The easiest way I find is to place a guide segment from the corner of the building perpendicular to the boundary first and then do the dimension.
It is a little easier in 19 as you don't need to place the guide segment.
Activate the guide snap (blue circle) on the corner and another guide snap (blue line) on the boundary and now when you dimension you should get a perpendicular snap guide.
The trick is you have to place the guides before you start to dimension.
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Barry.
One of the forum moderators.
Versions 6.5 to 27
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