Answer
What is the difference between a garden room and an extension?
A garden room is detached in the garden; an extension is built onto the house, with different rules.
In short
A garden room is a separate, detached building in the garden; an extension is built onto the house. They follow different rules, a garden room as an outbuilding and an extension as part of the house, and suit different needs. A garden room is often quicker, less disruptive and cheaper than extending.
A note
General guidance, not advice. Rules vary by site, and Article 4 directions or conditions can change what applies. Check with your local authority and the Planning Portal.
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