There's a moment that hits most British gardeners somewhere around mid-May. The seedlings on the windowsill are taking over the kitchen, the slugs got to last year's lettuces in record time, and you've decided. This is the year you finally buy a greenhouse.
Then you open a couple of tabs, look at the options, and realise greenhouse shopping is more complicated than it has any right to be. Polycarbonate or glass? Walk-in or cold frame? 6 by 2.5 feet or 8 by 12? Lean-to or freestanding? Aluminium frame or steel? Most buying guides answer all these questions in parallel and leave you no clearer. This one works through them in the order that actually matters, one question at a time.
Most British gardens have one good spot for a greenhouse and three okay ones. Work out the spot first, the rest follows.
Question one: where will it actually go?
Start here, not with the greenhouse itself. The single biggest mistake new greenhouse buyers make is choosing the model first and trying to make it fit the garden afterwards. The garden decides the greenhouse, every time.
Three placement factors matter, and they sometimes pull against each other.
Sunlight
Greenhouses work because they trap and amplify sunlight, so they need to be in the brightest spot in the garden. South-facing is ideal. East-facing works well for most crops because plants get the morning light when they need it most. West-facing is acceptable. North-facing gardens can still grow plenty in a greenhouse, but the structure needs to sit in the sunniest corner of the lawn rather than tight against the north wall.
Shelter
Wind is the second factor. Polycarbonate panels are tougher than glass but neither material loves a 50mph gust ripping through a poorly anchored greenhouse. If your garden is exposed (no fences, no mature trees, open countryside behind), look for sheltered corners. A position with a fence or hedge on the windward side genuinely extends the useful life of the structure.
Access
The third factor is the one most buyers underestimate. You'll be in and out of the greenhouse daily during the growing season. If it's at the bottom of a long, narrow path that gets boggy in autumn, you'll resent it by October. If it's near the back door, you'll use it three times more often.
If those three factors point to the same spot, brilliant. If they don't, prioritise sunlight first, then access, then shelter. You can shelter an exposed greenhouse with anchoring kits or windbreaks. You can't add sunlight to a shaded one.
Question two: how big should it actually be?
The standard advice is "buy the biggest one you can fit." This is almost right, but not for the reasons people give. The real reason bigger is usually better isn't ambition, it's airflow. Small greenhouses heat up faster, cool down faster, and develop humidity problems quicker than larger ones. A 6 by 8 walk-in with the door cracked open holds temperature far more comfortably than a 4 by 2 cold frame on the same day.
That said, three rough size brackets cover most British gardens:
Compact (6 by 2.5ft and similar lean-tos)
Small footprint, low cost, suits balconies, courtyard gardens, or tight side returns. The Outsunny 6 x 2.5ft Polycarbonate Greenhouse in our range is in this bracket: powder-coated aluminium frame, twin-wall polycarbonate panels, sliding door, an adjustable window with five ventilation levels, and a top gutter for rainwater drainage. At 70cm by 200cm internal, it's a starter greenhouse rather than a serious growing space. Good for seedlings and small tomatoes. Not for a year-round vegetable habit.
Mid-size walk-in (6 by 4ft to 6 by 6ft)
This is the sweet spot for most British gardens. Enough room to walk in, room for staging on both sides, capacity for seedlings, propagation, summer crops like tomatoes and chillies, and overwintering of tender plants. The Outsunny 6 x 4ft Lean-to Polycarbonate Greenhouse fits here, with the lean-to format using a house wall or fence to gain thermal mass without requiring a freestanding footprint.
Large walk-in (8 by 12ft and up)
Proper hobbyist territory. The Outsunny 8x12 ft Walk-In Greenhouse in our range gives you polycarbonate panels, double sliding doors, aluminium frame on a galvanised steel base, and importantly an open interior with no fixed shelves so you can tailor the layout. At this size you can run staging on both sides and still have a path down the middle for trolleys, watering cans, and the messier bits of greenhouse life.
