My idea of a garden room is one that would be warmer than what I would wish for plants like tomatoes. For tomatoes and propagation I would want 'frost free' but not warm enough to sit and read a book, watch the sunset or drink a glass of wine etc, at least not for very long. I don't think that two walls of glass would be any great problem, though.

So I guess there is the difference between a greenhouse and a 'hothouse', where tropical plants are happy. I suppose I should know what the maximum healthy temperature for tomatoes is and make sure ventilation occurs to control that, possibly using an automatic system. I've seen these operated by wax filled sensors.

Maybe in the end single glass would be better, and I'll just have to wear more layers to sit in there in the winter.

We have a half block wall gh topped with double glazed units reclaimed from the house, then a twin wall polycarb roof......I find that it evens out the temperatures and other than this poor year have success with toms and peppers, obviously well ventilated, also overwintering, last year osteospermum and pelargonium flowered almost throughout with no extra heat.....no good for the previous two bad winters though

There will be a certain amount of warmth from the wall.

Yes, as Dianebulley says, I am depending on the back wall and the solid floor to act as a thermal store, which will absorb some of the solar gain, and release it again at night. This should even things out a bit, and I'm encouraged by Pamg's success.

Also, as Bulbaholic says, there will just be 3/4 of the roof and the front glazing above a plinth wall, so it might not be any problem.

On balance, I think I will go for double glazing, and a bit of trial and error (hopefully not too much of the latter).

Thanks All for your thoughts.