Shadows are either on or off.
With the exception of morphs (and possibly some objects), you can't set if elements will cast or receive shadows.
So unless the neighbouring houses and fences are morphs, I think you are out of luck.
One of the forum moderators.
Versions 6.5 to 25
Dell XPS- i7-6700 @ 3.4Ghz, 16GB ram, GeForce GTX 960 (2GB), Windows 10
Dell Precision 3510 - i7 6820HQ @ 2.70GHz, 16GB RAM, AMD FirePro W5130M, Windows 10