To find an Extension builder, you can start by searching online for architects in your area. Many builders have websites that include information about their services, experience, and portfolio. You can also use a directory website that lists builders and their contact information. Additionally, you can ask for recommendations from friends, family, or other professionals who may have worked with architects in the past. When choosing a builder, it is important to consider their experience, qualifications, and portfolio, as well as their communication and design style. It is also a good idea to meet with multiple architects and compare their proposals before making a decision.