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I am a solicitor.
I am retired from conveyancing practice and am now a general solicitor.
In my opinion, the garden would not constitute part of the Premises. See definition of "Communal Areas" in your lease which includes any gardens on the "Development".
The area of your flat is outlines in red on Plan 2 which is the last page of the Lease.
Your question relates to the garden, not the terrace. If the terrace is outlined in red in Plan 2, that forms part of your flat.
Please see definition of "The Premises" as more particularly described in the Second Schedule-shown edged red in Plan 2. So whatever is shown edged red in Plan 2 constitutes the Premises.
Yes if it is marked in red on Plan 2.
All the best