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Hi thank you for your message, I appreciate this is important to you so I will endeavour to give you a quick but clear answer. Please note this is not a chat service so responses may not be instantaneous. This is not strictly a legal query that could be easily answered, it seems that the water drains into public drains. How that affects the drainage off the land and therefore, whether or not an extension or other building work would be approved on the land should planning permission be applied for is a matter for a chartered surveyor who could inspect the land in question. For more information on planning applications and drainage concerns see here: https://www.planningportal.co.uk/info/200130/common_projects/15/drains_and_sewers
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