Cold frames sit alongside all three brackets rather than competing with them. The Outsunny 3 Tier Greenhouse Cold Frame is genuinely useful for hardening off seedlings before transplanting, but it's not a substitute for a walk-in greenhouse. Don't think of cold frames as small greenhouses. They're a different tool for a different job.
Question three: polycarbonate or glass panels?
This is the question most buyers fixate on, and the honest answer is that for most British home gardens, polycarbonate is the better choice. Here's why, with the trade-offs spelled out so you can disagree if your situation is unusual.
Why polycarbonate usually wins
- Safety. Polycarbonate panels don't shatter into dangerous shards when something hits them. A football, a falling branch, or a determined child can all cause a glass panel to break catastrophically. Polycarbonate cracks or pops out of its frame at worst.
- Insulation. Twin-wall polycarbonate (the type used on quality greenhouses including the Outsunny range) has a built-in air gap that gives it noticeably better insulation than single-pane glass. Crops stay warmer overnight in spring and autumn.
- Light diffusion. Polycarbonate panels diffuse light rather than transmitting it directly. This sounds like a downside but it isn't: diffused light reaches all sides of a plant instead of just the side facing the sun. Plants grow more evenly.
- Weight and installation. Polycarbonate panels are dramatically lighter than glass, which makes solo or two-person assembly genuinely possible.
- UV protection. Quality polycarbonate panels (including the Outsunny twin-wall versions) include UV protective coatings that prevent the harshest rays from scorching plants while still letting in the spectrum they need.
Where glass still has an argument
- Light transmission. A clean glass pane transmits roughly 90% of available light. Polycarbonate transmits around 80%. In very low-light environments (north-facing gardens, dense shade), that difference matters.
- Appearance. Glass greenhouses look more traditional, with the clear-pane aesthetic of a Victorian conservatory. Polycarbonate has a slightly milky appearance that some gardeners find less attractive.
- Longevity. Quality horticultural glass lasts indefinitely. Polycarbonate panels typically last 10 to 15 years before UV exposure starts to yellow them noticeably.
If you have a north-facing garden, a traditional design preference, or you're planning a greenhouse you want to pass on to the next owner of the house, glass is defensible. For most other situations, polycarbonate is the practical choice.
Question four: walk-in or cold frame?
These two formats get conflated and they shouldn't be. They solve different problems.
What a walk-in does
A walk-in greenhouse is a proper growing environment. You stand inside it, you tend the plants by hand, you can leave them in there for the whole growing season. It supports tomatoes, chillies, aubergines, cucumbers, and the heat-loving crops that struggle outdoors in most British summers. It overwinters tender plants. It hardens off seedlings before transplanting.
What a cold frame does
A cold frame is a transitional space. Seedlings started indoors on a windowsill need to gradually adjust to outdoor conditions before being planted in beds, and that process (hardening off) takes a couple of weeks. A cold frame is where it happens. Some gardeners also use cold frames to overwinter alpines and other plants that need protection from rain but not from cold.
Which one you need
If you're just starting out and have a windowsill or two of seedlings each spring, a cold frame is probably enough. If you've moved past the seedling stage and want to grow things you can't grow outdoors (real tomatoes that ripen on the plant, chillies that turn red, cucumbers that don't taste of water), you need a walk-in.
Lots of gardeners eventually end up with both. The cold frame handles spring hardening-off, the walk-in handles summer crops and winter protection. If you're picking one to start, choose based on what you actually want to grow, not on what feels like the safer first purchase.
Question five: how will you anchor it?
This is the question that doesn't appear on glossy product pages and almost decides the lifespan of the greenhouse. A greenhouse is essentially a large kite with plants inside. In a 40mph gust, an unanchored greenhouse can lift, twist, or shift off its base. None of these outcomes are good. The good news is that proper anchoring is straightforward if you plan for it.
