Does CGT apply on annex once split to separate dwelling

[HC594]

I am considering whether to split the annex from my main house and sell both houses separately. Planning permission aside, my question is will CGT apply if I sell the annex on its own and what period would the CGT apply to? I.e would it apply to the whole time I have owned the whole property (main house &annex) or would it apply only from the point it is officially classed as a separate dwelling?

We bought the entire property 5 years ago for £450k. The main house is now worth £650k and the annex £325. So I’m guessing the whole annex would be deemed a ‘gain’ for CGT purposes? Thank you in advance.

[maths]

A sale of the annex plus surrounding land will constitute a partial disposal for CGT purposes. Any gain may be covered by principle private residence relief.

If however you build on the land with a view to sale any gain will likely be subject to income not capital gains tax.