What size greenhouse do I actually need?

Greenhouses are sold in imperial widths — 6ft, 8ft, 10ft — and lengths that step in roughly two-foot bays. Almost the only universal opinion among greenhouse owners is that they should have bought a bigger one. The counter-argument is heating cost, and it is a real one, but only if you actually heat the space.

Step by step

1. Understand what the width buys you.6ft wide gives staging down one side and a narrow path, or a border each side and no room to work. 8ft wide gives staging one side and a border the other with a usable path, which is the size at which a greenhouse becomes comfortable. 10ft gives both sides plus a proper path.
2. Work out what you actually want to grow.Raising seedlings and hardening off needs staging area, not floor. Growing tomatoes, cucumbers and peppers to maturity needs floor area and headroom. Overwintering tender plants needs floor and a frost-free volume you can afford to heat.
3. Count the beds and pots you will need to house.A cordon tomato needs roughly 45cm of border length. A grown cucumber needs more. Six tomatoes and two cucumbers is already a full 6ft by 8ft border. Sketch the layout at real scale before buying.
4. Consider the eaves height, not just the footprint.Low eaves make the side beds unusable for tall crops and make working at the edges uncomfortable. Taller models cost more and are far more pleasant. Check the eaves height in the specification, not just the ridge.
5. Understand the heating trade-off.Heating cost scales with the surface area you are losing heat through, so a big greenhouse is genuinely more expensive to keep frost-free. The standard answer is to partition — bubble-line and heat a section, and leave the rest cold.
6. Allow room around it.You need to walk right round the outside to clean the glass, replace panes, and clear gutters, and you need somewhere beside the door for pots and compost. A greenhouse squeezed into its exact footprint is a nuisance forever.
7. Buy the extendable design if you are unsure.Most aluminium greenhouses are built in bays and the manufacturer sells extension kits. Buying a model you can lengthen later is the practical hedge against getting the size wrong.

Tips

• The cost per square metre falls sharply as the greenhouse gets bigger. The base, the door, the ends and the labour are much the same whatever the length.
• If space is tight, a lean-to on a south-facing wall gives a surprising amount of usable growing area in a narrow footprint and is the cheapest type to heat.
• Check the door width if you use a wheelbarrow. Many standard doors are too narrow, and a double or sliding door is worth specifying at the outset.

Common mistakes

• Buying the smallest model to test the idea — A 6ft by 4ft fills within one season, overheats faster than anything larger, and you end up buying twice. If budget is the constraint, buy a longer, cheaper structure rather than a small expensive one.
• Sizing for summer only — Summer is when a greenhouse is emptiest, because everything has gone outside. The size test is February, when seed trays, overwintering plants and pots of cuttings are all competing for space.
• Ignoring how much staging costs — Staging, shelving, watering kit and a heater add a substantial amount to the price of the frame. Budget the whole installation, not the greenhouse alone.

Questions people ask