Inheritance tax - gift with reservation

......and of course the land on which it is built and the large garden.

The Son then builds a self-contained annex to the house for the mother's sole occupation and constructs an entirely separately fenced garden for the annex.

The Son occupies the house solely by himself.

A Gift with Reservation has clearly occurred (in the absence of mother paying son a commercail rent for the annex) in relation to the original gift - the question is, should the value to be taken into account on death of the mother be the value only of the annex or of the whole property originally gifted?