Use an existing wall
An attached or lean-to greenhouse limits your choices for positioning compared with a free-standing greenhouse. A north-facing site will likely require supplemental lighting, but if you have a south-facing wall, you will save the expense of one greenhouse wall while considerably reducing heating costs, as your house will block north winds. B.C. Greenhouse Builders, based in Surrey, offers a six- by 12-foot lean-to kit covered in twin wall (a double layer of corrugated polycarbonate with channels in between for air) for $2,795, plus freight. A single-layer glass lean-to is $2,350, while a thermal pane version is $5,325.
A greenhouse window is a smaller alternative; it should be professionally installed. Oran Industries, based in Woodbridge, Ontario, can install a 42- by 36-inch Suncatcher Garden Window for about $1,200. Greenhouse windows that include a vent and a shelf start at $1,500 from Vinylbuilt Windows based in Concord, Ontario.
Considering that you can get started with a small greenhouse for less than $50 (Lee Valley Tools has a patio greenhouse for $49.95), and less than $450 for a full-size portable one (Dekbrands, based in Cobourg, Ontario, has an eight- by eight-foot model for $429), the real question is not whether you can afford one, but rather how soon can you get yours started.
Get a jump on spring
The cheapest greenhouses are those that won't be used year-round, but still extend the season. Here are a few options:
Cold frames are great for starting seedlings, shrub and perennial cuttings, and for hardening off houseplants before moving them outdoors. The process couldn't be simpler: just nail together a bottomless box with a slanted, transparent top and place it on the ground with the slant facing south. Traditionally, the cover is simply a recycled window or two with a box built to fit, but you can also build your own top, then cover it with greenhouse film or rigid panelling.
This is simply a heated variant of the cold frame. The frame of a hot bed is higher because it needs to be dug into the ground at least 15 to 20 centimetres to keep cold air from seeping in around the edges. Traditionally, manure is poured into the bottom as a heat source or you can use a heating cable hooked up to a thermostat and covered in sand. Because of the extra heat, you can plant out a hot bed by mid-winter or use it to overwinter not-quite-hardy plants. Don't expect 22˚C temperatures on a cloudy day in January, but you can maintain minimum temperatures in the 4 to 10˚C range, ideal for overwintering bonsai, cacti, indoor azaleas and most subtropical plants.
There has been an explosion in the availability of portable greenhouses, from a simple hoop-type row cover, ideal for growing tomatoes and peppers in the North, to all manner of temporary greenhouses, some knee-high, some tall enough to walk into. Most kits include a solid but lightweight frame (aluminum, wood or PVC piping) with a greenhouse film or polycarbonate covering. Set it up in the spring to start your vegetables or annuals, then take it down in the summer (you'll more than double the life of your covering if it is only used a few months of the year). Set it up again in the fall to extend the season.
Finally, even smaller yet, are the most temporary structures of all: cloches. Place these over annuals and vegetables when planting them before the last frost date.