What Size Greenhouse Do I Need? Types and Sizes Explained
Size by what you want to grow, not by what the yard will technically fit. Plus the seven greenhouse types, who each one suits, and why our lengths read 6.5, 13 and 19.5 ft.
The one mistake almost everyone makes
Ask any group of greenhouse owners what they would change and the answer is monotonous: they would have bought a bigger one. Almost nobody says the opposite.
The reason is that a greenhouse fills faster than the plan predicts. The plan accounts for plants. Reality also includes trays waiting to be potted on, bags of compost, the watering can, a stack of empty pots, a heat mat, the propagator, the tools you started keeping in there because it was convenient, and a walkway you can actually turn round in. Growing space and floor space are not the same number.
So the working rule is: work out the size you think you need, then look seriously at the next one up. The cost gap between adjacent lengths is usually modest, because the doors, the end walls and the hardware are already paid for. The cost of replacing a greenhouse that turned out too small is the whole greenhouse, plus a second delivery and a second base.
Size by what you actually want to do
| What you want to do | Floor area | A real example |
|---|---|---|
| Start seeds for a normal vegetable garden | 40 - 70 sq ft | 10 x 6.5 ft = 65 sq ft |
| Overwinter a tender collection plus seed starting | 65 - 110 sq ft | 10 x 6.5 ft to 8.3 x 13 ft |
| Season extension plus a summer crop of tomatoes | 110 - 160 sq ft | 10 x 13 ft = 130 sq ft |
| Year-round growing with a proper work and storage zone | 160 - 260 sq ft | 10 x 19.5 ft = 195 sq ft |
| Feed a household most of the year | 260 - 400 sq ft | 10 x 26 ft = 260 sq ft to 10 x 39 ft = 390 sq ft |
| Market growing or small commercial production | 400 sq ft and up | 10 x 45.5 ft = 455 sq ft and longer |
Under about 8 ft of width you get one bench and a shuffle. At 10 ft you get a bench down each side and an aisle wide enough for a cart - the point at which a greenhouse becomes a room you work in rather than a box you reach into. Width is fixed for the life of the structure. Length is the cheap dimension and the one to spend spare budget on.
Greenhouse types, and who each suits
| Type | What it is | Suits | Limitations |
|---|---|---|---|
| Cold frame | A low glazed box, no standing room | Hardening off, winter salads, a first step | No working room at all |
| Mini or portable greenhouse | A small framed shelter, often 3 ft deep | Balconies, patios, tight yards | Very little thermal stability |
| Hoop house or high tunnel | Arched frame under polyethylene film | Cheap covered ground for in-soil crops | Film needs replacing; poor insulation; usually no glazing rigidity |
| Full arch, rigid panels | Curved frame glazed in twinwall polycarbonate | Best snow shedding and wind profile for the money | Headroom tapers towards the walls |
| Straight wall with arched roof | Vertical side walls, curved roof | Full-height benching at the walls plus good roof shedding | Slightly more material, slightly higher cost |
| Straight wall with gable roof | Vertical walls, pitched roof | The classic shape; maximum usable width at bench height | Needs a good pitch to shed snow |
| Lean-to | Built against an existing wall | Small yards; the wall gives free thermal mass | Light from one side only; needs a suitable wall |
| Glass house | Traditional glazing on a heavier frame | Appearance, clarity, permanence | Heaviest frame and base; highest cost; breakable |
Our range covers the three freestanding rigid-panel shapes. The Arc Series is a full arch at 10 ft wide and 7 ft at the peak - the best structural shape for snow and wind per dollar, and the one rated to 75 mph. The Loft Series is 10 ft wide with straight walls and an arched roof at 7.2 ft, which buys you full-height bench space right out to the wall. The Gable Series is 8.3 ft wide with straight walls and a pitched roof, which is the right family if what you actually want is an eight-foot-wide house. Loft and Gable are both rated to 65 mph; we state the ratings per series rather than averaging them, because they are not the same.
The real size ladder
Our lengths are metric - 2 m, 4 m, 6 m, 8 m and up - so the imperial figures are conversions rather than round numbers. On a 10 ft-wide frame the floor area steps like this: 65, 130, 195, 260, 325, 390, 455, 520, 585 and 650 sq ft. On the 8.3 ft-wide Gable Series: 54, 108, 162, 216, 270, 324, 378 and 432 sq ft.
That is why searches for round sizes land on pages that say something slightly different. If you searched for a 10x12, the honest answer is a 10 ft x 13 ft at 130 sq ft. If you searched for a 10x20, it is a 10 ft x 19.5 ft at 195 sq ft. We title the pages for the search and state the built dimension in the copy, because a rounded number on a freight order is a problem you discover with a tape measure on delivery day.
Many jurisdictions exempt small accessory structures below a floor-area threshold, and 100, 120 and 200 sq ft are all common figures. That can make the step from 65 sq ft to 130 sq ft a permit decision as well as a growing decision - and a permanent foundation or an electrical supply often triggers a permit regardless of size. It is genuinely local. Call your building department before you order.
How to decide, in ten minutes
- Write down the three things you most want the greenhouse to do, in order.
- Find the largest of those in the goal table above and note the floor area.
- Go out and mark that footprint on the ground with canes and string. Stand in it.
- Add the aisle: 30 to 36 inches down the middle, doors at both ends.
- Check the site takes the footprint plus a foot of working margin all round, on ground that does not pool.
- Check the delivery route - a pallet has to get reasonably close to the site.
- Now look at the next size up and price it. If the pad and the budget take it, take it.
Compare sizes side by side
Every listing states exact built dimensions and the real square footage.
8.3ft Gable Roof Walk-In Polycarbonate Greenhouse Kit - Gable Series
From $1,589.00
10ft Straight Wall Walk-In Polycarbonate Greenhouse Kit - Loft Series
From $1,589.00
10ft Walk-In Polycarbonate Greenhouse Kit - Arc Series
From $1,489.00
Greenhouse size questions
What size greenhouse do I need?
For seed starting alone, 40 to 70 sq ft. For season extension plus a summer crop, 110 to 160 sq ft. For genuine year-round growing with a work zone, 160 to 260 sq ft. Whatever you land on, price the next size up before deciding - buying too small is the near-universal regret in this category.
What are the main types of greenhouse?
Cold frames, mini and portable shelters, hoop houses and high tunnels under film, freestanding rigid-panel houses in full-arch, arched-roof and gable forms, lean-to structures built against a wall, and traditional glass houses. The rigid-panel freestanding types are what most home growers end up with.
Is it better to buy longer or wider?
Get the width right first, because it is fixed for the life of the structure and it decides whether you can have benches on both sides of a usable aisle. Then spend any remaining budget on length, which is much cheaper per square foot than width.
Is a 10x12 greenhouse big enough?
For most home growers, yes. Around 130 sq ft gives you benches down both sides, a real centre aisle and a potting corner - enough to start everything a large vegetable garden needs and grow a summer crop. Our closest real size is 10 ft x 13 ft.
Why are your lengths 6.5, 13 and 19.5 ft?
Because the frames are manufactured in metric lengths - 2 m, 4 m and 6 m - and we publish the converted figure rather than rounding it to a tidier imperial number. Rounding a 19.5 ft unit up to 20 ft would put a dimension on your freight order that does not match the pallet.