A good rule of thumb is that polytunnels are good for allotments, and greenhouses are good for gardens. The reason is that polytunnels are easier to erect on rough, uneven, or sloping ground, but perhaps are not the most beautiful things to look at, whilst greenhouses need a flat and durable base (like a paved or concreted area), and can be a very attractive feature in a garden.
A greenhouse has a number of advantages. They offer a protected environment to grow on seedlings in the spring, and provide ideal conditions for heat loving vegetables like tomatoes, chillies, aubergine, and sweet peppers in the summer, and extend the harvest season to as late as November. With the addition of a heat source, like an electric heater, there is no need to clutter windowsills inside the house trying to germinate seed in the spring.
A greenhouse can be a significant investment. The first investment is constructing a suitable strong, flat, and durable base. The second is the greenhouse itself, that ideally will not only be useful, but also be a good looking addition to a garden. With this in mind, and if money allows, it may be worth investing a little more to purchase a greenhouse that is strong and durable to last for many years.
A further consideration is what you intend to use a greenhouse for. A small greenhouse is ideal for tucking away in a corner of a garden, with its main use to help germination in the spring, and perhaps to protect a few cold sensitive plants overwinter. If this is the main motivation, then a greenhouse does not need to be big. However, if the intention is to use the greenhouse for summer growing of heat loving crops, or as an area for leisure and relaxation, then you will likely require a larger space.
