Modelling of interoperable and extensible design information is one of the key issues in computer-aided architectural design. IFC technology provides standardised mechanisms for the development of such information models. By using the IFC dynamic definition extension mechanism this paper presents a method for IFC-compliant design information modelling for architectural CAD objects. The method has been implemented as an addon toolkit to the Architectural Desktop CAD package. The use scenarios of the toolkit are discussed in the paper for CAD property modelling, property database management, and interoperable design information modelling with property set extensions.