mstg,
There is a very simple method that I employ to resolve your problem. I use it regularly to estimate gravel stockpiles.
Firstly you need a reference scale which will provide you with a benchmark from which all other measurments will be based, so the more detailed the scale the more accurate your results will be. it is best to scan the photo and enlarge to increase the level of detail. At the focal point of the picture (that which is closest to the photographer, pick out features from which a known measurement can be derived (bricks, a car, a person standing, a parking meter), something from which a known unit can be assigned.
Now, and this gets tricky, estimate the two perspective angle of the building, one perpendicular from the photographer (in planview) and two, perpendicular from the horizontal plane. You will use basic trig to determine distances relative to the point of photography.
If this seems like a daunting task, you can alternatively calculate a distance factor by employing the same "scaling" technique at each of the far corners of the building. Things like windows, door frames, sidewalk expansion joints, etc., can provide a good measurement at the "perspective" corner. Then you calculate the distance from physically measuring on the photograph from the "photographer" point and apply the scale factor. For instance, in the photograph, window with a known width measures 4 mm at the photographer, 2mm in the middle and again 1 mm at the end.
Knowing the dimension of the window, you can now calculate a factor for the rate of change of the scale. This will provide you with rough estimates.
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