Thinking of getting a green house is one that really needs a good think not just a passing thought,
The first thing is site, have you an open space to put the house on ?
Is the site shaded on any sides ? This can affect growing of some plants/seeds if to shaded all the time,
Are you having a base? concrete? solid base helps keep heat in during winter etc,
The vents are of upmost importance, ie bad ventilation will cause mold on a lot of plants and a free hotel for all kind of bugs
Auto mattic opening devices should be a must plus side hand opening devices,
Auto unless your going to be in the area 24 hours a day you'll be glad you had a device that opens when your not around it can save a lot of fried plants,
The door opening, slide door or opening doors?
How wide, ie do you have a need to get a wheelbarrow through the opening space? what is your size, large people can have trouble getting through slim spaces and if your carrying bags of compost you've only to knock the door frame a few times and you'll hear smashing glass etc "get a door size to fit you and your needs,
How fit are you ie can you manage large pots, heavy weights? All these things ref openings are important to think about now,
Again size is important ie do you intend winter growing and if so you'll need to think about heating the greenhouse,
Plants are not cheap and if you intend to over winter you'll find heat is also not cheap, the bigger the greenhouse the more heat cost,
If your a keen gardener and "like myself" live for gardening then ref space!!!! ? allow to buy a twice the size greenhouse your thinking of now, nothing worse than making the mistake of a to small greenhouse,
You can get all ,kinds of glass "some is made to retain heat better than others, take your time and study whats on offer and cost,
Some are self shading again to much bright sun can cause damage, self shading will sort the problem out when your away,
Electric "Are you able to have electric plumbed in ie are you near an outlet so your house can receive a power line, this again is a very good thing to have both sockets and lighting supply on tap,
Do you want the likes of grape vine in your greenhouse " the roots of a lot of vines are planted outside the greenhouse and so when building the foundation ensure you leave an entrance for the vine to be planted outside and led into greenhouse (plastic drainpipe placed in the foundation will sort this problem out,
And last but not least, is the site out of a wind tunnel, greenhouse can be damaged via strong winds? can you plant a windbreak near?