A conservatory is primarily built of PVC or glass walls and finishing while an orangery is built with glass and a percentage of it made of brick. An observatory is an open kind of structure while the orangery is more enclosed in terms of privacy. Orangeries have flat roofs, which will offer privacy from overlooking windows in the rest of the home while a conservatory can be viewed from any angle because of its clear walls.
Orangeries are considerably more expensive to build than the observatory because of their mix in building materials. With conservatories, it may be much cheaper to maintain and build because one can use low insulation materials like PVC or use double-glazing because traditional ones made of glass and other materials took long to heat. One should choose which one to construct based on the overall design of the home, weather conditions, how much one is willing to spend, and the uses of the structure one wishes to build.