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I would be grateful if you could confirm if it states in your tenancy agreement that the garden in question is a communal area? Kind regards
Your tenancy should define your rights and responsibilities regarding the garden. You may also pay a service charge for upkeep and maintenance of the garden.
If your rights in relation to the garden are defined in your tenancy agreement it would seem unjust for your landlord to provide the other tenant exclusive rights to the garden.
I would recommend you speak to your landlord directly regarding this (if you haven't done so already) and request a formal answer in writing.
Court action can be costly so I would recommend exhausting your landlord's complaints procedure and if you remain unsatisfied requesting that the housing ombudsman investigate matters for you.
Your landlord should provide you a copy of your tenancy agreement as you are entitled to this. In regards ***** ***** further documents regarding your tenancy, you could make a subject access request.
As stated above it really does depend on what your tenancy agreement says. Kind regards
I am happy to answer the additional questions you have raised and can offer a premium service for this. Kind regards