Greg, as you've advertised what you want to do on this forum, I presume you want to do it by the rules. Planning rules allow you to construct buildings ancillary to the main dwelling, subject to certain conditions, most notably that they cover no more than half the original garden. What you are proposing may be deemed to be a sub division of the original household, which is not allowed (though enforcement of these rules is another matter). The best thing to do is to go along to Perceval House and join the queue to speak to the duty planning officer. Take along any plans, however rudimentary, you've prepared to date. The planning officer will give you a good steer on what is permissable.
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