Folksinging started as a way to tell a story with music and now that music is largely focused on ticket sales and "draw," the intimate exchange between songwriter and audience is limited. House Concerts are a chance to hear the tales that never get told onstage, to chat, and to acknowledge our collective consciousness through the age-old tradition of storytelling and face-to-face communication. If I'm coming to your area, I'd love to play in your backyard or living room for 15 people or 100 people. I like to have pot lucks at these things and (if possible, but not necessary) with an emphasis on local, organic, vegan foods to promote a healthy lifestyle.
As far as planning, there's not a whole lot to plan. Letting everything go with flow is the best way to approach it. It helps to give people 1 hour to chat and eat before the music starts. The set can be as short as 50 minutes or as long 2 hours, depending on the vibe of the room. Usually somewhere around where intermission would be the host makes a small announcement about making sure I've got cash to keep me going on my journey and we "pass the hat" for donations, but you can do that at the door too.
No space (even an apartment) is too small
You choose the people invited (make a facebook event, choose a start time, email yer buds about it)
Hosting costs you nothing (artist costs are covered by a suggested "pass the hat" donation of $10-20 )
Optional Pot-Luck or BYO event (encourage people to bring their own drinks)