To add an extension to your flat you must apply for planning permission.
If your flat is a listed building it is likely you will need listed building consent and you should contact your local planning authority for advice before you start work.
Works to a listed building that affect its special historic character without consent is a criminal offence.
If your flat is in a conservation area and your proposals involve an element of demolition then you may need to apply for conservation area consent and you should contact your local planning authority for advice.
The majority of extensions or conversions of properties require approval under the Building Regulations. You may need to consult the Fire Service regarding issues relating to fire escapes.
Disclaimer - Wales
This is an introductory guide and is not a definitive source of legal information. This guidance relates to the planning regime for Wales. Policy in England may differ. If in doubt contact your Local Planning Authority.