Question 1160529
If this is drawn, the critical angle of elevation is when the tangent of theta is  < 15/13
One can take the inverse tan of that fraction, which is 49.08 degrees, so that any sun angle higher than that would produce a shorter shadow.

Or, look at the tangent 50 degrees with the building= opposite side and find its shadow
The is shadow=15/tan 50=12.59 m
It just misses the garden. 

The sun itself is a 1/2 degree in diameter (slightly less in summer), but even that would not change the result.