A PECAS land use transport model is being developed for the Baltimore Metropolitan Region. This model includes a representation of the actions of real estate developers in the modification (development, expansion, demolition, re-development) of floorspace. This is a key element in the entire PECAS model, providing representation of the evolution of the built-form, which is a major influence on the locations of activities and on the travel and transport between these activities into the future. The PECAS model simulates the evolution of the urban system in one-year steps. The space development component simulates the changes in space arising with developer actions in each time period from one year to the next. A microsimulation framework is used, providing explicit representation of developer actions at the level of individual parcels of land, and sub-divisions of these parcels in cases where the parcels are relatively large. Each parcel is considered in turn, with the developer action in each case determined using a Monte Carlo process where the selection probabilities are assigned using a nested logit model. The upper level logit model considers space type. The potential actions include keeping the space the same, changing the quantity of space or changing the type of space to one of the types permitted by the zoning rules for the parcel. The lower level logit model considers space quantity using a continuous logit formulation. The potential actions span the range of allowable quantities according to the zoning rules for the parcel.