Mine is over 100' away from my back doors. It just sort of happened that way. Would have preferred to have had it closer to my home like goldenrose's but I needed to site it properly to maximize the sun and needed to integrate it into the landscape design... what little there is. This is my second greenhouse.
Many factors to consider and try hard not to get hung up on aesthetics. Best to have a fully functional greenhouse as opposed to an eye catching purty greenhouse so goooooooooooooo slowly identifying what your needs are or end up like me... with a newer tear down. Glass is probably not the way to go in my area is about all I can say. If possible, consider a poly greenhouse. I think the name of the greenhouse I have is the American Classic and it's like 16 or 17' by 28'. I ordered it in powder white and it's on a concrete slab with footers below the frost level. Gas, electric, and water were brought to it.
Research research research! Prepare to sell you first born child to fund this. Just kidding but building a greenhouse can get kinda pricey and best to only have to build one once.
editing to add- these are the "bubbles" I use-
http://www.flickr.com/photos/megz/92401971/
They were cheap and I picked them up from Menards on clearance for around $9 a piece. I like that I can allow for a little air excahange.
