Posted: 05/05/2015 at 16:02
Hello Phil & welcome to the forum
I had a lean-to greenhouse at the back of my garage at my previous house. I suppose I could have used it as a garden room but - as Welsh Onion says - the number of days when the temperature was perfect for just sitting in it with a book & glass of wine were few & far between.
In the winter it was nice & sheltered for working (even sitting) in - but always with a coat on and it was insulated with bubble wrap. In the summer the ventilation needed tweaking several times a day to keep the temperature within a comfortable range.
As an alternative - do you have a suitable spot in the garden for a summer house? That would be cheaper than an extension and you could orient it to avoid prevailing winds / take advantage of the evening sun etc. I think you can even get one that rotates - so you can track the sun / shade / shelter as you prefer.