Mini Greenhouse vs Walk-In Greenhouse: Which Should You Buy? (UK Guide)

If you're only starting seeds, hardening off young plants, or keeping a few tender crops going a little longer into autumn, a mini greenhouse does the job and costs a fraction of the alternative.

If you actually want to grow tomatoes, chillies, cucumbers or salad through a full UK season, you need a walk-in greenhouse or polytunnel with real air volume and proper cross-ventilation, because a small plastic-cased mini greenhouse overheats fast on a sunny day and gives you nowhere to work standing up.

The honest dividing line isn't space or budget, it's what you're actually planning to grow and for how long.

Mini greenhouses at Opal Retail start from £26.99; walk-in greenhouses and polytunnels start from £64.99, with sturdier steel-framed options built for stronger UK winds further up the range.

In this article

• What's the real difference between a mini greenhouse and a walk-in greenhouse?
• What can you actually grow in each?
• Mini vs walk-in greenhouse: quick comparison
• Will it survive UK wind and rain?
• Do you need planning permission for a greenhouse?
• Which should you buy?
• FAQ

What's the real difference between a mini greenhouse and a walk-in greenhouse?

A mini greenhouse is a tall, shelved unit with a clear PE cover, sized to stand pots and seed trays on rather than to stand in yourself. You reach in through a roll-up front to water and check on plants.

A walk-in greenhouse or polytunnel is a full structure with a galvanised steel frame, room to stand and work inside, and ground space for raised beds, grow bags or a row of pots, with a proper door and mesh windows for airflow rather than a single roll-up panel.

That structural difference is what actually separates them, not just size. A mini greenhouse has a small enclosed air volume and one point of ventilation, so it heats up and cools down fast.

A walk-in structure has enough air volume and enough ventilation points to hold a more stable growing environment through the day.

What can you actually grow in each?

The Royal Horticultural Society is specific about what a mini greenhouse suits: it's "much better than windowsills for raising seedlings," and works well for summer crops that get too tall for a cold frame, such as aubergines, peppers and tomatoes.

The catch, in the RHS's own words, is that with only a small air volume they "need careful management to avoid extremes of temperature." In practice that means checking it daily in warm weather, not leaving it shut and walking away.

A walk-in greenhouse or polytunnel is built for more than that: a full season of tomatoes, cucumbers, chillies and salad crops, grown in the ground, in raised beds, or in a row of large pots, with enough space to actually tend them.

One RHS caveat worth knowing before you buy on the strength of "grows anything all year": polytunnels specifically are not well suited to overwintering tender plants, unlike a rigid, better-insulated structure.

If overwintering citrus, fuchsias or other tender plants is the plan, factor that in rather than assuming any walk-in structure covers it.

Budget pick

4-Tier Mini Portable Greenhouse with Lockable Wheels

Four shelves each rated to 10kg, a roll-up front for watering and air, and lockable wheels so you can move it to follow the sun.

From £26.99 Shop this greenhouse →

Mini vs walk-in greenhouse: quick comparison

Will it survive UK wind and rain?

This is where a lot of mini greenhouse buyers end up disappointed. Threads on UK gardening forums follow the same pattern: a lightweight plastic-cover mini greenhouse gets caught by wind, the cover splits or the frame tips over, and weighting the shelves with bricks or paving slabs only helps up to a point.

One long-running forum discussion on exactly this problem ends with several owners giving up on repeatedly patching and reinforcing their mini greenhouse and buying a proper walk-in structure instead. If your garden is exposed, that's worth knowing before you buy the cheapest option rather than after a windy night.

A walk-in greenhouse or polytunnel is a different proposition because it's actually designed to be anchored: a galvanised steel frame, extended cover edges that get buried or pegged into the soil, and guy ropes and ground stakes included in the box.

Opal Retail's own range carries manufacturer wind ratings that scale with size and frame strength: Level 4 on the 2x3m walk-in greenhouse, Level 5 on the 3x2m tunnel with its arched roof, and Level 6 (when fully fixed down) on the largest 6x3m polytunnel.

