Assuming the flat is in England or Wales.
Do you mean you, individually have the freehold of the whole property part of which is your flat or do you mean that the lessees have grouped together to form a comapny to buy the freehold or do you mean a freehold flat (which is very rare)?
Largely irrelevant though.
Flats do not have Permitted Development rights under the Planning system. So any changes you wish to make (an extension, changing the windows) are subject to seeking and gaining Planning Consent.
This aside structural changes along with alteration to the electrics, drainage, thermal element etc... will also require Building Regulation approval.
There is always scope for misinterpretation.
If my posts can be interpreted in two ways, one that makes you feel angry and one that doesn't, I meant the latter.
Everyday is an opportunity to learn something new.