Front porches are wonderful inventions; they provide an intermediate buffer zone that let you actually build more closely to the street, and they help bind neighbourhoods together. They are also often hit-or-miss; sometimes they work and sometimes they just sit there empty.
Steve Mouzon at The Original Green channels his inner Dr. McCoy as a qualifier (I'm an architect, Jim, not a social scientist!) but comes up with a formula for getting it right, changing what used to be an art into a bit of a science.
He describes his research:
For years, I measured porches and their relationship to the sidewalk. I looked for signs of life, not just furnishings... things like a coffee mug or newspaper that had been set aside when someone called, or a child's toy. What I found was that there is a clear distinction between porches people will sit on and ones they won't, and it's based on how close the front edge of the porch is to the sidewalk, and how far above the sidewalk it is.
He concludes that as the porch gets closer to the sidewalk it must also get higher or people won't sit on it, they feel too exposed. Mouzon suggests that it is "hardwired into the human mind," as are other factors like the height of hedges and the relationship between hedges and the height of the porch. It is important to get it right:
When you design a porch that is usable as an outdoor room, then it's a useful part of the living space of the house. And it's usually some of the least expensive space in the house because you don't have to heat and cool it, and it doesn't have walls or windows. But if it's not useful as living space, then a porch is just very expensive decoration.
More at Porches, Walkability, and Sustainability at The Original Green