An orangery sits between a conservatory and an extension, and the difference is not marketing. A conservatory is mostly glass. An extension is mostly building. An orangery has substantial masonry piers, a flat perimeter roof with a glazed lantern set into it, and a brick or rendered parapet, which makes it read as part of the house rather than as something attached to it.
Orangeries UK: the building, described plainly
The defining features are masonry piers or columns between the glazed bays, a flat insulated perimeter roof rather than a fully glazed pitched one, a glazed lantern or rooflight over the middle, and usually a parapet or cornice hiding the roof edge. Inside, the perimeter has a real ceiling and the light comes from the centre. The historical version was a masonry building for overwintering citrus trees on an estate, which is where the name comes from and why the proportions look the way they do.
Orangery vs conservatory
A conservatory is substantially glazed in its roof and walls, typically with a dwarf wall, glazing above it and a pitched glazed roof. It is lighter, cheaper and quicker, and it usually keeps the building regulations exemption. An orangery has more solid wall, more insulation and a much better thermal performance, feels like a room rather than a garden structure, and costs more. Where a conservatory is often too hot in July and too cold in January, a well built orangery is neither.
The difference between orangery and extension
An extension has a conventional roof and walls with normal windows. An orangery is an extension with a great deal more glass and a lantern, and in building control terms it is frequently treated as one, because the substantially glazed condition that exempts a conservatory usually is not met. So the honest summary is that an orangery is a highly glazed extension, priced and regulated much closer to an extension than to a conservatory, and sold in the same showroom as the conservatory.
Why the distinction has practical consequences
It decides whether the building regulations exemption applies, which decides whether you get a completion certificate, which a conveyancing solicitor will ask for. It decides the thermal standard the structure has to meet. And it decides whether the room counts as part of the house for the purposes of how a surveyor describes it. None of that is affected by what the brochure calls the product, which is why it is worth asking the supplier directly which regime they are building to.
Other questions people ask
Is an orangery just an expensive conservatory?
No. The masonry piers, the insulated flat perimeter roof and the lantern make it a substantially different building with far better thermal performance, and in building control terms it is usually treated as an extension rather than as an exempt conservatory.
Is an orangery warmer than a conservatory?
Considerably, because much less of its envelope is glass and the perimeter roof is insulated. That is the main practical reason people choose one.
Which adds more value, an orangery or a conservatory?
An orangery generally, because it is built and certified more like an extension and reads as part of the house. A conservatory is often treated by valuers as an addition rather than as living space.
Can an existing conservatory be turned into an orangery?
Sometimes, by replacing the glazed roof with an insulated flat perimeter roof and a lantern and adding piers. Whether the existing base and frame can carry it is the question, and it is assessed rather than assumed.