Robots operating in a constructed facility must possess knowledge about and a description of their environment. This information is organized into a domain model. This paper presents current work investigating issues in developing domain models of constructed facilities for robotic applications using an object-oriented approach. Requirements regarding model contents and functionality are identified and the basic architecture of a modeling system is outlined. An object-oriented approach, with Smalltalk-80 as the implementation language, is used in building the modeling system. Modeling issues under study arise from the descriptions of model objects, relations between objects, and services provided by the modeling system in support of planning activities. Additional modeling issues result from the mapping of the domain into the multi-layered network structure of the model. The paper concentrates on a discussion of these issues and alternative solution approaches to the problems which arise from this object-oriented approach to domain modeling.