled according to planning regulations usually present a correlation between plots and buildings dimensions. The representation of such correlation for a large number of plots requires repetitive work, thus suggesting the use of a computational tool to perform the task. Existing CAD, GIS, and VR software can generate accurate representations of the reality, but have no capabilities to simulate the impact of alternative urban regulations for large number of plots in a short period of time. CityZoom is a Decision Support System for urban planning, with a specific builtin city model, where data is represented in an object-oriented model representing the urban structure. CityZoom not only provide CAD tools, but a shell where different performance models can operate iteratively. It can simulate given urban regulations applied to a set of urban plots, as well as address environmental comfort issues such as shadow casting between buildings. Results can be displayed as tables, graphs, and in a 3D preview of the whole city or part of it. It's also possible to export them to commercial GIS tools, to perform different data analysis. The graphical outputs make for an easy understanding of the results by laymen, an important feature for participatory planning, while the display of the correspondent numerical data enable correlations with indicators and parameters of urban quality.