When using the ruler (measurement tool) or the dimension tool it adds a milimeter to the measurements and I don't know what is adding that extra milimeter.
If the outside measurements of the building is 5000 mm with walls 250 mm thick and a inside wall 120 mm thick somehow the inside measurements become for example 2631 mm. I've checked that all the walls are 90 degrees and that I measure exactly from the edge of the wall. There are no other objects but the walls and the foundation.
In Project Preferences > Dimensions I have a dimension setting that displays Linear Dimensions using Millimeter units to 1 decimal place and the "extra accuracy" to "01". Using this option either during development or through the quick view settings you can make see immediately where errors are in the drawing accuracy.