Google SketchUp is an easy-to-use modelling application that is particularly suitable for architectural models.
Reference Guide: http://sketchup.google.com/support/bin/answer.py?hl=en&answer=116008
SketchUp SDK: http://sketchup.google.com/community/developers.html
Unity Scale Calibration
1 meter in SketchUp = 1 unit in Unity.
Import the textures as assets into Unity, and apply them to the materials sketchup imports.
Exporting to Unity
If you discover that faces are missing in Unity, switch SketchUp into monochrome display mode (Edit→Face Style→Monochrome). In this mode, faces will appear white on the outside and blue on the inside. If you have any outward-pointing faces that appear in blue, right click on them and choose Reverse Faces from the contextual menu.
Export into Unity using FBX format.