[Alberta's Child]

I never heard of an HOA that wasn’t mandatory. In every situation I’ve been involved with, the HOA “membership” is written into the property title and can’t be changed unilaterally by the property owner.

[Ultra Sonic 007]

Usually, the only time I've seen a "voluntary" HOA is when the homeowners of a particular subdivision or development in a rural area combine resources to collectively maintain a private road or water/sewage services (especially if they're tied to a group well or septic systems).

In a rural area, this sort of "voluntary HOA" doesn't go as far as having CC&Rs filed for recording.