How to Choose the Right Garden Greenhouse: Types, Sizes, Materials and Siting

Choosing the Right Type of Garden Greenhouse

Most people searching for a garden greenhouse are choosing between four very different structures. A cold frame is the simplest option: a low box, often 30 to 60 cm high, with a hinged lid acting as a roof. It suits seed trays, cuttings and overwintering pots, but its low profile limits what can be grown vertically. A hobby greenhouse is the classic small glasshouse, usually 180 to 240 cm wide and 240 to 420 cm long, with vertical side walls and a pitched roof giving standing height along the ridge. A polycarbonate tunnel is longer, lower and rounder, cheaper per square metre and better at trapping heat, but the curved sides waste space at ground level and a hoop frame is weaker in wind. A small multi-span greenhouse, several bays joined under one roof, suits growers who want more usable area from one footprint and can run a fixed heating circuit through it.

The right answer depends less on status than on what you intend to grow. Cut flowers, tomatoes and salad crops suit a small multi-span. Herbs, chillies and overwintered perennials suit a hobby glass greenhouse. A tunnel works for rows of salad leaves and hardy annuals, while a cold frame handles propagation that would otherwise take up bench space indoors.

Greenhouse Sizes, Heights and Eaves

Dimensions are usually quoted as width by length, but height matters just as much. The three numbers worth checking on any garden greenhouse are the overall width, the ridge length and the height at the eaves, where the side wall meets the roof. A low eave of around 1.5 m is difficult to work in along the sides because the roof slopes down over the crop. An eave of 1.7 to 1.9 m with a ridge of 2.1 to 2.4 m allows comfortable standing and headroom over staging.

Width is the first number to fix, because it determines the frame you can buy and how the building sits on its foundation. Anything above roughly 2.4 m wide needs a multi-span design with intermediate support. Length is the cheapest dimension to add, though long structures need stronger anchorage and ventilation at each end.

Aluminium or Galvanized Steel Frames

Two frame materials dominate domestic greenhouses. Aluminium is the most common for hobby glasshouses. It is light, needs almost no maintenance, resists corrosion and its extruded sections hold glazing bars without sagging. The drawback is that aluminium conducts heat readily, so it loses more warmth than steel, and it costs more per square metre. Galvanized steel frames, usually powder-coated after galvanizing, are heavier, cheaper and stiffer. They suit longer structures because they resist deflection better than aluminium of the same profile.

Steel frames need attention at the base. Even powder-coated steel eventually chips at the ground line, and bare metal in damp soil will rust from the inside out. A well-made galvanized frame shows a continuous zinc coating, and fixings should be stainless or zinc-plated. Frames must also be anchored to the base at corners and at intervals along the sides, because an unanchored frame is what fails in a gale.

Glass, Twin-Wall Polycarbonate and Film Covers

The covering material defines how a garden greenhouse behaves in every other respect. Horticultural glass is the traditional choice. It has the highest light transmission of any covering, which matters for crop quality in winter, and it lasts for decades on a sound frame. Glass is heavy and unforgiving: a single broken pane on an old frame can cost more than a whole new roof, and panels need a supporting bar beneath them. Most hobby glasshouses use 3 mm glass in putty or silicone joints.

Twin-wall polycarbonate is the usual compromise for the home grower. The fluted structure traps a layer of still air inside the sheet, cutting heat loss while transmitting around 80 to 85 percent of light. It is lighter than glass, impact resistant and cheaper to replace, but it yellows with ultraviolet exposure and expands noticeably between seasons. Solid polycarbonate suits small roofs and vents. Film covers, usually 150 to 200 micron polyethylene, are cheapest and work best on tunnels and seasonal structures where replacement every few years is acceptable.

Foundations and Ground Anchoring

A garden greenhouse is only as strong as the ground it stands on. The usual approach for small hobby structures is a light base of treated timber or aluminium rails on paving slabs or compacted hardcore, with the frame pegged down using ground anchors. This works on level, sheltered sites, but it depends on anchors being properly driven and the soil firm enough to resist pull-out.

