I couldn't follow your description because I'm not sure what parts of the yard you are referring to. Regardless, the issue seems to be that you have neighbors 30 feet away who can look from their upper windows into some part of your yard - front, back or side.

I know that people have different levels of need for privacy, but a 20-foot privacy hedge to screen out the occasional glance from an upstairs window seems kind of extreme. It is possible that a single well-placed tree could give a sense of privacy while not giving your yard a fortress-like look that will block the sunlight for you and possibly some unfortunate neighbor.

Thanks Graywings for your response. I'll try to clarify this:

Our house and garage form one side of the space. Adjacent to the house/garage there is a yard space that is about 10' wide x 40' long, with a 6' fence that frames the space. (e.g., it's a rectangle with the house/garage on one side, and fence on the three other sides.) We are on a corner with the street/alley on two sides beyond the fence and a park beyond the street on the one side. Our neighbor's house faces the park; the side of our house faces it, as well as the long side of the space in question. The park is on a sloping hill, which segues to homes that sit about 40' above our lot on the other side of the park. It is very windy here, too, so a plant that would break the wind would be appreciated, especially in winter. I'd like to be able to create a small, private space in this area, right now it feels very exposed from all directions.

Behind our lot, about 30' from our fence, is our neighbor's front porch, which is elevated about 5', so their porch floor is at level with the top of our fence. Their porch, and their windows, look directly into our yard and at the back of our house (which is our family room, our bedroom, and our bathroom.) Their lot/house sits perpendicular to our lot, so the entire side of their house faces the back of our house; most of their windows face our yard/house due to floor plan configuration.

Because we are on a corner, there are no height constraints, 20' is fine, due to the garage/house heights nearby. We do need some year-round screening here if at all possible; an ornamental tree is not enough. I am not concerned about blocking sunlight in this case, because most of the direct sun comes from a different exposure and would be unaffected by this hedge/tree/whatever.

The opposite side of our lot, on the other side of the garage and facing their lot, has a viburnum hedge and a large maple tree that is great for privacy in the summer. The neighbors in question have planted leylands inside their back yard facing that side of our back yard/house, presumably to block our direct view into their yard from our back door. We're rather on top of each other here, due to small lot size.

Hope this clarifies a bit.

How about a row of sky pencil hollies? They are narrow and evergreen. Once established they will grow fast.