A computer simulation of land development subject to constraints is reported for the case of London. The study begins at a time when the ancient street system of the city enclosed blocks of open land which were developed only at the edges adjacent to the street. Despite a large subsequent increase in population, 16th century building was constrained by a series of proclamations and laws which encouraged covert development within the block, accessed by narrow passages. The historical process of development is interpreted in terms of a set of rules governing the evolution of passageway access within the internal segments of the blocks, and the relationship between new buildings and plots. A product initially of historical research and analysis, the development rules were revised and reformulated by the man machine interaction until the computer-generated developments were consistent with those observed in historical records. It was found that the original rules did not give sufficient weight to the street system which was of paramount importance in the spatial structure of the City: the maintenance of access to and from the street is seen as a crucial factor in the spatial evolution of London. The rules governing this particular development were considerably clarified by the computer simulation.