None of that replaces proper anchoring on the day you assemble it, but it does mean you're starting from a sturdier base than a mini greenhouse's few plastic shelves.

Entry-level walk-in

2x3m Walk-In Greenhouse, Galvanised Steel Frame

A UV-resistant PE cover, four mesh windows and a roll-up door, with guy ropes and ground stakes included for anchoring.

From £64.99 Shop this greenhouse →

Best for exposed gardens

3x2m Tunnel Greenhouse, Galvanised Steel Frame

An arched roof to shed water and a Level 5 wind rating, with extra-wide edges you can bury for added stability.

From £87.99 Shop this greenhouse →

Do you need planning permission for a greenhouse in the UK?

In most gardens, no. A domestic greenhouse falls under the same permitted development rules as other garden outbuildings:

• It needs to be single storey
• No higher than 2.5m at the eaves
• No more than 4m overall with a dual-pitched roof (3m for any other roof shape)
• No taller than 2.5m overall if it sits within 2 metres of a boundary

All your outbuildings combined, including any shed, also can't cover more than half the land around the original house.

Rules tighten if you're in a conservation area, a National Park or AONB, or if the property is listed, so check the Planning Portal's outbuildings guidance or your local council if you're at all unsure before you buy.

We've covered the same boundary and height thresholds in more detail, as they apply to sheds, in our guide to how big a garden shed can be without planning permission.

Which should you buy?

It comes down to what you're growing, how much space you have, and how exposed your garden is:

• Starting seeds, hardening off, or a few pots of herbs and chillies on a patio or balcony: a mini greenhouse is genuinely enough, and there's no reason to spend more.
• A full season of tomatoes, cucumbers or salad crops, or gardening in raised beds: step up to a walk-in greenhouse or polytunnel from the start, rather than outgrowing a mini one in a single summer.
• An exposed or windy garden: don't rely on weighting a mini greenhouse down. Go straight for a steel-framed walk-in model with a higher wind rating and anchor it properly with the guy ropes and stakes provided.
• Serious growing, or several rows of crops: the largest polytunnels give you the space to match.

Premium pick

6m x 3m Polytunnel Greenhouse

Two doors, nine mesh windows and a Level 6 wind rating when fully fixed, with room for several rows of crops.

From £171.99 Shop this greenhouse →

Frequently asked questions

What is a greenhouse actually for?

It creates a warmer, more sheltered environment than open ground, so you can start seeds earlier, protect tender plants from frost, and extend the growing season at both ends. How much of that you get depends heavily on which type you buy.

Is a mini greenhouse worth it?

Yes, for what it's actually designed to do: raising seedlings and hardening off young plants, or keeping a few pots of tender crops going through summer. It's not a substitute for a walk-in structure if you want to grow a full crop of tomatoes or cucumbers.

Can I overwinter tender plants in a walk-in greenhouse or polytunnel?

A rigid, well-insulated structure generally copes better than a thin polytunnel-style cover, which the RHS notes isn't well suited to overwintering tender plants. If overwintering is the main reason you're buying, factor that into which structure you choose rather than assuming any walk-in greenhouse covers it equally.

How do I stop a mini greenhouse blowing over?

Site it somewhere genuinely sheltered, not just what looks sheltered, keep it away from side-alley wind tunnels, and weight the base shelf. Be realistic about the limits of this: UK gardeners regularly report that even a weighted mini greenhouse can still go over in strong wind, because the structure itself is lightweight. In an exposed garden, a steel-framed walk-in model anchored with guy ropes and ground stakes is the more reliable option.

Do I need planning permission for a walk-in greenhouse?

Almost never, provided it's single storey, within the height limits for its position relative to your boundary, and your total outbuilding coverage stays under half the garden. See the planning permission section above for the exact thresholds.

What's the difference between a greenhouse and a polytunnel?

In everyday use the terms overlap. Strictly, a polytunnel has a tunnel-shaped frame with a single-layer plastic cover, while "greenhouse" is often used more loosely to include rigid-framed, walk-in structures like the ones in this guide. What matters more than the label is the frame strength, the cover material and the ventilation, which is why we've compared those directly above rather than by name alone.