The most Blender useful addons for architectural design

• Archipack - Add windows and doors - parametric and easily modifiable
• Tiny Cad meshtools - intersection between two edges

External Free Scripts

• Sun position - precisely locate your project - google maps gives you latitude and longitude of all places
• Offset Edges - useful for offsetting edges - more like we would expect as opposed to trying to scale
• NP Station - ability to move and align objects precisely
• Sure UVW Box - map materials without worrying about precisely mapping UV coordinates - similar the 3ds Max's UV box modifier
• BlenderGIS - create a surface from countours

External Paid scripts:

• OSM Data - $5.90 - import Open Street Map data directly into Blender