Polytunnel vs Polycarbonate Greenhouse UK: An Honest 10-Year Comparison

Buyer's guide · 2,280 words · ~10 min read · Published 21 May 2026 by Waldenhaus

TL;DR for the impatient: Polytunnel wins on lowest cost per square metre of growing space if you accept replacing the cover every 5–7 years. Cellular polycarbonate wins on permanence, storm resistance and allotment-policy compliance; the polytunnel wins on raw 10-year total cost. Most serious UK growers move from polytunnel to polycarbonate at the first cover replacement.

We make greenhouses, not polytunnels. That gives us a bias and you should know it. But we also lose customers to polytunnels every month, and we know exactly when we're the wrong answer. This guide is an honest walk through the comparison: what each is genuinely good for, what each costs across a decade, and where the trade-offs actually land.

If you came here looking for "polytunnels are rubbish" content, you'll be disappointed. Polytunnels are excellent for specific use cases and a £4,000 polytunnel will outgrow a £4,000 entry-tier aluminium greenhouse three times over. The real comparison is not polytunnel vs greenhouse. It is polytunnel vs cellular polycarbonate greenhouse, over ten years, in British weather, for the way you actually want to grow.

What we're comparing: exact specs, not categories

The category labels "polytunnel" and "greenhouse" both cover a huge range. To compare fairly, we'll pick representative examples at the same growing footprint.

These are representative figures from the UK retail market in 2026. Individual products vary; we'll come back to the variability below.

Is a "polycarbonate polytunnel" a real thing in the UK?

If you've searched for a polycarbonate polytunnel in the UK, you've spotted a genuine gap in the wording. A true polytunnel is defined by its cover: tensioned polythene film stretched over hoops. The moment you swap that film for rigid twin-wall polycarbonate, the structure stops behaving like a tunnel and starts behaving like a greenhouse, so what people picture as a "polycarbonate polytunnel" is really a polycarbonate tunnel greenhouse: an arched steel frame clad in cellular polycarbonate panels rather than film.

The distinction matters because it changes everything about ownership. Film is tensioned and anchored, so it can lift, flap and unzip; it also clouds and perishes under UV, which is why growers replace it every five to seven years. Twin-wall polycarbonate is mechanically screw-fixed to the frame panel by panel. It doesn't tension, doesn't unzip, and doesn't need replacing on a cycle. You also gain a twin-wall air gap that single-skin film simply cannot offer, which is where the warmer growing calendar comes from.

So the honest answer is: there is no such product as a "polytunnel" with a permanent polycarbonate skin. The two are different categories. What you actually want, if the tunnel shape appeals but the film replacement cycle doesn't, is an arched polycarbonate tunnel greenhouse. Our SteelRoot range is exactly that: a closed 40 × 20 mm galvanised steel arch frame on UK-spec 0.67 m spacing, clad in screw-fixed twin-wall polycarbonate, with galvanised Ground Screw Anchors included. SteelRoot starts from £1,199, and the comparison below shows how it stacks up against a film polytunnel over a decade.

The 10-year cost question

This is the comparison that genuinely matters and the one most buyers don't run until year 5.

For a 25 m² growing footprint over ten years of normal use, the cost comparison looks like this:

On pure £/m² per growing year, polytunnel wins by a clear margin. On cost per growing year with permanence, the gap closes considerably, and there are non-financial trade-offs the spreadsheet doesn't capture (next sections).

But the year-5 cover replacement is the moment most polytunnel owners reconsider the category. A new cover is £200–£400, installation takes a day of work (often two adults), and the next 5-year cycle starts from a brand new clock. Some owners go through three covers; others move to polycarbonate at the first replacement and never look back.

Storm and wind performance: the British weather reality

This is the comparison where the materials genuinely diverge.

UK weather since 2020 has trended toward more frequent severe named storms, with damaging gusts in elevated and coastal regions. Both polytunnels and entry-tier polycarbonate greenhouses are vulnerable in these conditions, which is where glazing retention and anchoring decide the outcome.

Polytunnel failure modes:

• Cover-strip in winter storms: a tensioned film cover that loses one anchor point can unzip across the entire structure in seconds. Threads describing a stripped polytunnel are common in February each year.
• Cover-perish at year 5+: UV degradation thins the film, particularly along the ridge where solar exposure is highest. A weakened cover fails sooner than a new one.
• Hoop deformation in heavy snow: wide hoop spacing (1.5–2 m) means snow loading is concentrated on long unsupported film sections.

