Software design models are routinely adapted to domains, companies, and applications. This requires customizable consistency checkers that allow engineers to dynamically adapt model constraints. To benefit from quick design feedback, such consistency checkers should evaluate the consistency of such changeable constraints incrementally with design changes. This paper presents such a freely customizable, incremental consistency checker. We demonstrate that constraints can be defined and re-defined at will. And we demonstrate that its performance is instant for many kinds of constraints without manual annotations or restrictions on the constraint language used. Our approach supports both model and meta-model constraints and was evaluated on over 20 software models and 24 types of constraints. It is fully automated and integrated into the IBM Rational Software Modeler tool.