I would start by examining your specific goals for just what it is you want to do with your garden, and how much time, money and effort you want to put forth. Do you want to grow vegetables? Do you want to use the greenhouse to start annuals, perennials or other plants? Many people find it easier to purchase these at nurseries. Are you going to grow some heat loving plants? Some uses will require that you heat the greenhouse as well.

As far as greenhouse vs. hoophouse, you'll have better heat retention with a prefabricated structure whose "windows" (fibebrglass, whatever) are of double thickness than you will with a single layer plastic hoophouse. Prefabricated greenhouses also offer more options for ventiliation, which is important when it gets hot in summer. You will get excellent heat retention in winter with a gravel floor. The downside is that they are more expensive.

The climate of Oregon is not really like England, except perhaps at the coast. England is cooler and breezier, with rainfall evenly distributed throughout the year. The Portland area has warm, sometimes hot summers, and is consistently dry in the summer. All planting should be done with this in mind: summer is very dry.

Other than that, should you get a greenhouse I can only suggest getting something bigger than you think you will need. You will fill it up with plants before you know it!

Portland is closer to England's weather than it is Southern California's or anywhere else in the US. It does not get severely cold here compared to the East Coast or Midwest. The dry summer is usually Mid July through Mid September. Sometimes it is dry and sunny for a couple weeks in February. I would say that most people do not have greenhouses.

Look for books by Ann Lovejoy, she lives in the Seattle area and has written a number of good books. Reader's Digest A Garden For All Seasons is an English book that I just love. Fabulous photos too. The plant information and availibility of the plants is correct for the Portland area.

British books often have plant selections that are scarce or absent here. It's the same as with the climate comparison, British books are generally better for us than those written elsewhere on our own continent but the fit is far from exact.

The difference in summer precipitation regimes, as well as minimum and maximum temperatures between Seattle and Portland (Vancouver is cooler and rainier) and most UK locations IS significant in its effects on gardening. A normal Seattle summer is a 100 year drought in London. 70F in Surrey, England is a heat wave. And British climates include coastal strips as mild in winter as parts of California. One map that has been linked to on this site shows the extreme western coast (Scilly Isles etc.) as USDA 11.

Most people don't have a greenhouse, or a hoophouse, or even a cold frame or sunroom. Luxury items. Wherever you find yourself gardening it's best to grow what doesn't need extreme life support.

A simple cold frame will expand veggie gardening considerably, allowing you to grow greens through the winter and start stuff earlier in spring. Portland is very mild compared to where I am so it may not matter as much for you but our springs are very cold and damp and I often don't even start my beans til July.

Compared to SoCal, Portland is very much like England. But if you have a Sunset Western Garden book that's still useful up here. Just don't bother buying any nationwide gardening books, they have no clue what climate is like here - warm and wet in winter, compared to most of the country, and cool and dry in summer. And way shorter days in winter, so between that and the clouds very little sunlight to heat a greenhouse with. If you do buy a greenhouse you need a different design than most of the country would use as maximizing insolation is much more important here than minimizing heat loss. Charlie's Greenhouses is a local greenhouse supplier.

Most commercial nurseries here just use unheated hoophouses to overwinter plants. Except of course for houseplants and other tropicals.

2 good sources for veggie growing would be Territorial Seeds and the book Growing Veggies West of the Cascades. Both are specific to the Oregon/Washington area.

Don't make any major gardening investments until you've been here at least a year.

If you have a particular interest there's alot of fun to be had. Probably what kills it for many is the upkeep, especially the heating cost.

A highly paid friend who loves tropical orchids has his greenhouse packed with them. Somebody else who got talked into a hobby greenhouse at a home show will be using theirs to store carboard boxes, discarded pots and wheelbarrows. That's the kind of thing most I see walking and driving around are being used for. It is the exception to see one at a residence actually being used for growing plants. Businesses will have an income being generated that pays for the cost of keeping greenhouses.