Cellular polycarbonate greenhouse failure modes (entry-tier):

• Spring W-clip glazing release: the most common entry-tier failure. Clips corrode, lose tension, and the panel pops out in moderate wind. This is a category-wide problem on the £200–£800 polycarbonate aluminium tier.
• Aluminium frame flex in moderate wind, particularly on wide spans without cross-bracing.
• Inadequate anchoring when ground anchors are sold separately and the buyer doesn't install them.

Engineered cellular polycarbonate greenhouse, SteelRoot specifically:

The SteelRoot answer to UK wind exposure is structural, not absolute. We don't claim "stormproof" because no honest manufacturer can. What we do offer is a four-mechanism engineered build:

1. Closed 40 × 20 mm RHS steel frame: closed box-section resists twist, not the open extrusion that flexes
2. 0.67 m UK-spec arch spacing: closer-arch pattern recommended for snowy and windy regions, against the 1 m default common on Continental retail
3. Mechanically screw-fixed 6 mm twin-wall polycarbonate panels: stainless screws with EPDM washers, no spring clips
4. Ground Screw Anchors INCLUDED: M12/M16 driven straight into typical UK soil, which removes the concrete step

It's the combination that matters: engineered structure plus correct anchoring, not tonnage. The SteelRoot ships at 65–125 kg (depending on length), which is roughly 1.5–2× a comparable entry-tier aluminium frame. That is meaningful, but not the multiplier you'd need if mass alone were the strategy.

Thermal performance: what each does to your growing calendar

Single-skin polythene film transmits roughly 90% of light. Twin-wall cellular polycarbonate transmits roughly 85–90%. On light alone they're close.

Where they diverge is heat retention. The twin-wall air gap in 6 mm cellular PC provides genuine thermal insulation, which counts for early spring (March), late autumn (October), and any attempt at year-round growing. A single-skin polytunnel cools to ambient within an hour of sunset; a cellular PC greenhouse holds 3–5°C above ambient for 4–6 hours after sunset, longer if you've thermal-mass installed (water butts, gravel beds).

What this means for your growing calendar:

• Polytunnel: adds roughly 4 weeks at each end of the season against outdoor growing. Excellent for spring and summer crops (tomatoes, peppers, salads).
• Cellular PC greenhouse: adds roughly 6–8 weeks at each end of the season and makes overwintering of hardy crops (winter salads, garlic, broad beans) genuinely viable.

If your ambition is May to September salad and tomatoes, the polytunnel is plenty. If your ambition is February to November production and overwintering, the cellular PC greenhouse changes the calendar in a way the film never quite manages.

Allotment and council policy: the practical reality

This is a category where polytunnel ownership has become genuinely complicated in UK contexts.

Allotment sites increasingly restrict polytunnels by policy. The 2023–2025 trend has been toward allotment associations banning polytunnels on aesthetic grounds (neighbour complaints), planning grounds (visual impact in conservation areas), or maintenance grounds (abandoned tunnels with shredded covers). Each site sets its own rules.

Conservation areas and listed properties rarely permit polytunnels without specific consent. Polycarbonate greenhouses are typically permitted under the same rules as garden sheds.

Permitted Development in domestic gardens generally allows both, subject to:

• Total outbuilding footprint under 50% of the residential curtilage
• Ridge height under 2.5 m within 2 m of any boundary
• No use as living accommodation

The SteelRoot 3.14 m line sits at 2.02 m ridge height; the SteelRoot 4 m line at 2.30 m ridge height, both well within the 2.5 m boundary cap on most domestic plots. A typical 8'×16' polytunnel sits at 2.4 m apex, also within the cap.

If you're growing on an allotment, check the site rules before purchase. If you're growing in your own garden under PD, both options are usually fine without permission, but if your house is listed or in a conservation area, written consent is sensible regardless of which you choose.

Aesthetic and integration: what each looks like in a real garden

Polytunnels look like polytunnels. The film catches light, the cover ages from clear to translucent to opaque over 5 years, and they read as "growing infrastructure" rather than "garden building." This is genuinely fine for an allotment plot or a dedicated kitchen garden behind a hedge. It's harder to integrate into a formal back garden or a property where street-facing visibility matters.

Cellular polycarbonate greenhouses with steel arch frames sit somewhere between a polytunnel and a heritage glasshouse aesthetically. They're more architectural than a polytunnel, less ornamental than a Hartley Botanic. For most UK gardens, whether suburban, rural, allotment or smallholding, they integrate without drama.

If aesthetic is the primary driver and budget allows, a timber-framed greenhouse (our NORDIC range, or comparable competitors) is genuinely the right answer, not a steel arch and not a polytunnel. The category of "beautiful greenhouse that's also fully engineered" is a different conversation.

