Polytunnel or greenhouse…
…is the first consideration and I found a few useful articles on what to take in to account when deciding which you want. In summary though; size of growing space, site/location, cost and lifespan are probably the key criteria, but other factors such as safety (if you have little ones or animals), type of crops to be grown and access to water/electricity may also influence your decision.
The overview article in Garden Answers was a really good starting point to help make your choice.
Key Considerations
1.Growing Space – not just how much space do you have but also what do you want to grow. If, for example, your interest is propagation from seeds, it may actually be that a lean-to greenhouse on the side of your property is better to suited to your garden and/or growing requirements.
2.Site/location – if you have a north-facing garden you may want to reconsider purchasing either a greenhouse or polytunnel as there’s likely to be limited sunlight in to your plot. After that you need to be mindful of things such as direction of sunlight, slope of the ground, wind direction, access to amenities and maintenance of the polytunnel or greenhouse (e.g. overhanging trees aren’t great for either). A brief explanation of what to be aware of, particularly for polytunnels is given in these two links; http://www.gardentunnels.co.uk/clever-bits/siting-your-haygrove-greenhouse/
and http://www.northernpolytunnels.co.uk/blog/cat/hobby-polytunnels/post/siting-a-polytunnel/
3.Costs – what do you want to afford? Greenhouses tend to be more expensive per metre sq of growing area, but they last longer (therefore the ground a greenhouse is sited on is basically tied up permanently, polytunnels are suggested to last for 10 – 15 years).
Selection
I have decided to get a polytunnel but there are a few more points to consider before buying:
1.Polythene quality for longevity, thickness for insulation and opaqueness. Toby Buckland also mentions a couple of other points: anti-drip coating and insulation tape (to keep the plastic clear of the metal hoops so the heat doesn’t cause the cover to crack)
2.Construction – there seem to be different methods for securing the plastic; ease of and durability to be considered.
Building
Both polytunnels and greenhouses can be self-build but as my 2ft x 4ft lean-to greenhouse took two of us the better part of a weekend to assemble I’m just saying this aspect needs some thought too…
Andy Mckee gives some good overall helpful hints about how to assemble a polytunnel – especially his first comment: ask to see a copy of the instructions before you buy!
General Handy Hints
The best explanation on buying, siting and erecting a polytunnel that I found was Mark Smith’s article – he’s someone who has obviously been through the process from start to finish!
Finally, the following articles also had some useful information if you’re look to research further this buying decision:
http://homefarmer.co.uk/buying-a-polytunnel/
http://www.telegraph.co.uk/gardening/gardeningadvice/8293920/Polytunnel-superpower-for-your-veg.html