Yes I have valuation for house and there are recent sales near by
Assume house is worth 1.5 million. Cost to build annexe say 100k. Monthly rental 1.25k
Just to clarify the rental value is for the annexe rather than the house which would be say 6,250.
I am therefore assuming that capital gains would be payable on the annexe based on 1250/(6250+1250) adjusted for time annexe is used