Thuja plicata 'green giant' is a variety of western red cedar that's becoming much more available due to its usefulness. It is a large fast growing evergreen similar in appearance to its cousin Thuja occidentallis (Northern White Cedar or arborvitae), but has a greater tolerance for shade and grows faster. It seems to be rapidly replacing Canadian Hemlock due to the latter's problem with Wooly Aldegid. You should be able to get these in large sizes (10'-12') without too much difficulty if the wallet allows. Good luck, changes like that in our home lives are tough to take. I have to sympathize on both sides as I would really like to throw a second floor on my tiny house. I'm sure my neighbors to the rear (with the pool) would be thrilled. I should worry about landscaping the place first, though.

Seconding Arborvitae for your area (I'm in northern Illinois, about an hour from you). Proper name: northern white-cedar --- Cupressaceae Thuja occidentalis L Cultivars: 'Aurea' - A golden globe that needs little shearing. 'Boisbraind' - A plant 10 feet tall with dark green, burn resistant foliage. 'Brabant' - This cultivar tends to form one central leader thus minimizing stem damage from winter snows. 'Brandon' - A fast-growing, cone-shaped plant about 15 feet tall and 4 feet wide. Reported as suitable for use on harsh sites. 'Chalet' - A narrow plant with a rounded top and lime- green foliage. It will be 10 to 15 feet tall. 'Compact American' - A compact, pyramidal form reaching a height of 25 feet and a spread of 8 feet. 'Danica' - A slow-growing, compact globe with dark green foliage. 'Degroot's Spire' - A narrow, columnar form. 'Douglasii Pyramidalis' - A dense, columnar form with twisted dark green foliage. 'Elegantissima' - A slow-growing, dense plant with yellow branch tips. 'Filiformis' - A pendulous plant with long, thread-like branchlets and bright green new growth. 'George Peabody' - A broad, upright plant with yellow foliage. The foliage becomes more orange in winter. 'Gold Cargo' - A cultivar with gold foliage that is reported to resist winterburn. The plant can be 25 feet tall and 6 feet wide. 'Golden' - A broad, upright form with bright yellow new growth. 'Golden Globe' - A uniform oval with foliage a soft yellow. 'Hetz Midget' - A slow-growing, compact plant. 'Hetz Wintergreen' - A narrow, upright form reaching a height of 60 feet. 'Holmstrup' - Dark green foliage that holds its color in winter. Broadly pyramidal. 'Little Gem' - A globe-shaped plant reaching a height of about 3 feet. 'Little Giant' - A slow-growing, globe-shaped plant with bright green foliage all year. 'Masonic' - Pyramidal form about 25 feet tall and 5 feet wide. 'Niagara Compact' - A globe-shaped plant more compact than 'Woodwardii'. 'Nigra' - A symmetrical form with good dark green foliage color. 'Pendula' - An open, weeping form with dark green foliage. 'Pygmy Globe' - A dwarf, globe-shaped plant with bright green foliage. Needs little shearing. 'Pyramidalis' - A narrow, columnar plant often used for screens. 'Rheingold' - Yellow foliage that becomes orange yellow in winter. 'Sherman' - A pyramidal form reaching a height of 30 feet and a spread of 12 feet. 'Sherwood Moss' - A dwarf plant 4 to 6 feet tall. 'Smaragd' ('Emerald') - Narrow habit with glossy green summer color. Reported as resistant to spider mites. Plants will be about 15 feet tall and 4 feet wide. 'Sudworthii' - Upright, broad plant with yellow foliage. 'Sunkist' - A compact, broad plant with orange-yellow foliage. 'Techny' ('Mission') - A fast-growing plant with dark green foliage. Usually about 15 feet tall. 'Tiny Tim' - A compact, upright mounding plant. 'Umbraculifera' - A rounded, irregular plant, nearly 3 feet tall and 5 feet wide. 'Unicorn' - A narrowly compact plant with dark green foliage. 'Wareana' - A slow-growing, broadly upright plant. 'Wintergreen' - A pyramidal form reaching a height of 30 feet that retains good green color in winter. 'Woodwardii' - A rounded form that retains its shape without shearing. 'Yellow Ribbon' - A narrow, upright habit is combined with yellow foliage all year.

It depends some on your dog, but I wouldn't count on a hedge to keep a dog in. At the least it will need a wire mesh fence in the inside of the hedge to keep the dog contained until the plants grow together. Even a well grown, mature hedge may have some gaps near the base which a dog could slide through, so it may continue to need a wire fence for the first two or three feet. This request may be a bit of a problem: "shaped like a box (not spherical and not pyramidal)" since in order to maintain dense foliage near the bottom of a hedge, you will need to taper it some so that the upper parts of the hedge don't shade out the lower parts of the hedge. To maintain a boxy tapered shape rather than a more rounded shape will require pruning to give you flat sides. Do you have deer? If not, there are forms of Thuja occidentalis that will stay shorter and will grow together to make a good hedge. I don't personally have experience with how they take to pruning since I like my plants unpruned for the most part. I have 'Sunkist' (green-gold) which will be easy to maintain in the 6'-10' range for quite some time based on mine as they seem to have slowed considerably at about 5'. I bought plants in the fall of 2007 at about 3'. As individual plants they are a bit floppy in heavy snow/ice situations, but that may be less of an issue with a hedge. 'Techny' is dark green and grows to 10'-15', and I have seen photos of it pruned into a hedge. I planted mine at about 4' in the fall of 2007 and they are about 6' now. I imagine that you can find other selections of this native evergreen that are dwarf in size compared to the regular species (which grows to around 40'.) If you have deer issues they aren't a good choice, but in my heavily wooded rural area, they haven't been bothered. spring 2010 From evergreen shrubs spring 2014 From evergreen shrubs Another possible choice to look into is a really hardy Rhododendron, though they will have a more rounded form and IMO don't look great pruned into boxy shapes and will make a more gently flowing shaped hedge. They will have the added benefit of spring flowers if you don't prune them. In really cold weather they don't provide a lot of sight privacy since the leaves tend to curl up, but most folks don't require a lot of privacy when the temperatures are below 15 degrees. The link below will take you to a search page from the American Rhododendron Society which allows you to put in height preferences. On left side of photoPJM rhododendrons in 3/4 shade (so not as dense as full sun,) about 5' at 6 or 7 years in the ground from >1' when planted. From evergreen shrubs You could ask this question on the shrub forum or the conifer forum for other suggestions beyond what you get here. It also might be worth visiting local nurseries (not big box stores which often have poor or inaccurate info and plants that may not be hardy here) to see what they have in stock that would fit your requirements. They could most probably order more to complete your hedge as it will take a rather large number of plants. Here is a link that might be useful: Rhododendron Plant Search

Wax myrtle could be a good option in your area if soil is neutral or acidic. Many culinary/(folk?) medicinal uses and I don't recall hearing of any toxicity issues for children or pets. Nursery person should be able to confirm. Loropetalum is another that might work. Most of the newer named cultivars will be smaller than what you're looking for. But the unnamed varieties found at the big box stores can usually make a large shrub or small tree. Major ice storms can be hard on these, but are pretty good about recovering.