This worked for me at least.

Close ark completely and return to the desktop.

Search for the ark .ini files for your game under the disk it was installed in.
\steamapps\common\ARK\ShooterGame\Saved\Config\WindowsNoEditor

[Open GameUserSettings.ini]

Find "OverrideStructurePlatformPrevention=False"
Change to "OverrideStructurePlatformPrevention=True" [this may already be set to True]

Find "PlatformSaddleBuildAreaBoundsMultiplier=1.000000"
Change to PlatformSaddleBuildAreaBoundsMultiplier=3.000000"

Restart ark and return to your single player game, problem should be fixed.

[if for some reason it doesn't, something else I did was park my raft near a foundation on the beach and logged off on my raft. Don't let your rudder touch the sand though or the raft may get stuck].