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Start With the Space You Actually Have

Before you look at a single catalogue page, walk the plot with a tape measure and a notebook. Measure the longest and widest rectangle that is genuinely free, not the patch of lawn that looks about right. Then subtract a working margin on every side — a minimum of 60cm, and 75cm if you want comfortable access for re-tensioning a cover or clearing guttering. Polytunnel covers need tightening and, eventually, replacing, and a frame jammed against a fence or hedge is a misery to maintain.

Take note of the surroundings, too. A north-facing wall casts shade for much of the winter, while a gap between a shed and a boundary hedge can funnel wind straight into an end wall. Slopes matter more than most people expect: a 1-in-20 fall across six metres is 30cm of level change, enough to leave one corner sitting in a puddle. Note the dimensions, slope and prevailing wind before deciding anything else.

Width, Height and the Door That Fits You

Standard polytunnel widths tend to fall around 2.4m, 3m, 4m, 4.5m and 6m, with ridge heights rising accordingly. The width is usually decided by how many beds you want side by side, but the height is decided by you and your crops.

Door height is the specification most often regretted. Many kits ship with a 1.8m doorway, which is fine until you are pushing a loaded wheelbarrow, carrying a water butt or growing cordon tomatoes that reach the ridge. If anyone in the household is over 1.75m, look for an opening of at least 1.9m, ideally 2m. Side wall height matters just as much. A tunnel with 1.5m sides is generous over open ground, but if you plan to garden in raised beds 40cm high, you have just lost most of that clearance. Add the bed height to your own working height and check the numbers add up.

Ventilation Is Not an Optional Extra

In the UK's damp, changeable climate, poor ventilation causes more polytunnel problems than cold does. Still, humid air and fluctuating temperatures are a recipe for botrytis, blight and damping off. As a rough rule, aim for ventilation openings totalling at least 15 to 20 per cent of the floor area, split between both ends and the sides.

• Sliding or hinged doors at both ends give a through-draught on still days.
• Side louvre vents help on hot afternoons when doors alone are not enough.
• Roof vents or roll-up side panels release the hot, moist air that gathers at the ridge.

There is a practical trade-off here. A larger tunnel is easier to ventilate relative to its volume, because the air has more room to move and heat builds less sharply. A small tunnel swings from cold to stifling within an hour, so it needs proportionally more opening area. If you are choosing between a 3m and a 4m tunnel and the site allows it, the wider one is often the kinder choice.

Plan Crop Rotation Before You Order

Rotation inside a polytunnel works differently from the open veg plot, because the structure stays put and the beds rarely move. What changes is what you grow in each bed, season after season. Sketch your beds on paper first: a bed width of 1.2m suits most people, since you can reach the middle from either side, and paths of 45cm to 60cm are comfortable for kneeling and for a wheelbarrow.

Two 1.2m beds with a 45cm path need 2.85m, which just fits a 3m tunnel. Three beds need roughly 4.5m, so a 5m or 6m tunnel. Group crops into families — fruiting crops such as tomatoes, peppers and cucumbers; leafy crops; roots; and legumes or green manures — and rotate them around the beds each year. Perennial plantings, such as herbs, strawberries or an asparagus bed, sit outside the rotation and deserve their own reserved bay. Deciding this now prevents the familiar mistake of a tunnel that is beautiful in its first year and impossible to plan by its fourth.

Orientation, Base and Anchoring

A ridge running north to south gives the most even light across both sides through the day, which suits most crops. An east–west ridge catches more low winter sun but leaves one side shaded for part of the day. Think about the doors, too: aligning them with the prevailing south-westerly gives good natural airflow, but on an exposed site you may need reinforced end frames or a windbreak planting to protect them.

Whatever the size, the frame needs a proper base. Timber soleplates or a concrete kerb keep the structure square and lift the cover clear of wet ground, while ground anchors or screw-in stakes hold it down in gusts. Drainage beneath the tunnel matters as much as the tunnel itself, so dig a shallow trench or French drain if the site holds water.

Your Pre-Order Checklist

Before you commit, write down and check the following:

• True footprint, plus a 60–75cm maintenance margin all round.
• Slope and drainage across the site.
• Door opening height and width — enough for you, a barrow and tall crops.
• Side wall height, measured against any raised beds you plan.
• Ventilation area as a percentage of floor area.
• Bed and path widths, and a rotation plan you can live with.
• Space for a water butt, staging and a small tool store, ideally near the door.

Get these numbers right and the polytunnel becomes a structure you work with rather than around. Order in haste and you will spend years ducking, sweating and shuffling beds about. A morning with a tape measure is the cheapest gardening investment you will ever make.