Greenhouses can be built from plans or you can buy greenhouse kits.  For most people, greenhouse kits are going to be the best choice for several reasons.

You will get more for your money when you use greenhouse kits.  Greenhouse kits are manufactured with everything needed.  When you decide to have a greenhouse built from a set of plans you will need to buy each thing needed for the greenhouse separately.  If you will not be building the greenhouse yourself you will pay more for a contractor to build the greenhouse from plans than from a kit.

If you buy a greenhouse kit you will be more likely to be able to build it yourself.  This will certainly save you even more money.

The guesswork is taken out of the building when you use a greenhouse kit.  You will have exactly what you need and the directions to build the greenhouse.  Greenhouse kits are usually easier to put together, too.

You will probably have more choices if you buy a greenhouse kit.  There are many shapes and sizes available when you buy a greenhouse kit. You will not have nearly as many options if you decide to use plans.

Greenhouse kits are available online.  Online sites that have greenhouse kits for sale also have people who can answer your questions and assist you with purchasing a greenhouse kit.  They will have information about the greenhouses and can tell you about greenhouse supplies and accessories that you may need.

Greenhouse kits are the best choice for most greenhouse gardeners.