Software development is a critical task that require comprehensive guidelines in the form of software development process model. Plan driven software development models were the earliest models that mostly provide sequential development approach. With the passage of time, changing business environment, needs and technologies made these models inappropriate for software development.Agile models were emerged to overcome the limitations of traditional plan driven software development models.These models received a huge acceptance from software industry due to their distinguishing features and practices. Extreme Programming is one of the renowned agile model that uses agile practices to handle unclear and changing requirements with good level of customer satisfaction.It uses iterative and incremental approach to develop a quality software in considerable short time duration. It works well for small scale projects however some of its limitation make it less suitable for medium and large scale projects. Lack of documentation, poor architectural structure and less focus on design are its major limitations that effect its performance. Furthermore some of its practices like on-site-customer and pair programming are a bit controversial that cannot be applied in every situation or may overburden development process when applied. To overcome these limitations and make XP suitable for medium scale projects, a customized version of XP called SXP (Simplified Extreme Programming Model is proposed in this research.Proposed model tried to cover these problem without effecting agility and simplicity of XP. SXP model introduced explicit analysis and design phase to solve design related issues. It also removes unnecessary norms of pair programming and on-site-customer practices. For validation and evaluation of this model a controlled case study is conducted. Two different teams developed medium scale projects using classical XP and Proposed SXP model respectively.Comparison of data collected against different quality parameters showed that SXP model provide considerably better results than classical XP.