I have a (relatively large) extension on my house, about 5.5m x 5.5 If you work out the 3 exposed walls + gable facing the garden, I have approx 52 sq m of 'wall'.
Of that, approximately 21 sq m of it is glazed including the gable, right up to the ridge. The room has a vaulted ceiling up to the ridge. Rafters are 200mm deep, packed with a very hi-po wood-based insulating mat. The remaining walls (between all the glass) are 100mm conc block cavity construction and is drylined with 19mm aeroboard between battens and 12.5mm plasterboard. Cavity is filled with bead insulation. Room has UFH over 150mm polystyrene insulation. There's a balanced flue Riva 70 fire in the room too.
Now, I originally planned to put a door between this, and the kitchen, but pressure of time means I didn't get around to it. Good job. It is the hottest room in the house. Fire rarely on, or on v.low just for looks. Not unknown in winter to have to open door to outside........I have no idea what the glazing is, so call it 'standard' double glazing in uPVC windows.
IMHO, the room doesn't cost anyting to heat, and it's the one everyone migrates to. In fact, the front of the house hardly gets used at all now.