Because of the additional construction cost I would guess that the breakeven time would be many years in the future.

The more important issue is whether or not you want to step down from the main floor level to use the space and the lower view to the outside.

You'll also want to address the higher moisture concerns in with the design/build.

Do you know what the below grade distance is in your are to obtain steady state temps?
Where I am in zone 6 that's around four feet. Not a serious building concern but depending on size and fenestration, a possible issue with sunlight especially at a low winter angle.

That's true, those underground greenhouses I've seen are in slightly warmer areas, and hardly above ground, whereas I'm in zone 4 and planning something well above ground. I'm mainly wondering if going underground would help stabilize the temperature, or whether it would just be more feet of cold space to have to heat. Maybe it wouldn't make much difference either way.

(lighting, drainage, cost, building code might not matter b/c plants could be raised up, and it's on a dry hill, and we'd do most or all of it ourselves, and it would be a 'cottage' on a parent's property)

Actually this is sounding silly as I type it. What about in a normal totally above-ground case, when it's -20 celsius/-4 F at night, I wonder how hard it is to keep a 10x10 all good glass--except for four lower feet of stone wall, if it's above-ground--room at like 13c/55F? This would be wood heat piped into a radiator (with propane backup). Is that crazy? I don't know anything about it.

I guess you don't really ever see plant rooms built partly underground.

Laura- I'm hoping to put a greenhouse on my remodel, too. It's been called an 'orangery' on another post, but it's just a fancy greenhouse. There will be one door from the mudroom and one window, from the pantry. This part of the home is the only place we're adding on...so we could use brick or some other material for the greenhouse wall, if that would help.

I think sinking the greenhouse will be even more trouble, when the snow melts. You'd probably have to do french drains and other things to try to take the water around the greenhouse. I think your temperature could be more easily regulated (and more affordable) with some heavy shades for night and maybe even a little heat from the house...or a baseboard heat. I believe plants will still be fine at about 55 degrees (maybe even a little cooler).

Good luck with your research and please let us know what you find. I'm still getting ideas and working out a few details, too :)

Oh, and my greenhouse is just for plants...not a breakfast room or family room, which is often a concern on the forums.

Definetely not as an addition but here's something that might be of interest to you, (sorta off topic), but relates.

Here is a link that might be useful: inground greenhouse

How much glass are you talking about? One thing you have to take into consideration is the R value of your windows is going to be a lot less than your walls or roof. You need to take this into consideration when designing your building envelope. Would it make more sense to have the greenhouse attached but outside the building envelope of the main house?

thanks for your answers

Lol lavenderlass, I think I was trying to avoid the word 'orangery' to make it sound less ambitious or something. Ya I'm thinking going below ground wouldn't be a good idea. I was always planning on having plenty of heat from the house when it's needed plus good ventilation and so on.

That link looks really neat and I'd like to try that sometime. For this thing, though, I'd really like it attached to the house.

Thanks for everyone's response; I'll post back if anything comes of it :)