Which is right for you: honest framing

Buy a polytunnel if:

• Your budget is genuinely under £1,000 and you want maximum growing area for that money
• You have a dedicated growing space (allotment, smallholding) where aesthetic isn't the priority
• You're willing to replace the cover every 5–7 years and treat it as part of the running cost
• Your growing ambition is seasonal, spring through autumn, not year-round
• You're starting out and want to test whether grow-your-own is for you before committing to a permanent structure

Buy a cellular polycarbonate greenhouse if:

• You want a permanent structure with no replacement cycle
• Your site is exposed (coastal, elevated, or in a known wind corridor)
• Your allotment site bans polytunnels or you're growing in a visible garden setting
• Year-round growing matters to you, including overwintering, early starts and late finishes
• You're upgrading from a starter aluminium greenhouse you lost in a storm
• You've owned a polytunnel and the year-5 cover replacement is now due

Buy a SteelRoot specifically if:

• You want the cellular PC permanence with the UK-specific engineering decisions (closer-arch spacing, screw-fix glazing, included ground anchors) rather than the Continental defaults
• You want 25–48 m² of serious growing space at a price below the heritage glasshouse category
• Modular extension matters to you, buying at 4 m today and extending to 8/10/12 m as your ambition develops

Frequently asked questions

Polycarbonate greenhouse or polytunnel: which should I choose?
Choose a polytunnel if your budget is genuinely under £1,000, you want the largest possible growing area for the money, and you're comfortable replacing the polythene cover every five to seven years. Choose a polycarbonate greenhouse (including an arched polycarbonate tunnel greenhouse) if you want a permanent structure with no replacement cycle, you're on an exposed or allotment site where film struggles, or you want the warmer twin-wall growing calendar. In short: polytunnel for lowest upfront cost per square metre, polycarbonate for permanence, warmth and a tidier garden presence. Our SteelRoot range covers the polycarbonate route from £1,199.

Is a polytunnel cheaper than a polycarbonate greenhouse over 10 years? Yes, by roughly £450–£1,150 across a 25 m² footprint over a decade, depending on the specific products. The trade-off is the cover replacement cycle, the storm-failure risk, and the allotment-policy considerations.

What's the lifespan of polytunnel film? UV warranties on commercial-grade UK polytunnel film are typically 4 years, with real-world lifespan of 5–7 years before significant clouding or weakening. Coastal and elevated sites tend toward the shorter end.

Are polycarbonate greenhouses noisier in rain? Cellular polycarbonate is genuinely quieter than corrugated metal but louder than glass in heavy rain. Polytunnel film is the loudest of the three. In practice, none are loud enough to be a problem unless you're spending extended time inside during downpours.

Do polytunnels need planning permission in the UK? Generally no, under Permitted Development for domestic gardens, subject to the standard outbuilding limits. Allotment sites vary by individual policy; check your site rules.

Will a polycarbonate greenhouse blow away in a storm? Entry-tier aluminium greenhouses with spring-clip glazing have a documented history of panel loss in moderate UK winds. Engineered cellular PC greenhouses with screw-fixed panels, closed steel frames, and correct anchoring (such as the SteelRoot range) are engineered for British storm conditions, assuming the included anchors are correctly installed.

Can I extend a polytunnel? Most polytunnels are fixed-length; replacement with a longer model is the usual upgrade path. Cellular PC greenhouses with modular extensions (such as the SteelRoot Solid Extension Module +2 m) allow you to extend in 2 m increments without replacing the original structure.

What about the SteelRoot warranty? 2-year manufacturing defect warranty on cellular polycarbonate panels + 10-year anti-corrosion warranty on the galvanised steel frame. Full terms published on the SteelRoot product page before checkout.

The honest summary

Polytunnels are excellent for specific use cases, particularly dedicated growing where aesthetic doesn't matter, budget is tight, and the buyer accepts the cover replacement cycle as part of the running cost.

Cellular polycarbonate greenhouses make more sense for buyers who want permanence, who're growing on an allotment with policy restrictions, who're upgrading from a storm-damaged starter, or whose growing calendar extends beyond the May to September peak.

SteelRoot specifically, galvanised, arched, quiet about it, sits in the engineered category: closer-arch spacing for UK wind exposure, screw-fixed 6 mm cellular polycarbonate, Ground Screw Anchors INCLUDED. From £1,199 entry, with modular extension up to 12 m as your growing ambition develops.

Choose the structure you'd genuinely use in winter, not the one that looks cheapest in May.

See also: Polycarbonate vs Glass Greenhouse UK · Greenhouse Permitted Development UK · SteelRoot collection · Wooden Greenhouse Buying Guide

Internal links: → SteelRoot 3.14 m · → SteelRoot 4 m · → Greenhouse size calculator

Related: polytunnel vs greenhouse, the full UK comparison.