All over the world, local authorities are moving towards managing and storing urban data in digital form. But the data storage devices used are heterogeneous and typically include relational database management systems (DBMS), GIS and CAD files. As a result, data are present in different locations on different platforms and under different schemas. This poses a problem for software applications meant to support decision making in urban design that require input from more than one data source. We demonstrate in our paper how data warehousing—combined with object-oriented data modeling—is able to provide a general solution for this problem. Data warehousing is a technique initially developed for business applications, but is equally useful for urban design: The data warehouse constitutes a communication layer between the urban design applications and data sources. It makes the data available through a unified interface that hides the sources themselves and represents that data in terms of a general-purpose, preferably object-oriented, model. We also describe an implementation prototype that supports different applications. The City of Makkah in Saudi Arabia provides us with real-world data and a context to test our prototype.