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As long as there are no planning restrictions or other regulations preventing you from doing this then there is no general reason why you cannot subdivide your property and sell the annex as a separate property indeed a great many people have made a good deal of money doing just that. I would suggest you contact a local conveyancing solicitor to check if there are any local restrictions preventing this plan, but as a general statement of law you can subdivide and sell in the way envisaged.