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Good morning, if the local authority are not willing to enforce any action against breach of the planning conditions then you would have to take matters into your own hands. It would probably not be a breach of the lease but you could argue that as a co-freeholder you did not consent to the works to be to this boundary. You need to consider the legal costs and time implications of bringing such a claim forward, and also have reasonable grounds why this is causing you nuisance or detriment. Particularly because reducing the height of an extension would be a major job so they would probably not give in easily. Furthermore you needs to consider whether the external boundaries are communal (owned by both of you) unless they have built internally and taken away from your use and enjoyment of property that you own. Hope this helps.
The Lease will specify the extent of your boundary - it is rare that would include the space outside particularly for a first floor flat. Something like the garden (if any) is demised to the ground floor unless specified it is shared - but that does not seem like the case here if they have built an extension. so if they are building on their garden which is demised to them then that is fine. If they build upwards essentially they should obtain your consent as a co-freeholder because yes it is a breach to proceed without consent. However this would only really affect you if they created a second floor to the extension e.g. blocking your view or being completely parrallel to say your window. Hope this makes sense.
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