I am aware that anyone who is over 18 should get a plus one whether they are single or not... and I plan on doing that.
My question is, if these people still live with their parents I have to I send an entire separate invite or send one invite, addresses the envelope to Mr. & Mrs. Head of household and family and in the "X number of seats reserved for you" include that plus one.
For example, I'll be inviting a friend of the family. Husband and wife SMITH for sake of conversation. Mr. & Mrs. Smith have two college age daughters (A Smith and B Smith), single but I'll be giving a plus one to each. So can I address the envelope to Mr. & Mrs. Smith and family? Then on the invite say "we have reserved 6 seats in your honor..."
It just seems redundant to send three invitations to the same household. There are only a handful of people with these circumstances, and I'm close enough to call them all and clear up any confusion.