Choose an extension when connection to the house is the point
If the goal is a larger kitchen, a connected family room or extra circulation space, an extension often makes more sense because it changes the way the existing house works. It can also be the better route where garden space is limited and every square metre outside matters.
Extensions usually involve more interaction with the existing structure, services and finishes, so the disruption and regulatory route can be more involved.
Choose a garden room when separation is useful
A detached room is especially strong when the use benefits from being away from the main house: focused work, training, music, hobbies, cinema, games or entertaining. That physical separation can be the feature rather than a compromise.
Construction can also be less disruptive to day-to-day life because much of the work happens outside the home.
Planning and building regulations differ
Both routes can sometimes proceed without a full planning application, but the rules are different and depend on the property and design. Garden rooms can fall within outbuilding permitted-development rights, while extensions have their own limits and conditions.
Building-regulation requirements also differ by size, use, construction and proximity to boundaries. Check the specific project rather than assuming one route is automatically simpler.
Compare the space you actually need
Before comparing costs, write down the activities the new room needs to support. If it needs direct access from the kitchen every hour of the day, an extension may win. If it needs quiet, noise separation or a different atmosphere from the house, a garden room can be the better piece of architecture.