The paper explores a number of issues related to lot size and urban density. First, trends in single-family residential lot size over the past 35 years are examined in eight counties in the state of Maryland. We find that there was a trend toward larger lot sizes in many suburban counties in the mid to late 1990s, and that there has been a general flattening of the density gradient in urban areas over the last few decades. We then examine the extent to which lot size is being constrained by regulation by comparing actual subdivision density to the allowable density under zoning rules. This analysis is done for three counties with different degrees of suburbanization. We find that only in the areas with the very large lot zoning does zoning seem to be constraining actual lots size. There is a good deal of excess capacity in the density that could be built, especially in the more densely zoned areas. Finally, recognizing that households have preferences for lot size and other housing characteristics, we provide some evidence about the strength of household preferences over lot size and their willingness to trade off lot size for other characteristics.