A realistic comparison of cost, disruption and year-round usability before you commit either way.
Both add space. They add it in very different ways, over very different timescales, at very different costs — and the right answer depends on what the room is for.
This guide compares them on the criteria homeowners tell us actually mattered once the work was finished.
Weighed against each other:
A conservatory with a solid insulated roof, thermally efficient glazing and proper heating sits much closer to extension comfort than the draughty glass boxes the format is remembered for.
Equally, an orangery bridges the two. If you like the conservatory budget but need the extension comfort, that middle ground is where to look.
Ask what the room must do in January. If the answer is 'be a kitchen, bedroom or everyday living space', build the extension. If it's 'be a bright, comfortable sitting space', a well-specified conservatory will likely serve you better per pound spent.
We'll quote the conservatory and, where it's the better answer, tell you plainly that an extension is what you actually need.
Sometimes the base and dwarf walls can be reused, but not always — it depends on how the original was built. Have it assessed before assuming.
A conservatory, by a clear margin — fewer trades, fewer inspections and a largely prefabricated envelope.
Both can fall under permitted development within limits on size, height and boundaries, with different rules in conservation areas and on listed buildings. Check with your local authority first.