It's been a awhile since I have worked with vinyl siding, but I would be concerned with the max temps your GH will reach in the summer warping the siding. Even if you ventilate it will likely hit upper 90's to 100F air temps, and depending on the color of the siding it could get quite a bit hotter on the siding itself. It might be safest to remove the vinyl siding from inside the GH when you add the attachment struct for the GH. Also textured vinyl lap siding tends to have lots of grooves, hollows, and spots to collect mold, etc, so you might want a better surface in there regardless of the heat issue, and a white one to reflect maximum light back to the plants.

I would think that if you are putting a long 2x4 (or whatever) up along the upper attachment point with connections into each stud along its length, that would be adequate. Of course it will depend upon the snow and wind loads you get. You could check with a builder or civil engineer. If you could put a room on the size of your GH and the wall would hold, then it should be fine with the GH. Although you will likely need to use similar attachments for the GH as one would use for a room extension (remove siding under the attachment members, nail/screw into existing garage framing, adding it inside if needed).

One concern may be frost heaving. If your gh "foundation" will be nothing more than some timbers laying on the ground, then it is likely that it will move as the ground freezes and thaws over the winter. That will cause added stress to GH where it attaches to the not moving garage. Something to consider.

As far as insulation goes, if you don't insulate the garage wall to the GH, then you will loose/gain more heat thru it. If you were planning on using the garage as a heat sink that might be OK (although you'd probably need the rest of the garage insulated to hold that heat). But in general it is probably easier to insulate the GH-garage wall and put in fans/vents if you decide to use the garage as a heat sink/store. If you know how cold the garage gets at night and what the GH temp and outside temp is, you can do a quick calculation of the heat losses thru each surface. That would give you guideline as to whether insulating that wall would be worth it or not. Depending upon the numbers, you might find that insulation there gets you some additional above freezing time at each end of the season.

Thanks for the reply! We went back to the place we bought the greenhouse and were advised to remove the siding and secure the greenhouse to the wall directly (without even adding 2x4s. Fortunately, we were able to hit studs on both ends, so I feel pretty good about it.

For the foundation we were recommended to use 4x6s anchored to the ground with rebar. I am still concerned with frost heaves as we get significant freeze/thaw cycles throughout the winter, but our entire yard is a conflict zone with utilities so we can't anchor anything below the frost line anyway. At the moment the timbers are sitting in trenches dug into compacted and levelled dirt, but we are going to backfill with gravel for drainage. Fortunately (?) our soil is awfully heavy and clay-filled, so it does compact very well. We'll have to bring in some topsoil for the rest of the garden though.

Thanks for the idea on using the garage as a heat sink - I've been so focused on maintaining heat that I hadn't given much thought to cooling. At the moment the black tarpaper is exposed, but we will have to cover that with something. I'm thinking insulation for shoulder season, but I could see our problem turning out to be over rather than underheating. Perhaps I'll dust off the heat & mass transfer book and see what makes sense.

Thanks again for your suggestions!

If you are choosing between insulating the back wall of the
attached greenhouse versus adding a thermal shade system, the thermal shade
system will probably provide more return on your investment. This can be used
for a dual purpose. It will help the greenhouse retain heat in the colder times
when the daily period of solar heat is over or if it never happens because of
cloud cover. In addition, the shade system will block out too much solar gain
during the warmer months.