The proportion of planning activity that concerns existing buildings is increasing steadily throughout Europe. In Germany it is estimated that already two thirds of architectural planning tasks are related to revitalisation, modernisation, conversion and reconstruction. For these types of projects in particular, clients are interested in obtaining an estimate of the building costs as early as possible. Because many details are unknown or can only be guessed, it is often difficult to make a reliable estimate. This article describes a software concept for the rapid, sketch-based capture of data on site with a view to obtaining a cost-assessment for a series of different revitalisation variants using a stochastic approach.