Concrete foundations are the most durable option and the right choice for larger glasshouses, multi-spans or exposed sites. A strip footing under each wall line, or a full slab for a growing area, gives a level base and resists frost heave. Timber bases raise the frame off the soil, which helps drainage and keeps pests out, but need regular treatment against rot. Soil screws suit sites where excavation is difficult: long screws are twisted into the ground and frame brackets bolt to them.

Siting and Orientation in the Garden

Position makes a bigger difference to performance than any single component. The ideal site is level, in full sun, sheltered from prevailing wind, close to a water supply and within easy reach of the house. A north-facing site will be gloomy for most of the day, and a greenhouse in the shadow of a wall or mature tree will disappoint repeatedly, no matter how good the glass is. Watch out for overhanging branches that drip and drop leaf litter, and for downpipes that discharge onto the roof.

Orientation is also about light and heat. A greenhouse with the ridge running north to south gives the broadest, most even distribution of daylight across the floor. Running the ridge east to west puts more light on the south side but shades the north edge at midday. Either way, the sunniest end should hold the tallest or most heat-demanding crops. Leave working space around the building so vents can open, doors can swing and glass can be cleaned; a metre on the sunward side and half a metre on the shaded side is a sensible minimum.

Wind, Snow and Load Considerations

Domestic greenhouses are engineered for sheltered gardens, and it is worth being honest about that. Check the stated wind load for any structure you consider, usually a design speed in metres per second or miles per hour, and compare it with your site exposure. A greenhouse on a coastal or hilltop plot may face gusts that overwhelm a design intended for a sheltered suburb. Solid glazing bars, a rigid ridge and proper anchoring make more difference than a heavier frame.

Snow is the other common failure point. A roof that sags under wet snow will not recover its shape, and a warped frame is usually beyond repair. Check the roof pitch, glazing bar spacing and whether a snow load rating is given, usually in kilograms per square metre of ground area. Polycarbonate roofs shed snow more readily than glass because they are slicker and flex slightly, but they can be lifted by wind if fixings are too light. In exposed locations, plan where a temporary brace would go and clear snow rather than letting it sit.

Kit Builds Versus Custom Installations

A kit greenhouse arrives as a flat pack with a written assembly sequence, glazing, fixings and a set of tools. Kits suit most home growers because the design is already resolved: parts are cut to fit, the instructions are written, and the frame geometry is known to work. A typical hobby kit can be assembled in a weekend with a helper and basic tools, and many manufacturers offer extended or wider kits on the same base frame.

Custom installation becomes sensible when a garden has awkward dimensions, sloping ground, unusual access or a requirement for integrated heating, irrigation and staging. It also makes sense for larger multi-spans where a professional crew can level a base, pour footings and tension the covering correctly. The trade-off is cost and lead time: a custom build costs more than the equivalent area of kit, and lead times can stretch considerably, so confirm delivery and installation dates in writing. For most gardeners, a mid-sized kit with the option of extending length and adding a second door is the sweet spot.

Frequently Asked Questions

How big should my first garden greenhouse be?

Choose a width you can reach across comfortably, about 180 to 240 cm for most people, and add length rather than width. More floor area on one axis gives more usable bench length, which limits most home growers, and costs proportionally less. Check the ridge height too if you grow tall plants.

Is a glass or polycarbonate greenhouse better for a home garden?

Glass gives the best light and the longest service life, and rewards a sheltered site with a solid frame. Polycarbonate is lighter, cheaper to repair and loses far less heat, making it the safer choice on exposed ground or for overwintering tender plants.

Do I need a base or can a greenhouse sit straight on grass?

A greenhouse can stand on level ground with anchors, but it will settle unevenly and the frame will twist, the main cause of glazing bar failure and glass breakage. A timber base on paving slabs is the usual compromise, while concrete footings are worth the cost for exposed builds.

Can a garden greenhouse be moved or extended later?

Small aluminium hobby greenhouses are designed to be dismantled and relocated, provided you take the glazing off first and treat the frame joints. Length can usually be extended with a matching kit, but mixing profiles from different suppliers causes gaps and load problems, so check the extension suits your frame carefully.