Splitting my house and capital gains tax

Here is the deal:

I bought my primary residence several years ago.

I added an apartment which was not considered a separate dwelling by the council at the time.

I recently applied for and received planning permission to carry out works to make the apartment a separate property in its own right.

I am selling my primary residence and keeping the apartment.

In anyone's opinion, am I goign to have a capital gains issue?
Has anyone else had this situation or heard of it?

(yes i will be getting professional advice too)