My understanding of planning law is that you can seek to extend a permission for a house depending on how complete the house is, within the last year of the grant of permission.
The requirement for obtaining this grant of extension of permission over the past few years was that it needed to be built up to certain levels, depending on -
(i) the local authority
(ii) the planning oficer and
(iii) the revised legislation involved.
If I recall correctly, it started and went along with this kind of increasing order of work completed; -
(a) ground floor cills level
(b) walls up to first floor level
(c) walls completed to roof level.
My understanding of the current position is that if the walls are complete to wall plate level (roof level) you would normally get an extension to allow you to complete the dwelling.
In the present case the house isn't built or commenced yet - I'm not certain that you can apply for an extension of the existing grant.
You could apply for a new permission, but that may not be what you would want to do.
The other party could introduce matters of ownership into the planning process.
It's difficult to advise on something so remote and only outline information.
If you talk to the planners, you should prepare your comments carefully.
ONQ.
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All advice on AAM is remote from the situation and cannot be relied upon as a defence or support - in and of itself - should legal action be taken.
Competent legal and building professionals should be asked to advise in Real Life with rights to inspect and issue reports on the matters at hand.