[starry]

to have a lean to green house that has 3 solid brick walls, the roof and half of one side and the front being glass or similar ?   and how high would the walls have to be to take a sloping roof ?

[8doubles]

Yes very possible.
The wider the room the higher the back wall need to be . Triplewall polycarbonate is probably your best choice for the roof as it is safer than glass and does not need a lot of pitch to get the rainwater away.
Brickwork can act as a storage heater and be very useful in keeping a steady temperature for growing in the colder weather.

[Trillium]

I remember seeing a pic of Toby Buckland's greenhouse and if memory serves, it was a 3 sided brick structure that seemed quite cozy.