The purpose of this paper is to study on the architecture and extension project for the Church of St. Lorenzo. This study is composed of a through analysis of the extension elements on the church building and extension type for extension project of the Church of St. Lorenzo. The results of study are as follows: 1. The extension project for the Church of St. Lorenzo was basically consisted of the concept of a horizontal extension plan through both the interior and exterior space of the church building. This project was the plan of concept of additional or affixing extension through existing spaces with necessary spaces to make up for the previous simple space and form as well as functional aspect. 2. The unit spaces of the nave, the aisle, the small chapels bilaterally adorning the aisles, the transept and the chapels around transepts, and the sacristies by the extension project for the Church of St. Lorenzo is composed of regulation, balance and harmony as geometric space composition according to simple proportion system on whole and parts as well as parts and parts, through the definite articulation by each space zoning. 3. The most important innovative aspect different to previous churches in the extension project for the Church of St. Lorenzo is the compositive system of each other symmetrical spaces through long centrical axis of central part, applying the module system for horizontal arrangement. In particular, the use of regular module on repetition and proportion of rhythm from architectural composition system was precisely composed with the alter that consists of visual focus to express more accurate perspective.