If it is sufficiently important, you might want to get an architect to word the objection.
You do realise of course that there is no way of getting out of the current situation without some form of falling out with your neighbour.
The usage as accommodation would not be allowed - is your neighbour getting planning for a toilet etc.? If yes, it should raise some flags for the Planning Dept.
You might include lines in your objection such as "we would be concerned that the proposed structure would be used for more permanent occupation than suggested in the Planning Application". Telling the Planner more directly that your neighbour cannot be trusted and will not comply with Planning is not a particularly good course of action without a prior history of such action (e.g. where the neighbour has repeatedly tried for retention permission), and in any case may allow you to be sued for libel.
Standard grounds for objection can be used in any case - the overlooking, loss of privacy, loss of residential amenity, loss of use of back garden, impact on value of your house, if the building overshadows your garden etc.
You might also criticise the use of materials if they are not in keeping with the existing.
How much garden space will be left when they are finished?