Apartments have been the most prevalent type of housing in Korea for the past few decades. During those times, apartments have emerged as one of the most attractive real estate properties in Korea, with the still growing population living in them. Now that over twenty years have passed since the 1970s and 1980s when apartment construction started booming, building stocks are increasing that need refurbishment. In this concern, apartment extension has raised public interest for the past few years, as the remodeling of them has mainly been considered in terms of increasing asset value in Korea. It comes through in our study that these extension types are examined by such items as building core types, number of rooms and front and rear bays in unit, and other design features related to extension techniques. The objective of this study is to suggest a way to produce the best quality possible of unit plan through extension.