Base options
- Concrete slab. The most secure option. Pour a slab slightly larger than the greenhouse footprint and bolt the frame down through the integrated holes. Permanent, completely stable, never an issue again.
- Paving slabs. The middle ground. Lay paving slabs on a compacted hardcore base, level them carefully, and bolt or weight the frame. Less permanent than concrete but plenty stable for most British conditions.
- Galvanised steel base. The Outsunny 8x12 ft walk-in greenhouse ships with a galvanised steel base built into the structure, which simplifies installation considerably. Combined with ground anchors, this is enough for normal garden conditions.
- Ground anchors. Spike-in or screw-in anchors that secure the frame to the soil directly. Acceptable as a supplement, not as a sole anchoring method.
What not to do
Don't sit a greenhouse on bare grass or compacted soil and hope for the best. Don't rely on the greenhouse's own weight. Don't skip the anchoring brackets that came in the box. Don't put it directly under a tree where falling branches become a real risk.
The assembly reality check
Every greenhouse box says it can be assembled in a few hours by two people. Almost none of them can. The Outsunny 6 x 2.5ft Polycarbonate Greenhouse explicitly recommends 2-3 people for assembly, which is honest. For larger models, a realistic time estimate is closer to a full Saturday.
A few practical notes that save grief:
- Lay all the parts out before you start. Polycarbonate panels look almost identical and putting one in the wrong position means dismantling the section to fix it.
- Check the parts list first. Manufacturing errors are rare but they happen, and finding out a critical bracket is missing after you've started assembly is worse than finding out before.
- Build on the level base, not in the garden. You can't level a greenhouse after the fact. Get the base right, then build on top of it.
- Have two people on each side of the frame as the panels go in. The aluminium frames are flexible until everything's locked together, and a panel sliding in pushes against the frame as it goes.
- Don't fully tighten the bolts until everything's in place. Slight movement in the frame is what lets panels slide into their slots correctly. Tighten everything at the end, not as you go.
What happens after the greenhouse is up
Most buying guides stop at the point of purchase. Here's the more useful question: what's the first three months of greenhouse ownership actually look like?
Week one
The greenhouse is bare. Get the staging or shelving in if your model didn't come with it. Open the ventilation window in the morning and close it before evening for the first few days to monitor how warm it gets. UK summer greenhouses regularly hit 35 to 40 degrees inside on a clear day, which is too hot for most plants. Knowing your model's temperature behaviour matters before you put anything important in.
Week two to four
Move your existing seedlings in. They'll grow faster than they did indoors. Water more often than you think you need to. Greenhouse air is genuinely drier than indoor air despite being warmer, because the ventilation that prevents overheating also dries the atmosphere out. Daily watering during warm spells is normal.
Month two onward
Add slow-growing summer crops (tomatoes, chillies, cucumbers if you have space). Watch for pests. The closed environment that protects plants from slugs and rain also concentrates problems if aphids or whitefly get in. Open the door wide for at least an hour on warm days to let air move through.
Going into autumn
The greenhouse extends the season by roughly six weeks at each end. Crops that would normally finish in early September keep going into mid-October. Seedlings can start three to four weeks earlier in spring than they could on a windowsill. That's the actual return on the purchase.
Where to look next
If you've worked through these five questions and have a clear sense of what you need, the greenhouses collection has the range from compact cold frames through 6 by 2.5 starter greenhouses to the 8 by 12 walk-in models. Most of the range is from Outsunny, and our Outsunny brand review covers what to expect from the build quality and finish generally if you'd like that broader context. The garden storage collection and our outdoor storage guide pair well if you also need somewhere to keep tools, compost, and the bits and pieces that accumulate around greenhouse use.
Free UK mainland delivery applies on every order, dispatched in three to five working days, with no hidden fees. Greenhouses ship on substantial pallets and benefit from two or three people on hand for unloading and assembly day, so plan delivery accordingly. The polycarbonate panels are well-packed but worth checking on arrival for any shipping marks before signing for the delivery.
