You will need the services of a civil engineer (and possibly an architect) to make the plans and apply for building permission. We provide these services. Building permission can be fairly costly as the Greek Government is anxious to get its hands on as much money as possible. Whilst building, care must be taken to keep all receipts for materials (to prove that VAT has been paid) and for labour. Social insurance for the labour must also be paid. You would be well advised to take a reputable builder for this as otherwise you could end up in a nightmare of Greek bureaucracy. We can advise on this as well.