I have a 24x14 polytunnel.
I got mine from Northern Polytunnels 8 years ago and still have the original cover on in perfect condition,best thing I've ever ever bought..
I'd definately recommend burying the polythene in a trench as this way it makes the whole structure stronger and you can get the polythene much tighter.
Make sure you get the thickest guage possible steel hoops and don't get them any more than 6 feet apart.
And get one which has straight sides at least 2ft6 - 3ft before the curve starts otherwise you lose so much growing space.
Anti hotspot tape is essential as it protects the polythene from the steel hoops,heat wise and wear wise.
I got Thermal anti fog polythene which holds more heat in at night and the condensation doesn't drip,it runs down the polythene instead.
I made my own doors easily out of 3x2 treated wood as the companies charge an arm and a leg if you buy them off them.
And make sure you get one which has double doors at each end otherwise it will be like an oven in summer
I personally prefer raised beds so you never end up digging too close to the polythene which can slacken it off.
They do get very hot in summer so you can get some with short net vents about 1ft high all around the base but then you can't trench the polythene in.
If you decide what you want and know what spec you require then I'd get Northern Polytunnels to give you a quote.
I couldn't find anyone any cheaper when I was looking.
That's about it.
Hope that helps a bit.
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