Hi,
We cannot comment on any situation until after the fact as we might not be in possession of all of the facts and also cannot provide financial advice.
However, we can advise that if the titles between the annex and the original house were split, the apportionment of the initial purchase price between both, would need to be on a just and reasonable basis and consistent between both.
The new kitchen and bathroom would be a capital cost and again, if no receipts can be provided, a just and reasonable estimate could be used.
You can see guidance on Private Residence Relief here:
CG64311 - Private residence relief: the entity of the dwelling-house: part of the building let
HS283 Private Residence Relief (2024)
Thank you.