Answer: The best spacing will depend on how wide you want the planting to be -- whether or not you plan on trimming the trees to keep them short and need to have the width be in proportion to a reduced height, or if you want to let them go to their full mature height and look "natural" over time. Leyland Cyprus is a fast grower but can break apart in wind/ice storms, so it may be that you will have some natural trimming happening over the years. If you want to leave them untrimmed, spacing them fifteen feet apart should leave a small gap between the individual trees. At ten feet, they should overlap and touch at the branch tips. At six feet, they will soon form a solid hedge effect but will be somewhat crowded as they age; this can seem disproportionate unless the height is also restrained.
Generally speaking, it is not a good idea to plant at/on the property line unless you are certain the long term ownership on the other side will allow the trees to overhang their property. You might check with owner of the common area and their maintenance staff to be sure they will allow this encroachment over time -- it might interfere with their mowing or trimming efforts, for example, and then they would be within their rights to trim the trees back to the property line. This would be stressful for the trees if they are planted on the line.
Speaking as a designer, you might consider planting them in an informal cluster or grouping rather than a straight line. This might blend better visually with the less formal area behind them. It will also help mask any nonsymmetrical growth.
The other consideration I should mention is that these trees will not thrive in an area that is constantly wet or extra moist or swampy. They need a well drained soil. Based on your description, if these are going in a low spot next to an area that grows cattails (these need wet soil to survive and are an indicator plant for swampy areas) then the spot may be too wet (in a non drought year) for these.
I hope this helps!
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