Before buying a greenhouse, try to think carefully about the type of work that you intend to carry out there; a greenhouse used mainly for sheltering tender plants will usually have lower space requirements to one used primarily for propagation and growing on. Try not to skimp on the size of the greenhouse you choose, as it will certainly seem much smaller once it is filled with plants. However, do bear in mind that a larger greenhouse will cost more to heat in the winter than a smaller one. One option would be to buy a greenhouse that can be extended; alternatively, thermal screens may be used to partition off unused sections during the winter to save heating costs.

A good minimum practical size for a general-propose greenhouse would be 2 m (6 ft) by 2.5 m (8 ft); smaller structures will be prone to sudden fluctuations in temperature, especially in summer when they may undergo a rapid build-up of heat. A six foot wide greenhouse will allow for a set of 2 ft wide staging on each side, with a 2 foot working area running between them. You will need a wider structure (8 ft) if you need wheelchair access or if you want to be able to bring a wheelbarrow inside.

You may find that many small greenhouses have fairly low eaves and ridges, which may mean that you have to stoop when working at the staging. This can become very tiring even after a short period; consider digging out a sunken base or raising the structure up onto a brick layer to gain more height.