Expansion and modification processes are ordinary matters that applied to many houses in Bangkok, Thailand. Evidences are depicted in various scales of construction, from floor extension to the modification of house structure, in order to accommodate with the new function. Although, there are studies that addressed the topic of expansion in architectural field but those studies have not focused on the issues of users’ satisfaction toward the expansion and modification process. From the study, there are various types of existing structure which can affect the modification process. However, in this paper, the author categorised the existing detached houses in Bangkok into 3 different categories; 1. Interlocking and joint 2. Masonry and concrete casting 3. Prefabricated load bearing wall and modular unit. The purpose is to contemplate the future development with suitable construction techniques. There are data collecting from 30 house expansion projects from detached house unites in Bangkok. These study subjects are selected alongside with the questionnaire survey from people who are involved with the construction process. The questionnaire surveys are administered with 1. Detached house owners 2. Architects/Engineers 3. Contractors, in order to figure out the pros and cons of the existing structure toward the expansion process. Nevertheless, the surveys have shown, the prefabricated load bearing wall and modular structure is difficult to modify. The reason is about the inflexibility of the structure. Therefore, it is needed to think about the new possibilities of the prefabrication structure for the future modification of detached house. In contrast, conventional structural system such as 1. Interlocking and joint 2. Masonry and concrete casting, provides more flexibilities in expansion and modification. However, these conventional structural systems require expert to control expansion quality. Purpose / Context – This paper conducted to find better solutions of expansion and modification process of detached houses in Bangkok, alongside with the improvement of existing structures. In order to provide the design possibilities which can accommodate the future modification. Methodology / Approach - This paper categorised existing detached house structure into 3 different categories; 1. Interlocking and joint 2. Masonry and concrete casting 3. Prefabricated load bearing wall and modular unit. Therefore, to analyse the random selected 30 expansion projects to detached house units in Bangkok. Together with questionnaire surveys involving parties in expansion process. In order to investigated pros and cons of existing structure toward expansion projects. Results – The study reveals prefabrication system has the most difficulties with the future modification. Due to the fact that there is inflexibility of the structure itself. On the other hands, conventional system such as 1. Interlocking and joint 2. Masonry and concrete, provide more flexibility in modification. However, house owners still need to consult with engineer. The quality of long-term expansive construction requires expertise. Key Findings / Implications - The research focus on the advantage and disadvantage effects of various existing structure of detached houses toward modification process. Originality – The research contributes to improve current expansion and modification of detached houses in Bangkok. This study also states in the potential status of structure, expanding material and modification construction process.