Greenhouses and Polytunnels: The UK Grower's Guide

Choosing between a mini greenhouse, a full-size greenhouse and a polytunnel comes down to how much you want to grow and how much space and budget you have — not which is "best" in the abstract. A mini greenhouse from £47.12 suits a balcony or small patio; a walk-in polytunnel or lean-to greenhouse from around £270 suits anyone ready to grow standing up, in beds rather than pots. This guide covers what the RHS actually recommends for siting, sizing and ventilation, alongside three real, currently available options.

Mini greenhouse, greenhouse, or polytunnel — which do you actually need?

The RHS's own guidance on growing under cover draws a clear line between the three. A full greenhouse is "the most versatile protected growing environment," since glass "transmits and holds more light than plastic glazing," making it suited to seedlings and light-hungry plants year-round. A polytunnel works well for tomatoes and cucumbers over summer, but the RHS is direct about its limits: it's "not usually economical to heat and keep frost-free over winter," so it isn't the right choice if you want to overwinter tender plants. A mini greenhouse — a small, plastic-covered shelving unit — is "better than windowsills" for starting seedlings and suits taller summer crops like peppers and tomatoes in a small space, though the RHS notes it needs "careful management to avoid extremes of temperature" because there's so little air inside it to buffer changes.

In practice: pick a mini greenhouse if you're working with a balcony or a couple of square metres of patio; a polytunnel if you want a full-size growing space for summer crops at a lower cost than glass; and a full greenhouse — glass or polycarbonate — if you want the most reliable, year-round option and have the budget and space for it.

Where to put it: siting and orientation

Position matters as much as the structure itself. RHS advice on choosing greenhouses recommends a spot that gets "uninterrupted sun throughout the day," sheltered from cold northerly or easterly winds. An east-west orientation extends the amount of light you get in winter, while a north-south orientation suits summer crops like tomatoes, since both sides of the structure catch several hours of sun. The same principle applies to a polytunnel or a mini greenhouse: more open sky above it and less exposure to prevailing wind means less heating and repair work later.

Size and headroom: what the numbers actually mean

The RHS is specific about the dimensions that make a greenhouse usable rather than just decorative: eaves should be "at least 1.5m (5ft) tall, and ideally 1.8m (6ft) or more" for good light transmission, with the ridge at least 60cm (2ft) above the eaves for proper door access and rain drainage. A structure below that height lets in noticeably less light and is harder to work in comfortably. Of the three options below, the walk-in polytunnel (2m height) and the lean-to greenhouse (eave height 134cm) both meet or approach this recommendation; the mini greenhouse is a shelving unit rather than a walk-in space, so the RHS's eaves guidance doesn't apply to it in the same way.

Glazing: glass, polycarbonate or polythene

According to the RHS, glass is the benchmark: it transmits around 90% of light and reflects interior heat back inside, giving "the fullest range of uses." Polycarbonate, used in our lean-to greenhouse, transmits roughly 83% of light — a real but modest drop, and one that's usually a fair trade for panels that are lighter, cheaper and far less likely to shatter than glass. Polytunnels use a polythene or PE cover instead of rigid panels, which is the cheapest option per square metre of growing space and quick to put up, but it degrades faster in UV and wind than glass or polycarbonate, which is why reinforcement grids (like the 140gsm reinforced cover on the polytunnel below) matter for how long the cover actually lasts.

Ventilation matters more than people think

A sealed structure on a sunny day heats up fast enough to damage plants, which is why the RHS treats ventilation as essential rather than optional. Their guidance puts roof vent area at 15–20% of floor area as the target for a full-size greenhouse, noting that "roof vents are the most useful" and that side vents alone aren't sufficient. This is also why multiple doors and mesh windows matter on a polytunnel: a single entrance at one end means walking the full length to reach the other side, while doors and windows spaced along the tunnel let air move through and let you reach any bed without disturbing the rest.

Three real options to start from

Prices below are live prices on our own site at the time of writing — always check the product page for the current price, since these can change.

In short: a mini greenhouse (from £47.12) suits a balcony or small patio for pots and seedlings; a walk-in polytunnel (from £269.60) gives the most growing space for the money over summer; and a lean-to polycarbonate greenhouse (from £270.32) is the more reliable year-round option for a small garden with a spare wall. All three should go in the sunniest, most sheltered spot available, per RHS siting advice.

Frequently asked questions

Is a polytunnel or a greenhouse better for a UK garden?

It depends on the season you care most about. A polytunnel is cheaper for the growing space it gives you and works well for summer crops, but the RHS notes it's "not usually economical to heat and keep frost-free over winter." A greenhouse, especially glass, is more reliable for year-round growing.

Is a mini greenhouse actually useful, or just a decorative shelving unit?

It's genuinely useful for starting seedlings and growing summer crops like peppers and tomatoes in a small space — the RHS rates it as better than a windowsill for this. It does need closer attention to temperature than a larger structure, since there's less air inside to buffer changes.

How tall should a greenhouse be?

The RHS recommends eaves of at least 1.5m (5ft), and ideally 1.8m (6ft) or more, for good light transmission, with the ridge at least 60cm (2ft) above that for proper door access and rain drainage.

Does polycarbonate let in enough light compared to glass?

Yes, for most purposes. Glass transmits around 90% of light versus roughly 83% for polycarbonate. The difference is real but modest, and polycarbonate's lower cost and shatter resistance are a fair trade-off for most home growers.

Which way should a greenhouse or polytunnel face?

The RHS recommends a spot with uninterrupted sun throughout the day, sheltered from cold northerly or easterly winds. East-west orientation extends winter light; north-south suits summer crops like tomatoes by catching sun on both sides.

How much ventilation does a greenhouse need?

The RHS's target is roof vent area equal to 15–20% of the floor area, since roof vents are more effective than side vents alone. This is also why doors and mesh windows spaced along a polytunnel matter for keeping it from overheating.

Getting started

Match the structure to the season you care about most and the space you actually have — a mini greenhouse to start small, a polytunnel for summer growing space on a budget, or a polycarbonate greenhouse for a more reliable year-round setup. You can browse the full range, including all three options above, in our greenhouses collection.

Written and checked by the Hackerprice® editorial team. Product details and prices are checked against our live catalogue at the time of publication and reviewed quarterly; siting, sizing and ventilation guidance is drawn from the RHS. Last updated: 15 September 2026.