A system is said to be extensible; if any modifications can be made to any of the existing system functionalities in addition of new functionalities with minimum impact. To attain extensibility, it has to be scheduled properly starting from the initial stage of the system development life cycle. Keeping in mind all the probable upcoming modifications to be made, the software designer should select the appropriate design patterns and finish the design for the application. As soon as the application design is finished, it must be examined to make sure that the application is extensible. This research paper recommends a research framework for Extensibility evaluation process and does an extensive review on extensibility of object oriented software. A metrics based Extensibility Evaluation Model for Object Oriented Design" has been recommended by creating the relationship in the middle of design properties. Consecutively researchers justifying the model correlation with the help of statistical measures, which shows that for all the System, Coupling, Cohesion, Inheritance, Polymorphism are highly correlated with Extensibility. The developed model has been authenticated by experimental tryout. Finally, it incorporates the empirical validation of the extensibility evaluation model as the author"s most important contribution. In Conclusion, Hypothesis testing is performed to test the significance of r (Correlation Coefficient) using 2-tailed test for a 95% confidence interval with different degrees of freedom. As a result, the researcher"s claim of correlating Extensibility with Coupling, Cohesion, Inheritance and Polymorphism at design phase is statistically extremely justified.