Are they offering the 'roof terrace' in the details ?
Unless it has the requisite permissions they cannot call it such.............it's just the flat roof of the extension below.
Roof joists are slimmer than floor joists and thus the terrace 'floor' may not be fit for purpose. If you wish to use it, you would be best advised to have further investigations by a surveyor and then IF remedial work IS required to conform to Building Regulations you may be able to negotiate with the vendors as to who should be liable for the costs of the works to bring it up to standard.